Dermpath Diagnosis

Folliculotropic Mycosis Fungoides

Folliculotropic mycosis fungoides (FMF) is a variant of mycosis fungoides (MF) that occurs mostly in adults with a male predilection. The disease clinically favors the head and neck. Patients commonly present with pruritic papules that often are grouped, alopecia, and frequent secondary bacterial infections. Less commonly patients present with acneiform lesions and mucinorrhea. Patients often experience more pruritus in FMF than in classic MF, which can provide a good means of assessing disease activity. Disease-specific 5-year survival is approximately 70% to 80%, which is worse than classic plaque-stage MF and similar to tumor-stage MF.¹

Treatment of FMF differs from classic MF in that the lesions are less responsive to skin-targeted therapies due to the perifollicular nature of dermal infiltrates. Superficial skin lesions can be treated with psoralen plus UVA (PUVA) therapy. Other options include PUVA in combination with interferon alfa or retinoids and local radiotherapy for solitary thick tumors; however, in patients who have more infiltrative skin lesions or had PUVA therapy that failed, total skin electron beam therapy may be required.²

On histologic examination, there typically is perivascular and periadnexal localization of dermal infiltrates with varied involvement of the follicular epithelium and damage to hair follicles by atypical small, medium, and large hyperchromatic lymphocytes with cerebriform nuclei. Mucinous degeneration of the follicular epithelium can be seen, as highlighted on Alcian blue staining, and a mixed infiltrate of eosinophils and plasma cells often is present (quiz image and Figure 1). Frequent sparing of the epidermis is noteworthy.^2-4 In most cases, the neoplastic T lymphocytes are characterized by a CD3⁺CD4⁺CD8^-immunophenotype as is seen in classic MF. Sometimes an admixture of CD30⁺ blast cells is seen.¹

Histologic differential considerations for FMF include eosinophilic pustular folliculitis (EPF), primary follicular mucinosis, lupus erythematosus, and pityrosporum folliculitis.

Eosinophilic pustular folliculitis has several clinical subtypes, such as classic Ofuji disease and immunosuppression-associated EPF secondary to human immunodeficiency virus. Histologically, EPF is characterized by spongiosis of the hair follicle epithelium with exocytosis of a mixed infiltrate of lymphocytes and eosinophils extending from the sebaceous gland and its duct to the infundibulum with formation of hallmark eosinophilic pustules (Figure 2). Infiltration of neutrophils in inflamed lesions generally is seen. Eosinophilic pustular folliculitis is an important differential for FMF, as follicular mucinosis has been observed in lesions of EPF.⁵ Both EPF and FMF can exhibit eosinophils and lymphocytes in the upper dermis, spongiosis of the hair follicle epithelium, and mucinous degeneration of follicles,⁶ though lymphocytic atypia and relatively fewer eosinophils are suggestive of the latter.

Photograph courtesy of Brian Swick, MD (Iowa City, Iowa).

 

Primary follicular mucinosis (PFM) tends to occur as a solitary lesion in younger female patients in contrast to the multiple lesions that typically appear in older male patients with FMF. Histologically, PFM usually manifests as large, cystic, mucin-filled spaces and polyclonal perivascular and periadnexal lymphocytic infiltrate without notable cellular atypia or epidermotropism (Figure 3). Because follicular mucinosis is a common feature of FMF, its distinction from PFM can be challenging and often is aided by the absence of cellular atypia and relatively mild lymphocytic infiltrate in the latter.⁷

Photograph courtesy of Brian Swick, MD (Iowa City, Iowa).

Cutaneous lupus erythematosus with its characteristic folliculocentric lymphocytic infiltration and associated dermal mucin also qualifies as a potential differential possibility for FMF; however, the perivascular and periadnexal pattern of lymphocytic infiltration as well as the localization of mucin to the reticular dermal interstitium^8,9 are key histopathologic distinctions (Figure 4). Furthermore, although the histologic presentation of lupus erythematosus can be variable, it also classically shows interface dermatitis, basement membrane thickening, and follicular plugging.

Photograph courtesy of Brian Swick, MD (Iowa City, Iowa).

Pityrosporum folliculitis is the most common cause of fungal folliculitis and is caused by the Malassezia species. On histology, there typically is an unremarkable epithelium with plugged follicles and suppurative folliculitis. Serial sections of the biopsy specimen often are required to identify dilated, follicle-containing, budding yeast cells (Figure 5). Organisms are located predominantly within the infundibulum and orifice of follicular lumen, are positive for periodic acid-Schiff, and are diastase resistant.¹⁰

Photograph courtesy of Brian Swick, MD (Iowa City, Iowa).

Folliculotropic Mycosis Fungoides

Folliculotropic mycosis fungoides (FMF) is a variant of mycosis fungoides (MF) that occurs mostly in adults with a male predilection. The disease clinically favors the head and neck. Patients commonly present with pruritic papules that often are grouped, alopecia, and frequent secondary bacterial infections. Less commonly patients present with acneiform lesions and mucinorrhea. Patients often experience more pruritus in FMF than in classic MF, which can provide a good means of assessing disease activity. Disease-specific 5-year survival is approximately 70% to 80%, which is worse than classic plaque-stage MF and similar to tumor-stage MF.¹

Treatment of FMF differs from classic MF in that the lesions are less responsive to skin-targeted therapies due to the perifollicular nature of dermal infiltrates. Superficial skin lesions can be treated with psoralen plus UVA (PUVA) therapy. Other options include PUVA in combination with interferon alfa or retinoids and local radiotherapy for solitary thick tumors; however, in patients who have more infiltrative skin lesions or had PUVA therapy that failed, total skin electron beam therapy may be required.²

On histologic examination, there typically is perivascular and periadnexal localization of dermal infiltrates with varied involvement of the follicular epithelium and damage to hair follicles by atypical small, medium, and large hyperchromatic lymphocytes with cerebriform nuclei. Mucinous degeneration of the follicular epithelium can be seen, as highlighted on Alcian blue staining, and a mixed infiltrate of eosinophils and plasma cells often is present (quiz image and Figure 1). Frequent sparing of the epidermis is noteworthy.^2-4 In most cases, the neoplastic T lymphocytes are characterized by a CD3⁺CD4⁺CD8^-immunophenotype as is seen in classic MF. Sometimes an admixture of CD30⁺ blast cells is seen.¹

Histologic differential considerations for FMF include eosinophilic pustular folliculitis (EPF), primary follicular mucinosis, lupus erythematosus, and pityrosporum folliculitis.

Eosinophilic pustular folliculitis has several clinical subtypes, such as classic Ofuji disease and immunosuppression-associated EPF secondary to human immunodeficiency virus. Histologically, EPF is characterized by spongiosis of the hair follicle epithelium with exocytosis of a mixed infiltrate of lymphocytes and eosinophils extending from the sebaceous gland and its duct to the infundibulum with formation of hallmark eosinophilic pustules (Figure 2). Infiltration of neutrophils in inflamed lesions generally is seen. Eosinophilic pustular folliculitis is an important differential for FMF, as follicular mucinosis has been observed in lesions of EPF.⁵ Both EPF and FMF can exhibit eosinophils and lymphocytes in the upper dermis, spongiosis of the hair follicle epithelium, and mucinous degeneration of follicles,⁶ though lymphocytic atypia and relatively fewer eosinophils are suggestive of the latter.

Photograph courtesy of Brian Swick, MD (Iowa City, Iowa).

 

Primary follicular mucinosis (PFM) tends to occur as a solitary lesion in younger female patients in contrast to the multiple lesions that typically appear in older male patients with FMF. Histologically, PFM usually manifests as large, cystic, mucin-filled spaces and polyclonal perivascular and periadnexal lymphocytic infiltrate without notable cellular atypia or epidermotropism (Figure 3). Because follicular mucinosis is a common feature of FMF, its distinction from PFM can be challenging and often is aided by the absence of cellular atypia and relatively mild lymphocytic infiltrate in the latter.⁷

Photograph courtesy of Brian Swick, MD (Iowa City, Iowa).

Cutaneous lupus erythematosus with its characteristic folliculocentric lymphocytic infiltration and associated dermal mucin also qualifies as a potential differential possibility for FMF; however, the perivascular and periadnexal pattern of lymphocytic infiltration as well as the localization of mucin to the reticular dermal interstitium^8,9 are key histopathologic distinctions (Figure 4). Furthermore, although the histologic presentation of lupus erythematosus can be variable, it also classically shows interface dermatitis, basement membrane thickening, and follicular plugging.

Photograph courtesy of Brian Swick, MD (Iowa City, Iowa).

Pityrosporum folliculitis is the most common cause of fungal folliculitis and is caused by the Malassezia species. On histology, there typically is an unremarkable epithelium with plugged follicles and suppurative folliculitis. Serial sections of the biopsy specimen often are required to identify dilated, follicle-containing, budding yeast cells (Figure 5). Organisms are located predominantly within the infundibulum and orifice of follicular lumen, are positive for periodic acid-Schiff, and are diastase resistant.¹⁰

Photograph courtesy of Brian Swick, MD (Iowa City, Iowa).

Folliculotropic Mycosis Fungoides

Folliculotropic mycosis fungoides (FMF) is a variant of mycosis fungoides (MF) that occurs mostly in adults with a male predilection. The disease clinically favors the head and neck. Patients commonly present with pruritic papules that often are grouped, alopecia, and frequent secondary bacterial infections. Less commonly patients present with acneiform lesions and mucinorrhea. Patients often experience more pruritus in FMF than in classic MF, which can provide a good means of assessing disease activity. Disease-specific 5-year survival is approximately 70% to 80%, which is worse than classic plaque-stage MF and similar to tumor-stage MF.¹

Treatment of FMF differs from classic MF in that the lesions are less responsive to skin-targeted therapies due to the perifollicular nature of dermal infiltrates. Superficial skin lesions can be treated with psoralen plus UVA (PUVA) therapy. Other options include PUVA in combination with interferon alfa or retinoids and local radiotherapy for solitary thick tumors; however, in patients who have more infiltrative skin lesions or had PUVA therapy that failed, total skin electron beam therapy may be required.²

On histologic examination, there typically is perivascular and periadnexal localization of dermal infiltrates with varied involvement of the follicular epithelium and damage to hair follicles by atypical small, medium, and large hyperchromatic lymphocytes with cerebriform nuclei. Mucinous degeneration of the follicular epithelium can be seen, as highlighted on Alcian blue staining, and a mixed infiltrate of eosinophils and plasma cells often is present (quiz image and Figure 1). Frequent sparing of the epidermis is noteworthy.^2-4 In most cases, the neoplastic T lymphocytes are characterized by a CD3⁺CD4⁺CD8^-immunophenotype as is seen in classic MF. Sometimes an admixture of CD30⁺ blast cells is seen.¹

Histologic differential considerations for FMF include eosinophilic pustular folliculitis (EPF), primary follicular mucinosis, lupus erythematosus, and pityrosporum folliculitis.

Eosinophilic pustular folliculitis has several clinical subtypes, such as classic Ofuji disease and immunosuppression-associated EPF secondary to human immunodeficiency virus. Histologically, EPF is characterized by spongiosis of the hair follicle epithelium with exocytosis of a mixed infiltrate of lymphocytes and eosinophils extending from the sebaceous gland and its duct to the infundibulum with formation of hallmark eosinophilic pustules (Figure 2). Infiltration of neutrophils in inflamed lesions generally is seen. Eosinophilic pustular folliculitis is an important differential for FMF, as follicular mucinosis has been observed in lesions of EPF.⁵ Both EPF and FMF can exhibit eosinophils and lymphocytes in the upper dermis, spongiosis of the hair follicle epithelium, and mucinous degeneration of follicles,⁶ though lymphocytic atypia and relatively fewer eosinophils are suggestive of the latter.

Photograph courtesy of Brian Swick, MD (Iowa City, Iowa).

 

Primary follicular mucinosis (PFM) tends to occur as a solitary lesion in younger female patients in contrast to the multiple lesions that typically appear in older male patients with FMF. Histologically, PFM usually manifests as large, cystic, mucin-filled spaces and polyclonal perivascular and periadnexal lymphocytic infiltrate without notable cellular atypia or epidermotropism (Figure 3). Because follicular mucinosis is a common feature of FMF, its distinction from PFM can be challenging and often is aided by the absence of cellular atypia and relatively mild lymphocytic infiltrate in the latter.⁷

Photograph courtesy of Brian Swick, MD (Iowa City, Iowa).

Cutaneous lupus erythematosus with its characteristic folliculocentric lymphocytic infiltration and associated dermal mucin also qualifies as a potential differential possibility for FMF; however, the perivascular and periadnexal pattern of lymphocytic infiltration as well as the localization of mucin to the reticular dermal interstitium^8,9 are key histopathologic distinctions (Figure 4). Furthermore, although the histologic presentation of lupus erythematosus can be variable, it also classically shows interface dermatitis, basement membrane thickening, and follicular plugging.

Photograph courtesy of Brian Swick, MD (Iowa City, Iowa).

Pityrosporum folliculitis is the most common cause of fungal folliculitis and is caused by the Malassezia species. On histology, there typically is an unremarkable epithelium with plugged follicles and suppurative folliculitis. Serial sections of the biopsy specimen often are required to identify dilated, follicle-containing, budding yeast cells (Figure 5). Organisms are located predominantly within the infundibulum and orifice of follicular lumen, are positive for periodic acid-Schiff, and are diastase resistant.¹⁰

Photograph courtesy of Brian Swick, MD (Iowa City, Iowa).

Cutaneous Metastasis of Pulmonary Adenocarcinoma

Cutaneous metastasis of pulmonary adenocarcinoma (CMPA) is a rare phenomenon with an overall survival rate of less than 5 months.1,2 Often, CMPA can be the heralding feature of an aggressive systemic malignancy in 2.8% to 22% of reported cases.^2-4 Clinically, CMPAs often present as fixed, violaceous, ulcerated nodules on the chest wall, scalp, or site of a prior procedure.^3,5,6 Other clinical presentations have been described including zosteriform and inflammatory carcinomalike CMPA and CMPA on the tip of the nose.⁷ Histologically, CMPA presents as a subdermal collection of atypical glands arranged as clustered aggregates of infiltrative glands penetrating the dermal stroma (quiz image). The atypical glands have large oval nuclei with high nuclear to cytoplasm ratios with scant pale cytoplasm.

Cutaneous metastasis of pulmonary adenocarcinoma is difficult to distinguish from other metastatic or primary glandular malignancies based on histology alone. Immunohistochemical analysis can aid in the diagnosis of the primary tumor. Pulmonary adenocarcinomas are positive for cytokeratin (CK) 7 and thyroid transcription factor 1 (TTF-1), and they are negative for CK5/6 and CK20.⁷ The differential diagnosis for CMPA includes other internal malignancies such as invasive ductal adenocarcinoma of the breast and gastrointestinal adenocarcinomas (eg, gastric or colorectal carcinoma [CRC]). Additionally, endometriosis and primary sebaceous carcinomas can mimic cutaneous metastatic adenocarcinomas.

Endometriosis can mimic adenocarcinoma, especially when presenting as a subdermal nodule. However, the scattered dermal glands are cytologically banal and are surrounded by uterine-type stroma and extravasated hemorrhage, a classic presentation of endometriosis (Figure 1).

Invasive ductal carcinoma of the breast is one of the most common cutaneous metastases of internal malignancy.³ Clinically, these lesions present on the chest wall or abdomen as flesh-colored nodules. Histopathology generally reveals either tubular or single tumor cells infiltrating the dermis with surrounding desmoplastic fibrosis (Figure 2). Immunohistochemistry typically is positive for CK7, estrogen receptor, and mammaglobin, and negative for CK20, CK5/6, and TTF-1.

Gastrointestinal adenocarcinomas encompass a variety of primary sites that can metastasize to the skin including CRC. Clinically, cutaneous metastases of CRC present as multiple nodules on the trunk, abdomen, or umbilicus (also known as Sister Mary Joseph nodule).^7,8 Distinguishing CRC as the primary site of origin can be difficult; however, there are subtle differences depending on the histologic subtype. In well-differentiated CRCs, well-defined atypical glands are haphazardly arranged within the dermis (Figure 3), while poorly differentiated lesions can present as single cells or with a signet ring-like morphology (Figure 4). For perianal lesions, extramammary Paget disease should be considered when biopsies show large, amphophilic, intraepithelial cells. These lesions often present with mucin and CK20 expression and are frequently associated with colorectal malignancies.⁹ Another characteristic feature of CRC is central necrosis with karyorrhectic debris, known as dirty necrosis. Immunohistochemical analysis typically shows expression of caudal type homeobox 2 and CK20 with infrequent expression of CK7 and no expression of TTF-1; however, additional clinical history (eg, history of colorectal adenocarcinoma, positive fecal occult blood test) often is the best distinguishing feature. 

 

Primary sebaceous carcinoma also can mimic metastatic adenocarcinoma within the skin and is histologically similar to metastatic adenocarcinomas. The most distinguishing feature is sebaceous differentiation characterized by sebocytes, which have a vacuolated cytoplasm giving the nucleus a scalloped appearance, frequently with adjacent ductlike structures (Figure 5). Epidermotropism sometimes is present in sebaceous carcinomas but cannot be relied on as a distinguishing feature. Immunohistochemical analysis also is a helpful tool; these tumors typically are positive for p63 and podoplanin, distinguishing them from negative-staining metastatic adenocarcinomas.^10,11 

Cutaneous Metastasis of Pulmonary Adenocarcinoma

Cutaneous metastasis of pulmonary adenocarcinoma (CMPA) is a rare phenomenon with an overall survival rate of less than 5 months.1,2 Often, CMPA can be the heralding feature of an aggressive systemic malignancy in 2.8% to 22% of reported cases.^2-4 Clinically, CMPAs often present as fixed, violaceous, ulcerated nodules on the chest wall, scalp, or site of a prior procedure.^3,5,6 Other clinical presentations have been described including zosteriform and inflammatory carcinomalike CMPA and CMPA on the tip of the nose.⁷ Histologically, CMPA presents as a subdermal collection of atypical glands arranged as clustered aggregates of infiltrative glands penetrating the dermal stroma (quiz image). The atypical glands have large oval nuclei with high nuclear to cytoplasm ratios with scant pale cytoplasm.

Cutaneous metastasis of pulmonary adenocarcinoma is difficult to distinguish from other metastatic or primary glandular malignancies based on histology alone. Immunohistochemical analysis can aid in the diagnosis of the primary tumor. Pulmonary adenocarcinomas are positive for cytokeratin (CK) 7 and thyroid transcription factor 1 (TTF-1), and they are negative for CK5/6 and CK20.⁷ The differential diagnosis for CMPA includes other internal malignancies such as invasive ductal adenocarcinoma of the breast and gastrointestinal adenocarcinomas (eg, gastric or colorectal carcinoma [CRC]). Additionally, endometriosis and primary sebaceous carcinomas can mimic cutaneous metastatic adenocarcinomas.

Endometriosis can mimic adenocarcinoma, especially when presenting as a subdermal nodule. However, the scattered dermal glands are cytologically banal and are surrounded by uterine-type stroma and extravasated hemorrhage, a classic presentation of endometriosis (Figure 1).

Invasive ductal carcinoma of the breast is one of the most common cutaneous metastases of internal malignancy.³ Clinically, these lesions present on the chest wall or abdomen as flesh-colored nodules. Histopathology generally reveals either tubular or single tumor cells infiltrating the dermis with surrounding desmoplastic fibrosis (Figure 2). Immunohistochemistry typically is positive for CK7, estrogen receptor, and mammaglobin, and negative for CK20, CK5/6, and TTF-1.

Gastrointestinal adenocarcinomas encompass a variety of primary sites that can metastasize to the skin including CRC. Clinically, cutaneous metastases of CRC present as multiple nodules on the trunk, abdomen, or umbilicus (also known as Sister Mary Joseph nodule).^7,8 Distinguishing CRC as the primary site of origin can be difficult; however, there are subtle differences depending on the histologic subtype. In well-differentiated CRCs, well-defined atypical glands are haphazardly arranged within the dermis (Figure 3), while poorly differentiated lesions can present as single cells or with a signet ring-like morphology (Figure 4). For perianal lesions, extramammary Paget disease should be considered when biopsies show large, amphophilic, intraepithelial cells. These lesions often present with mucin and CK20 expression and are frequently associated with colorectal malignancies.⁹ Another characteristic feature of CRC is central necrosis with karyorrhectic debris, known as dirty necrosis. Immunohistochemical analysis typically shows expression of caudal type homeobox 2 and CK20 with infrequent expression of CK7 and no expression of TTF-1; however, additional clinical history (eg, history of colorectal adenocarcinoma, positive fecal occult blood test) often is the best distinguishing feature. 

 

Primary sebaceous carcinoma also can mimic metastatic adenocarcinoma within the skin and is histologically similar to metastatic adenocarcinomas. The most distinguishing feature is sebaceous differentiation characterized by sebocytes, which have a vacuolated cytoplasm giving the nucleus a scalloped appearance, frequently with adjacent ductlike structures (Figure 5). Epidermotropism sometimes is present in sebaceous carcinomas but cannot be relied on as a distinguishing feature. Immunohistochemical analysis also is a helpful tool; these tumors typically are positive for p63 and podoplanin, distinguishing them from negative-staining metastatic adenocarcinomas.^10,11 

Cutaneous Metastasis of Pulmonary Adenocarcinoma

Cutaneous metastasis of pulmonary adenocarcinoma (CMPA) is a rare phenomenon with an overall survival rate of less than 5 months.1,2 Often, CMPA can be the heralding feature of an aggressive systemic malignancy in 2.8% to 22% of reported cases.^2-4 Clinically, CMPAs often present as fixed, violaceous, ulcerated nodules on the chest wall, scalp, or site of a prior procedure.^3,5,6 Other clinical presentations have been described including zosteriform and inflammatory carcinomalike CMPA and CMPA on the tip of the nose.⁷ Histologically, CMPA presents as a subdermal collection of atypical glands arranged as clustered aggregates of infiltrative glands penetrating the dermal stroma (quiz image). The atypical glands have large oval nuclei with high nuclear to cytoplasm ratios with scant pale cytoplasm.

Cutaneous metastasis of pulmonary adenocarcinoma is difficult to distinguish from other metastatic or primary glandular malignancies based on histology alone. Immunohistochemical analysis can aid in the diagnosis of the primary tumor. Pulmonary adenocarcinomas are positive for cytokeratin (CK) 7 and thyroid transcription factor 1 (TTF-1), and they are negative for CK5/6 and CK20.⁷ The differential diagnosis for CMPA includes other internal malignancies such as invasive ductal adenocarcinoma of the breast and gastrointestinal adenocarcinomas (eg, gastric or colorectal carcinoma [CRC]). Additionally, endometriosis and primary sebaceous carcinomas can mimic cutaneous metastatic adenocarcinomas.

Endometriosis can mimic adenocarcinoma, especially when presenting as a subdermal nodule. However, the scattered dermal glands are cytologically banal and are surrounded by uterine-type stroma and extravasated hemorrhage, a classic presentation of endometriosis (Figure 1).

Invasive ductal carcinoma of the breast is one of the most common cutaneous metastases of internal malignancy.³ Clinically, these lesions present on the chest wall or abdomen as flesh-colored nodules. Histopathology generally reveals either tubular or single tumor cells infiltrating the dermis with surrounding desmoplastic fibrosis (Figure 2). Immunohistochemistry typically is positive for CK7, estrogen receptor, and mammaglobin, and negative for CK20, CK5/6, and TTF-1.

Gastrointestinal adenocarcinomas encompass a variety of primary sites that can metastasize to the skin including CRC. Clinically, cutaneous metastases of CRC present as multiple nodules on the trunk, abdomen, or umbilicus (also known as Sister Mary Joseph nodule).^7,8 Distinguishing CRC as the primary site of origin can be difficult; however, there are subtle differences depending on the histologic subtype. In well-differentiated CRCs, well-defined atypical glands are haphazardly arranged within the dermis (Figure 3), while poorly differentiated lesions can present as single cells or with a signet ring-like morphology (Figure 4). For perianal lesions, extramammary Paget disease should be considered when biopsies show large, amphophilic, intraepithelial cells. These lesions often present with mucin and CK20 expression and are frequently associated with colorectal malignancies.⁹ Another characteristic feature of CRC is central necrosis with karyorrhectic debris, known as dirty necrosis. Immunohistochemical analysis typically shows expression of caudal type homeobox 2 and CK20 with infrequent expression of CK7 and no expression of TTF-1; however, additional clinical history (eg, history of colorectal adenocarcinoma, positive fecal occult blood test) often is the best distinguishing feature. 

 

Primary sebaceous carcinoma also can mimic metastatic adenocarcinoma within the skin and is histologically similar to metastatic adenocarcinomas. The most distinguishing feature is sebaceous differentiation characterized by sebocytes, which have a vacuolated cytoplasm giving the nucleus a scalloped appearance, frequently with adjacent ductlike structures (Figure 5). Epidermotropism sometimes is present in sebaceous carcinomas but cannot be relied on as a distinguishing feature. Immunohistochemical analysis also is a helpful tool; these tumors typically are positive for p63 and podoplanin, distinguishing them from negative-staining metastatic adenocarcinomas.^10,11 

The Diagnosis: Subcutaneous Phaeohyphomycosis

Subcutaneous phaeohyphomycosis (SP), also called mycotic cyst, is characterized by a painless, nodular lesion that develops in response to traumatic implantation of dematiaceous, pigment-forming fungi.¹ Similar to other fungal infections, SP can arise opportunistically in immunocompromised patients.^2,3 More than 60 genera (and more than 100 species) are known etiologic agents of phaeohyphomycosis; the 2 main causes of infection are Bipolaris spicifera and Exophiala jeanselmei.^4,5 Given this variety, phaeohyphomycosis can present superficially as black piedra or tinea nigra, cutaneously as scytalidiosis, subcutaneously as SP, or disseminated as sinusitis or systemic phaeohyphomycosis.

Coined in 1974 by Ajello et al,⁶ the term phaeohyphomycosis translates to “condition of dark hyphal fungus,” a term used to designate mycoses caused by fungi with melanized hyphae. Histologically, SP demonstrates a circumscribed chronic cyst or abscess with a dense fibrous wall (quiz image A). At high power, the wall is composed of chronic granulomatous inflammation with foamy macrophages, and the cystic cavity contains necrotic debris admixed with neutrophils. Pigmented filamentous hyphae and yeastlike entities can be seen in the cyst wall, in multinucleated giant cells, in the necrotic debris, or directly attached to the implanted foreign material (quiz image B).⁷ The first-line treatment of SP is wide local excision and oral itraconazole. It often requires adjustments to dosage or change to antifungal due to recurrence and etiologic variation.⁸ Furthermore, if SP is not definitively treated, immunocompromised patients are at an increased risk for developing potentially fatal systemic phaeohyphomycosis.³

Chromoblastomycosis (CBM), also caused by dematiaceous fungi, is characterized by an initially indolent clinical presentation. Typically found on the legs and lower thighs of agricultural workers, the lesion begins as a slow-growing, nodular papule with subsequent transformation into an edematous verrucous plaque with peripheral erythema.⁹ Lesions can be annular with central clearing, and lymphedema with elephantiasis may be present.¹⁰ Histologically, CBM shows pseudoepitheliomatous hyperplasia and intraepidermal pustules as the host rids the infection via transepithelial elimination. Dematiaceous fungi often are seen in the dermis, either freestanding or attached to foreign plant material. Medlar bodies, also called copper penny spores or sclerotic bodies, are the most defining histologic finding and are characterized by groups of brown, thick-walled cells found in giant cells or neutrophil abscesses (Figure 1). Hyphae are not typically found in this type of infection.¹¹

Granulomatous foreign body reactions occur in response to the inoculation of nonhuman material and are characterized by dermal or subcutaneous nodules. Tissue macrophages phagocytize material not removed shortly after implantation, which initiates an inflammatory response that attempts to isolate the material from the uninvolved surrounding tissue. Vegetative foreign bodies will cause the most severe inflammatory reactions.¹² Histologically, foreign body granulomas are noncaseating with epithelioid histiocytes surrounding a central foreign body (Figure 2). Occasionally, foreign bodies may be difficult to detect; some are birefringent to polarized light.¹³ Additionally, inoculation injuries can predispose patients to SP, CBM, and other fungal infections.

Tattoos are characterized by exogenous pigment deposition into the dermis.¹⁴ Histologically, tattoos display exogenous pigment deposited throughout the reticular dermis, attached to collagen bundles, within macrophages, or adjacent to adnexal structures (eg, pilosebaceous units or eccrine glands). Although all tattoo pigments can cause adverse reactions, hypersensitivity reactions occur most commonly in response to red pigment, resulting in discrete areas of spongiosis and granulomatous or lichenoid inflammation. Occasionally, hypersensitivity reactions can induce necrobiotic granulomatous reactions characterized by collagen alteration surrounded by palisaded histiocytes and lymphocytes (Figure 3).^15,16 There also may be focally dense areas of superficial and deep perivascular lymphohistiocytic infiltrate. Clinical context is important, as brown tattoo pigment (Figure 3) can be easily confused with the pigmented hyphae of phaeohyphomycosis, melanin, or hemosiderin.

Subcutaneous hyalohyphomycosis is a nondemat-iaceous (nonpigmented) infection that is caused by hyaline septate hyphal cells.¹⁷ Hyalohyphomycosis skin lesions can present as painful erythematous nodules that evolve into excoriated pustules.¹⁸ Hyalohyphomycosis most often arises in immunocompromised patients. Causative organisms are ubiquitous soil saprophytes and plant pathogens, most often Aspergillus and Fusarium species, with a predilection for affecting severely immunocompromised hosts, particularly children.¹⁹ These species tend to be vasculotropic, which can result in tissue necrosis and systemic dissemination. Histologically, fungi are dispersed within tissue. They have a bright, bubbly, mildly basophilic cytoplasm and are nonpigmented, branching, and septate (Figure 4).¹¹

The Diagnosis: Subcutaneous Phaeohyphomycosis

Subcutaneous phaeohyphomycosis (SP), also called mycotic cyst, is characterized by a painless, nodular lesion that develops in response to traumatic implantation of dematiaceous, pigment-forming fungi.¹ Similar to other fungal infections, SP can arise opportunistically in immunocompromised patients.^2,3 More than 60 genera (and more than 100 species) are known etiologic agents of phaeohyphomycosis; the 2 main causes of infection are Bipolaris spicifera and Exophiala jeanselmei.^4,5 Given this variety, phaeohyphomycosis can present superficially as black piedra or tinea nigra, cutaneously as scytalidiosis, subcutaneously as SP, or disseminated as sinusitis or systemic phaeohyphomycosis.

Coined in 1974 by Ajello et al,⁶ the term phaeohyphomycosis translates to “condition of dark hyphal fungus,” a term used to designate mycoses caused by fungi with melanized hyphae. Histologically, SP demonstrates a circumscribed chronic cyst or abscess with a dense fibrous wall (quiz image A). At high power, the wall is composed of chronic granulomatous inflammation with foamy macrophages, and the cystic cavity contains necrotic debris admixed with neutrophils. Pigmented filamentous hyphae and yeastlike entities can be seen in the cyst wall, in multinucleated giant cells, in the necrotic debris, or directly attached to the implanted foreign material (quiz image B).⁷ The first-line treatment of SP is wide local excision and oral itraconazole. It often requires adjustments to dosage or change to antifungal due to recurrence and etiologic variation.⁸ Furthermore, if SP is not definitively treated, immunocompromised patients are at an increased risk for developing potentially fatal systemic phaeohyphomycosis.³

Chromoblastomycosis (CBM), also caused by dematiaceous fungi, is characterized by an initially indolent clinical presentation. Typically found on the legs and lower thighs of agricultural workers, the lesion begins as a slow-growing, nodular papule with subsequent transformation into an edematous verrucous plaque with peripheral erythema.⁹ Lesions can be annular with central clearing, and lymphedema with elephantiasis may be present.¹⁰ Histologically, CBM shows pseudoepitheliomatous hyperplasia and intraepidermal pustules as the host rids the infection via transepithelial elimination. Dematiaceous fungi often are seen in the dermis, either freestanding or attached to foreign plant material. Medlar bodies, also called copper penny spores or sclerotic bodies, are the most defining histologic finding and are characterized by groups of brown, thick-walled cells found in giant cells or neutrophil abscesses (Figure 1). Hyphae are not typically found in this type of infection.¹¹

Granulomatous foreign body reactions occur in response to the inoculation of nonhuman material and are characterized by dermal or subcutaneous nodules. Tissue macrophages phagocytize material not removed shortly after implantation, which initiates an inflammatory response that attempts to isolate the material from the uninvolved surrounding tissue. Vegetative foreign bodies will cause the most severe inflammatory reactions.¹² Histologically, foreign body granulomas are noncaseating with epithelioid histiocytes surrounding a central foreign body (Figure 2). Occasionally, foreign bodies may be difficult to detect; some are birefringent to polarized light.¹³ Additionally, inoculation injuries can predispose patients to SP, CBM, and other fungal infections.

Tattoos are characterized by exogenous pigment deposition into the dermis.¹⁴ Histologically, tattoos display exogenous pigment deposited throughout the reticular dermis, attached to collagen bundles, within macrophages, or adjacent to adnexal structures (eg, pilosebaceous units or eccrine glands). Although all tattoo pigments can cause adverse reactions, hypersensitivity reactions occur most commonly in response to red pigment, resulting in discrete areas of spongiosis and granulomatous or lichenoid inflammation. Occasionally, hypersensitivity reactions can induce necrobiotic granulomatous reactions characterized by collagen alteration surrounded by palisaded histiocytes and lymphocytes (Figure 3).^15,16 There also may be focally dense areas of superficial and deep perivascular lymphohistiocytic infiltrate. Clinical context is important, as brown tattoo pigment (Figure 3) can be easily confused with the pigmented hyphae of phaeohyphomycosis, melanin, or hemosiderin.

Subcutaneous hyalohyphomycosis is a nondemat-iaceous (nonpigmented) infection that is caused by hyaline septate hyphal cells.¹⁷ Hyalohyphomycosis skin lesions can present as painful erythematous nodules that evolve into excoriated pustules.¹⁸ Hyalohyphomycosis most often arises in immunocompromised patients. Causative organisms are ubiquitous soil saprophytes and plant pathogens, most often Aspergillus and Fusarium species, with a predilection for affecting severely immunocompromised hosts, particularly children.¹⁹ These species tend to be vasculotropic, which can result in tissue necrosis and systemic dissemination. Histologically, fungi are dispersed within tissue. They have a bright, bubbly, mildly basophilic cytoplasm and are nonpigmented, branching, and septate (Figure 4).¹¹

The Diagnosis: Subcutaneous Phaeohyphomycosis

Subcutaneous phaeohyphomycosis (SP), also called mycotic cyst, is characterized by a painless, nodular lesion that develops in response to traumatic implantation of dematiaceous, pigment-forming fungi.¹ Similar to other fungal infections, SP can arise opportunistically in immunocompromised patients.^2,3 More than 60 genera (and more than 100 species) are known etiologic agents of phaeohyphomycosis; the 2 main causes of infection are Bipolaris spicifera and Exophiala jeanselmei.^4,5 Given this variety, phaeohyphomycosis can present superficially as black piedra or tinea nigra, cutaneously as scytalidiosis, subcutaneously as SP, or disseminated as sinusitis or systemic phaeohyphomycosis.

Coined in 1974 by Ajello et al,⁶ the term phaeohyphomycosis translates to “condition of dark hyphal fungus,” a term used to designate mycoses caused by fungi with melanized hyphae. Histologically, SP demonstrates a circumscribed chronic cyst or abscess with a dense fibrous wall (quiz image A). At high power, the wall is composed of chronic granulomatous inflammation with foamy macrophages, and the cystic cavity contains necrotic debris admixed with neutrophils. Pigmented filamentous hyphae and yeastlike entities can be seen in the cyst wall, in multinucleated giant cells, in the necrotic debris, or directly attached to the implanted foreign material (quiz image B).⁷ The first-line treatment of SP is wide local excision and oral itraconazole. It often requires adjustments to dosage or change to antifungal due to recurrence and etiologic variation.⁸ Furthermore, if SP is not definitively treated, immunocompromised patients are at an increased risk for developing potentially fatal systemic phaeohyphomycosis.³

Chromoblastomycosis (CBM), also caused by dematiaceous fungi, is characterized by an initially indolent clinical presentation. Typically found on the legs and lower thighs of agricultural workers, the lesion begins as a slow-growing, nodular papule with subsequent transformation into an edematous verrucous plaque with peripheral erythema.⁹ Lesions can be annular with central clearing, and lymphedema with elephantiasis may be present.¹⁰ Histologically, CBM shows pseudoepitheliomatous hyperplasia and intraepidermal pustules as the host rids the infection via transepithelial elimination. Dematiaceous fungi often are seen in the dermis, either freestanding or attached to foreign plant material. Medlar bodies, also called copper penny spores or sclerotic bodies, are the most defining histologic finding and are characterized by groups of brown, thick-walled cells found in giant cells or neutrophil abscesses (Figure 1). Hyphae are not typically found in this type of infection.¹¹

Granulomatous foreign body reactions occur in response to the inoculation of nonhuman material and are characterized by dermal or subcutaneous nodules. Tissue macrophages phagocytize material not removed shortly after implantation, which initiates an inflammatory response that attempts to isolate the material from the uninvolved surrounding tissue. Vegetative foreign bodies will cause the most severe inflammatory reactions.¹² Histologically, foreign body granulomas are noncaseating with epithelioid histiocytes surrounding a central foreign body (Figure 2). Occasionally, foreign bodies may be difficult to detect; some are birefringent to polarized light.¹³ Additionally, inoculation injuries can predispose patients to SP, CBM, and other fungal infections.

Tattoos are characterized by exogenous pigment deposition into the dermis.¹⁴ Histologically, tattoos display exogenous pigment deposited throughout the reticular dermis, attached to collagen bundles, within macrophages, or adjacent to adnexal structures (eg, pilosebaceous units or eccrine glands). Although all tattoo pigments can cause adverse reactions, hypersensitivity reactions occur most commonly in response to red pigment, resulting in discrete areas of spongiosis and granulomatous or lichenoid inflammation. Occasionally, hypersensitivity reactions can induce necrobiotic granulomatous reactions characterized by collagen alteration surrounded by palisaded histiocytes and lymphocytes (Figure 3).^15,16 There also may be focally dense areas of superficial and deep perivascular lymphohistiocytic infiltrate. Clinical context is important, as brown tattoo pigment (Figure 3) can be easily confused with the pigmented hyphae of phaeohyphomycosis, melanin, or hemosiderin.

Subcutaneous hyalohyphomycosis is a nondemat-iaceous (nonpigmented) infection that is caused by hyaline septate hyphal cells.¹⁷ Hyalohyphomycosis skin lesions can present as painful erythematous nodules that evolve into excoriated pustules.¹⁸ Hyalohyphomycosis most often arises in immunocompromised patients. Causative organisms are ubiquitous soil saprophytes and plant pathogens, most often Aspergillus and Fusarium species, with a predilection for affecting severely immunocompromised hosts, particularly children.¹⁹ These species tend to be vasculotropic, which can result in tissue necrosis and systemic dissemination. Histologically, fungi are dispersed within tissue. They have a bright, bubbly, mildly basophilic cytoplasm and are nonpigmented, branching, and septate (Figure 4).¹¹

Lepromatous Leprosy

Lepromatous leprosy (LL) is a chronic, cutaneous, granulomatous infection caused by Mycobacterium leprae or the newly discovered Mycobacterium lepromatosis, both acid-fast, intracellular, bacillus bacterium.¹ Although decreasing in prevalence due to effective treatment with antimicrobials, LL continues to be endemic in warm tropical or subtropical areas in Southeast Asia, sub-Saharan Africa, the Indian subcontinent, and South America.¹ The mode of transmission of infection is not well established.

The cutaneous manifestation of leprosy was previously classified based on the cell-mediated immune response of the patient, as described by Ridley and Jopling,² ranging from tuberculoid leprosy (TT) to LL. In this spectrum of leprosy are the borderline lesions including borderline tuberculoid, borderline, and borderline lepromatous.^2,3 Although this classification is popular, in 2012 the World Health Organization implemented a new 2-category classification system to standardize treatment regimens: paucibacillary (2–5 lesions or 1 nerve involvement) and multibacillary (>5 lesions or multiple nerve involvement).⁴

In LL, a cell-mediated immune response is not mounted against the infection in the patient. Clinically, the disease can manifest as macular and nodular erythematous cutaneous lesions with poorly defined borders that are preferentially located on the face, earlobes, and nasal mucosa. Chronic infections are associated with sensory loss. Histologically, the dermis is densely infiltrated by foamy macrophages (Virchow cells or lepra cells), which do not form granulomas (quiz image A). The infiltrate may have varying accompanying lymphocytes and plasma cells, which can extend deep into the subcutaneous adipose tissue. Between the dermal infiltrate and epidermis is an uninvolved band of superficial dermis called the Grenz zone. The epidermis is flattened and atrophic. Nerves often are surrounded by macrophages with degrees of hyalinization but rarely are swollen. On acid-fast staining (Wade-Fite or Ziehl-Neelsen), numerous acid-fast bacilli are present within dermal cells in densely packed, intracellular collections called globi (quiz image B).^2,3,5

In TT, the robust immune response causes epithelioid granuloma formation, similar to cutaneous sarcoidosis, and few, if any, organisms can be found on special stains. The remaining borderline lesions have varying numbers of bacilli and varying amounts of granuloma formation.^3,6,7 Many cases of TT resolve without specific treatment. For most leprous diseases, the World Health Organization currently recommends a regimen of dapsone, rifampin, and clofazimine combination treatments for 6 to 12 months depending on the type of leprosy.⁸

Cutaneous leishmaniasis should be included in the differential diagnosis for patients from LL endemic areas. Early lesions can have a histiocytic infiltration with associated mixed inflammation and prominent epidermal hyperplasia. These early lesions usually have parasitic organisms located within the periphery of the cytoplasm of macrophages (“marquee sign”) to help differentiate it from leprous diseases (Figure 1).⁹

In nonendemic areas, leprous diseases often are mistaken for sarcoidosis, xanthomas, granular cell tumors, paraffinomas, or other histiocytic-rich lesions.¹⁰ Cutaneous sarcoidosis may be difficult to distinguish from TT, as both have noncaseating granulomas (Figure 2). Rare acid-fast bacilli may aid in the diagnosis, and sarcoid granulomas are not typically associated with cutaneous nerve involvement. New diagnostic tools such as polymerase chain reaction or genome sequencing can pick up rare organisms.

Xanthogranuolomas and xanthomas may histologically resemble LL with a dense dermal infiltrate of foamy histiocytes. No organisms are found in the infiltrate. Histologically, xanthogranulomas (juvenile or adult) will be a mixed infiltrate with foamy histiocytes; giant cell formation, especially Touton giant cells; lymphocytes; and granulocytes (Figure 3). Touton giant cells have a wreathlike formation of nuclei and an outer vacuolated cytoplasm. Xanthomas have sheets of large histiocytes with a foamy, lipid-filled interior and mild lymphocytic infiltrate (Figure 4).

Lepromatous Leprosy

Lepromatous leprosy (LL) is a chronic, cutaneous, granulomatous infection caused by Mycobacterium leprae or the newly discovered Mycobacterium lepromatosis, both acid-fast, intracellular, bacillus bacterium.¹ Although decreasing in prevalence due to effective treatment with antimicrobials, LL continues to be endemic in warm tropical or subtropical areas in Southeast Asia, sub-Saharan Africa, the Indian subcontinent, and South America.¹ The mode of transmission of infection is not well established.

The cutaneous manifestation of leprosy was previously classified based on the cell-mediated immune response of the patient, as described by Ridley and Jopling,² ranging from tuberculoid leprosy (TT) to LL. In this spectrum of leprosy are the borderline lesions including borderline tuberculoid, borderline, and borderline lepromatous.^2,3 Although this classification is popular, in 2012 the World Health Organization implemented a new 2-category classification system to standardize treatment regimens: paucibacillary (2–5 lesions or 1 nerve involvement) and multibacillary (>5 lesions or multiple nerve involvement).⁴

In LL, a cell-mediated immune response is not mounted against the infection in the patient. Clinically, the disease can manifest as macular and nodular erythematous cutaneous lesions with poorly defined borders that are preferentially located on the face, earlobes, and nasal mucosa. Chronic infections are associated with sensory loss. Histologically, the dermis is densely infiltrated by foamy macrophages (Virchow cells or lepra cells), which do not form granulomas (quiz image A). The infiltrate may have varying accompanying lymphocytes and plasma cells, which can extend deep into the subcutaneous adipose tissue. Between the dermal infiltrate and epidermis is an uninvolved band of superficial dermis called the Grenz zone. The epidermis is flattened and atrophic. Nerves often are surrounded by macrophages with degrees of hyalinization but rarely are swollen. On acid-fast staining (Wade-Fite or Ziehl-Neelsen), numerous acid-fast bacilli are present within dermal cells in densely packed, intracellular collections called globi (quiz image B).^2,3,5

In TT, the robust immune response causes epithelioid granuloma formation, similar to cutaneous sarcoidosis, and few, if any, organisms can be found on special stains. The remaining borderline lesions have varying numbers of bacilli and varying amounts of granuloma formation.^3,6,7 Many cases of TT resolve without specific treatment. For most leprous diseases, the World Health Organization currently recommends a regimen of dapsone, rifampin, and clofazimine combination treatments for 6 to 12 months depending on the type of leprosy.⁸

Cutaneous leishmaniasis should be included in the differential diagnosis for patients from LL endemic areas. Early lesions can have a histiocytic infiltration with associated mixed inflammation and prominent epidermal hyperplasia. These early lesions usually have parasitic organisms located within the periphery of the cytoplasm of macrophages (“marquee sign”) to help differentiate it from leprous diseases (Figure 1).⁹

In nonendemic areas, leprous diseases often are mistaken for sarcoidosis, xanthomas, granular cell tumors, paraffinomas, or other histiocytic-rich lesions.¹⁰ Cutaneous sarcoidosis may be difficult to distinguish from TT, as both have noncaseating granulomas (Figure 2). Rare acid-fast bacilli may aid in the diagnosis, and sarcoid granulomas are not typically associated with cutaneous nerve involvement. New diagnostic tools such as polymerase chain reaction or genome sequencing can pick up rare organisms.

Xanthogranuolomas and xanthomas may histologically resemble LL with a dense dermal infiltrate of foamy histiocytes. No organisms are found in the infiltrate. Histologically, xanthogranulomas (juvenile or adult) will be a mixed infiltrate with foamy histiocytes; giant cell formation, especially Touton giant cells; lymphocytes; and granulocytes (Figure 3). Touton giant cells have a wreathlike formation of nuclei and an outer vacuolated cytoplasm. Xanthomas have sheets of large histiocytes with a foamy, lipid-filled interior and mild lymphocytic infiltrate (Figure 4).

Lepromatous Leprosy

Lepromatous leprosy (LL) is a chronic, cutaneous, granulomatous infection caused by Mycobacterium leprae or the newly discovered Mycobacterium lepromatosis, both acid-fast, intracellular, bacillus bacterium.¹ Although decreasing in prevalence due to effective treatment with antimicrobials, LL continues to be endemic in warm tropical or subtropical areas in Southeast Asia, sub-Saharan Africa, the Indian subcontinent, and South America.¹ The mode of transmission of infection is not well established.

The cutaneous manifestation of leprosy was previously classified based on the cell-mediated immune response of the patient, as described by Ridley and Jopling,² ranging from tuberculoid leprosy (TT) to LL. In this spectrum of leprosy are the borderline lesions including borderline tuberculoid, borderline, and borderline lepromatous.^2,3 Although this classification is popular, in 2012 the World Health Organization implemented a new 2-category classification system to standardize treatment regimens: paucibacillary (2–5 lesions or 1 nerve involvement) and multibacillary (>5 lesions or multiple nerve involvement).⁴

In LL, a cell-mediated immune response is not mounted against the infection in the patient. Clinically, the disease can manifest as macular and nodular erythematous cutaneous lesions with poorly defined borders that are preferentially located on the face, earlobes, and nasal mucosa. Chronic infections are associated with sensory loss. Histologically, the dermis is densely infiltrated by foamy macrophages (Virchow cells or lepra cells), which do not form granulomas (quiz image A). The infiltrate may have varying accompanying lymphocytes and plasma cells, which can extend deep into the subcutaneous adipose tissue. Between the dermal infiltrate and epidermis is an uninvolved band of superficial dermis called the Grenz zone. The epidermis is flattened and atrophic. Nerves often are surrounded by macrophages with degrees of hyalinization but rarely are swollen. On acid-fast staining (Wade-Fite or Ziehl-Neelsen), numerous acid-fast bacilli are present within dermal cells in densely packed, intracellular collections called globi (quiz image B).^2,3,5

In TT, the robust immune response causes epithelioid granuloma formation, similar to cutaneous sarcoidosis, and few, if any, organisms can be found on special stains. The remaining borderline lesions have varying numbers of bacilli and varying amounts of granuloma formation.^3,6,7 Many cases of TT resolve without specific treatment. For most leprous diseases, the World Health Organization currently recommends a regimen of dapsone, rifampin, and clofazimine combination treatments for 6 to 12 months depending on the type of leprosy.⁸

Cutaneous leishmaniasis should be included in the differential diagnosis for patients from LL endemic areas. Early lesions can have a histiocytic infiltration with associated mixed inflammation and prominent epidermal hyperplasia. These early lesions usually have parasitic organisms located within the periphery of the cytoplasm of macrophages (“marquee sign”) to help differentiate it from leprous diseases (Figure 1).⁹

In nonendemic areas, leprous diseases often are mistaken for sarcoidosis, xanthomas, granular cell tumors, paraffinomas, or other histiocytic-rich lesions.¹⁰ Cutaneous sarcoidosis may be difficult to distinguish from TT, as both have noncaseating granulomas (Figure 2). Rare acid-fast bacilli may aid in the diagnosis, and sarcoid granulomas are not typically associated with cutaneous nerve involvement. New diagnostic tools such as polymerase chain reaction or genome sequencing can pick up rare organisms.

Xanthogranuolomas and xanthomas may histologically resemble LL with a dense dermal infiltrate of foamy histiocytes. No organisms are found in the infiltrate. Histologically, xanthogranulomas (juvenile or adult) will be a mixed infiltrate with foamy histiocytes; giant cell formation, especially Touton giant cells; lymphocytes; and granulocytes (Figure 3). Touton giant cells have a wreathlike formation of nuclei and an outer vacuolated cytoplasm. Xanthomas have sheets of large histiocytes with a foamy, lipid-filled interior and mild lymphocytic infiltrate (Figure 4).

Granular Cell Basal Cell Carcinoma

Basal cell carcinoma (BCC) is the most common human epithelial malignancy. There are several histologic variants, the rarest being granular cell BCC (GBCC).¹ Granular cell BCC is reported most commonly in men with a mean age of 63 years. Sixty-four percent of cases develop on the face, with the remainder arising on the chest or trunk. Granular cell BCC has distinct histologic features but has no specific epidemiologic or clinical features that differentiate it from more common forms of BCC. Treatment of GBCC is identical to BCC and demonstrates similar outcomes. The presence of granular cells can make GBCC difficult to differentiate from other benign and malignant lesions that display similar granular histologic changes.^1,2 Rarely, tumors that are histologically similar to human GBCC have been reported in animals.¹

Histologically, GBCC commonly demonstrates the architecture of a nodular BCC or may extend from an existing nodular BCC (quiz images A and B). Granular cell BCC is comprised of large islands of basaloid cells extending from the epidermis with rare mitotic activity. Certain variants showing no epidermal attachments have been described,^1,3 as in the current case. Classically, BCC and GBCC both demonstrate a peripheral palisade of blue basal cells; however, GBCC may lack this palisading feature in some cases. Therefore, GBCC may be comprised of granular cells only, which may be more easily confused with other tumors with granular cell differentiation. Even when GBCC retains the traditional peripheral palisade of blue basal cells, the central cells are filled with eosinophilic granules.^1,2

Electron microscopy of GBCC usually reveals bundles of cytoplasmic tonofilaments and desmosomes in both granular cells and the peripherally palisaded cells. Electron microscopy imaging also demonstrates 0.1- to 0.5-µm membrane-bound lysosomelike structures. In certain reports, these structures show focal positivity for lysozymes.^1,2 The etiology of the granules is unclear; however, they are thought to represent degenerative changes related to metabolic alteration and accumulation of lysosomelike structures. These lysosomelike structures have been highlighted with CD68 staining, which was negative in our case.^1,2 The lesional cells in GBCC stain positively for cytokeratins, p63, and Ber-EP4, and negatively for S-100 protein, epithelial membrane antigen, and carcinoembryonic antigen. The granules in GBCC generally are positive on periodic acid–Schiff staining.^1-4

The histologic differential diagnosis for GBCC includes granular cell tumor as well as other tumors that can present with granular cell changes such as ameloblastoma, leiomyoma, leiomyosarcoma, angiosarcoma, malignant peripheral nerve sheath tumor, and granular cell trichoblastoma. Granular cell ameloblastomas have histologic features and staining patterns that are identical to GBCC; however, ameloblastomas are distinguished by their location within the oral cavity. Granular cell tumors and malignant peripheral nerve sheath tumors stain positive for S-100 protein, and angiosarcomas stain positive for D2-40 and CD31. Leiomyomas and leiomyosarcomas can be differentiated by staining with smooth muscle actin or desmin.¹ Granular cell trichoblastomas can be differentiated by the follicular stem cell marker protein PHLDA1 positivity.⁵

Desmoplastic trichilemmoma is difficult to distinguish from BCC. These tumors are comprised of superficial lobules of basaloid cells with a perilobular hyaline mantel surrounding a central desmoplastic stroma (Figure 1). The basaloid cells in desmoplastic trichoepithelioma demonstrate clear cell change; however, granular features are not seen. The cells within the desmoplastic areas are arranged haphazardly in cords and nests and can mimic an invasive carcinoma; however, nuclear atypia and mitotic activity generally are absent in desmoplastic trichilemmoma.⁶

Granular cell tumors generally are poorly circumscribed dermal nodules comprised of large polygonal cells with an eosinophilic granular cytoplasm (Figure 2). The nuclei are generally small and round, and cytological atypia, necrosis, and mitotic activity are uncommon. The cells are positive for S-100 protein and neuron-specific enolase but negative for CD68. The granules are positive for periodic acid–Schiff stain and are diastase resistant. Rarely, these tumors can be malignant.⁷

Sebaceous adenoma is a well-circumscribed tumor comprised of lobules of characteristic mature sebocytes with bubbly or multivacuolated cytoplasm and crenated nuclei (Figure 3). There is an expansion and increased prominence of the peripherally located basaloid cells; however, in contrast to sebaceous epithelioma, less than 50% of the tumor usually is comprised of these basaloid cells.⁸

Xanthogranuloma demonstrates a dense collection of histiocytes in the dermis, commonly with Touton giant cell formation (Figure 4). The cells often have a foamy cytoplasm and cytoplasmic vacuoles are observed. The histiocytes are positive for factor XIIIa and CD68, and generally negative for S-100 protein and CD1a, which allows for differentiation from Langerhans cells.⁹

Granular Cell Basal Cell Carcinoma

Basal cell carcinoma (BCC) is the most common human epithelial malignancy. There are several histologic variants, the rarest being granular cell BCC (GBCC).¹ Granular cell BCC is reported most commonly in men with a mean age of 63 years. Sixty-four percent of cases develop on the face, with the remainder arising on the chest or trunk. Granular cell BCC has distinct histologic features but has no specific epidemiologic or clinical features that differentiate it from more common forms of BCC. Treatment of GBCC is identical to BCC and demonstrates similar outcomes. The presence of granular cells can make GBCC difficult to differentiate from other benign and malignant lesions that display similar granular histologic changes.^1,2 Rarely, tumors that are histologically similar to human GBCC have been reported in animals.¹

Histologically, GBCC commonly demonstrates the architecture of a nodular BCC or may extend from an existing nodular BCC (quiz images A and B). Granular cell BCC is comprised of large islands of basaloid cells extending from the epidermis with rare mitotic activity. Certain variants showing no epidermal attachments have been described,^1,3 as in the current case. Classically, BCC and GBCC both demonstrate a peripheral palisade of blue basal cells; however, GBCC may lack this palisading feature in some cases. Therefore, GBCC may be comprised of granular cells only, which may be more easily confused with other tumors with granular cell differentiation. Even when GBCC retains the traditional peripheral palisade of blue basal cells, the central cells are filled with eosinophilic granules.^1,2

Electron microscopy of GBCC usually reveals bundles of cytoplasmic tonofilaments and desmosomes in both granular cells and the peripherally palisaded cells. Electron microscopy imaging also demonstrates 0.1- to 0.5-µm membrane-bound lysosomelike structures. In certain reports, these structures show focal positivity for lysozymes.^1,2 The etiology of the granules is unclear; however, they are thought to represent degenerative changes related to metabolic alteration and accumulation of lysosomelike structures. These lysosomelike structures have been highlighted with CD68 staining, which was negative in our case.^1,2 The lesional cells in GBCC stain positively for cytokeratins, p63, and Ber-EP4, and negatively for S-100 protein, epithelial membrane antigen, and carcinoembryonic antigen. The granules in GBCC generally are positive on periodic acid–Schiff staining.^1-4

The histologic differential diagnosis for GBCC includes granular cell tumor as well as other tumors that can present with granular cell changes such as ameloblastoma, leiomyoma, leiomyosarcoma, angiosarcoma, malignant peripheral nerve sheath tumor, and granular cell trichoblastoma. Granular cell ameloblastomas have histologic features and staining patterns that are identical to GBCC; however, ameloblastomas are distinguished by their location within the oral cavity. Granular cell tumors and malignant peripheral nerve sheath tumors stain positive for S-100 protein, and angiosarcomas stain positive for D2-40 and CD31. Leiomyomas and leiomyosarcomas can be differentiated by staining with smooth muscle actin or desmin.¹ Granular cell trichoblastomas can be differentiated by the follicular stem cell marker protein PHLDA1 positivity.⁵

Desmoplastic trichilemmoma is difficult to distinguish from BCC. These tumors are comprised of superficial lobules of basaloid cells with a perilobular hyaline mantel surrounding a central desmoplastic stroma (Figure 1). The basaloid cells in desmoplastic trichoepithelioma demonstrate clear cell change; however, granular features are not seen. The cells within the desmoplastic areas are arranged haphazardly in cords and nests and can mimic an invasive carcinoma; however, nuclear atypia and mitotic activity generally are absent in desmoplastic trichilemmoma.⁶

Granular cell tumors generally are poorly circumscribed dermal nodules comprised of large polygonal cells with an eosinophilic granular cytoplasm (Figure 2). The nuclei are generally small and round, and cytological atypia, necrosis, and mitotic activity are uncommon. The cells are positive for S-100 protein and neuron-specific enolase but negative for CD68. The granules are positive for periodic acid–Schiff stain and are diastase resistant. Rarely, these tumors can be malignant.⁷

Sebaceous adenoma is a well-circumscribed tumor comprised of lobules of characteristic mature sebocytes with bubbly or multivacuolated cytoplasm and crenated nuclei (Figure 3). There is an expansion and increased prominence of the peripherally located basaloid cells; however, in contrast to sebaceous epithelioma, less than 50% of the tumor usually is comprised of these basaloid cells.⁸

Xanthogranuloma demonstrates a dense collection of histiocytes in the dermis, commonly with Touton giant cell formation (Figure 4). The cells often have a foamy cytoplasm and cytoplasmic vacuoles are observed. The histiocytes are positive for factor XIIIa and CD68, and generally negative for S-100 protein and CD1a, which allows for differentiation from Langerhans cells.⁹

Granular Cell Basal Cell Carcinoma

Basal cell carcinoma (BCC) is the most common human epithelial malignancy. There are several histologic variants, the rarest being granular cell BCC (GBCC).¹ Granular cell BCC is reported most commonly in men with a mean age of 63 years. Sixty-four percent of cases develop on the face, with the remainder arising on the chest or trunk. Granular cell BCC has distinct histologic features but has no specific epidemiologic or clinical features that differentiate it from more common forms of BCC. Treatment of GBCC is identical to BCC and demonstrates similar outcomes. The presence of granular cells can make GBCC difficult to differentiate from other benign and malignant lesions that display similar granular histologic changes.^1,2 Rarely, tumors that are histologically similar to human GBCC have been reported in animals.¹

Histologically, GBCC commonly demonstrates the architecture of a nodular BCC or may extend from an existing nodular BCC (quiz images A and B). Granular cell BCC is comprised of large islands of basaloid cells extending from the epidermis with rare mitotic activity. Certain variants showing no epidermal attachments have been described,^1,3 as in the current case. Classically, BCC and GBCC both demonstrate a peripheral palisade of blue basal cells; however, GBCC may lack this palisading feature in some cases. Therefore, GBCC may be comprised of granular cells only, which may be more easily confused with other tumors with granular cell differentiation. Even when GBCC retains the traditional peripheral palisade of blue basal cells, the central cells are filled with eosinophilic granules.^1,2

Electron microscopy of GBCC usually reveals bundles of cytoplasmic tonofilaments and desmosomes in both granular cells and the peripherally palisaded cells. Electron microscopy imaging also demonstrates 0.1- to 0.5-µm membrane-bound lysosomelike structures. In certain reports, these structures show focal positivity for lysozymes.^1,2 The etiology of the granules is unclear; however, they are thought to represent degenerative changes related to metabolic alteration and accumulation of lysosomelike structures. These lysosomelike structures have been highlighted with CD68 staining, which was negative in our case.^1,2 The lesional cells in GBCC stain positively for cytokeratins, p63, and Ber-EP4, and negatively for S-100 protein, epithelial membrane antigen, and carcinoembryonic antigen. The granules in GBCC generally are positive on periodic acid–Schiff staining.^1-4

The histologic differential diagnosis for GBCC includes granular cell tumor as well as other tumors that can present with granular cell changes such as ameloblastoma, leiomyoma, leiomyosarcoma, angiosarcoma, malignant peripheral nerve sheath tumor, and granular cell trichoblastoma. Granular cell ameloblastomas have histologic features and staining patterns that are identical to GBCC; however, ameloblastomas are distinguished by their location within the oral cavity. Granular cell tumors and malignant peripheral nerve sheath tumors stain positive for S-100 protein, and angiosarcomas stain positive for D2-40 and CD31. Leiomyomas and leiomyosarcomas can be differentiated by staining with smooth muscle actin or desmin.¹ Granular cell trichoblastomas can be differentiated by the follicular stem cell marker protein PHLDA1 positivity.⁵

Desmoplastic trichilemmoma is difficult to distinguish from BCC. These tumors are comprised of superficial lobules of basaloid cells with a perilobular hyaline mantel surrounding a central desmoplastic stroma (Figure 1). The basaloid cells in desmoplastic trichoepithelioma demonstrate clear cell change; however, granular features are not seen. The cells within the desmoplastic areas are arranged haphazardly in cords and nests and can mimic an invasive carcinoma; however, nuclear atypia and mitotic activity generally are absent in desmoplastic trichilemmoma.⁶

Granular cell tumors generally are poorly circumscribed dermal nodules comprised of large polygonal cells with an eosinophilic granular cytoplasm (Figure 2). The nuclei are generally small and round, and cytological atypia, necrosis, and mitotic activity are uncommon. The cells are positive for S-100 protein and neuron-specific enolase but negative for CD68. The granules are positive for periodic acid–Schiff stain and are diastase resistant. Rarely, these tumors can be malignant.⁷

Sebaceous adenoma is a well-circumscribed tumor comprised of lobules of characteristic mature sebocytes with bubbly or multivacuolated cytoplasm and crenated nuclei (Figure 3). There is an expansion and increased prominence of the peripherally located basaloid cells; however, in contrast to sebaceous epithelioma, less than 50% of the tumor usually is comprised of these basaloid cells.⁸

Xanthogranuloma demonstrates a dense collection of histiocytes in the dermis, commonly with Touton giant cell formation (Figure 4). The cells often have a foamy cytoplasm and cytoplasmic vacuoles are observed. The histiocytes are positive for factor XIIIa and CD68, and generally negative for S-100 protein and CD1a, which allows for differentiation from Langerhans cells.⁹

Nuchal-Type Fibroma

Nuchal-type fibroma (NTF) is a rare benign proliferation of the dermis and subcutis associated with diabetes mellitus and Gardner syndrome.^1,2 Forty-four percent of patients with NTF have diabetes mellitus.² The posterior aspect of the neck is the most frequently affected site, but lesions also may present on the upper back, lumbosacral area, buttocks, and face. Physical examination generally reveals an indurated, asymptomatic, ill-defined, 3-cm or smaller nodule that is hard and white, unencapsulated, and poorly circumscribed.

Histopathologic examination of NTF typically reveals a nodular paucicellular proliferation of thick collagen bundles with inconspicuous fibroblasts, radiation of collagenous septa into the subcutaneous fat, and entrapment of mature adipose tissue and small nerves (quiz image A). Collagen bundles are thickened with entrapment of adipose tissue without increased cellularity (quiz image B). S-100 staining can show the entrapped nerves.

Similar to NTF, sclerotic fibroma is a firm dermal nodule with histologic examination usually demonstrating a paucicellular collagenous tumor. In sclerotic fibromas, the collagen pattern resembles Vincent van Gogh’s painting “The Starry Night” and may be a marker for Cowden disease (Figure 1).³ Solitary fibrous tumors are distinguished by more hypercellular areas, patternless pattern, and staghorn-shaped blood vessels (Figure 2).⁴ Spindle cell lipoma classically demonstrates a mixture of mature adipocytes and bland spindle cells in a mucinous or fibrous background with thick collagen bundles with no storiform pattern (Figure 3). Some variants of spindle cell lipoma have minimal or no fat.⁵ All of these conditions have positive immunohistochemical staining for CD34.

However, dermatofibroma is CD34‒. Dermatofibroma is characterized by an interstitial spindle cell proliferation with a loose storiform pattern, collagen trapping at the outer edges of the tumor, overlying platelike acanthosis, and sometimes follicular induction (Figure 4).

Nuchal-type fibroma also can resemble scleredema. Both lesions can show increased and thickened collagen bundles without notable fibroblast proliferation; the difference is the occurrence of mucin in scleredema. However, incases of late-stage scleredema, mucin is not always demonstrated. Therefore, one can conclude that histologically NTF is closely associated with late-stage scleredema.⁶

Nuchal-Type Fibroma

Nuchal-type fibroma (NTF) is a rare benign proliferation of the dermis and subcutis associated with diabetes mellitus and Gardner syndrome.^1,2 Forty-four percent of patients with NTF have diabetes mellitus.² The posterior aspect of the neck is the most frequently affected site, but lesions also may present on the upper back, lumbosacral area, buttocks, and face. Physical examination generally reveals an indurated, asymptomatic, ill-defined, 3-cm or smaller nodule that is hard and white, unencapsulated, and poorly circumscribed.

Histopathologic examination of NTF typically reveals a nodular paucicellular proliferation of thick collagen bundles with inconspicuous fibroblasts, radiation of collagenous septa into the subcutaneous fat, and entrapment of mature adipose tissue and small nerves (quiz image A). Collagen bundles are thickened with entrapment of adipose tissue without increased cellularity (quiz image B). S-100 staining can show the entrapped nerves.

Similar to NTF, sclerotic fibroma is a firm dermal nodule with histologic examination usually demonstrating a paucicellular collagenous tumor. In sclerotic fibromas, the collagen pattern resembles Vincent van Gogh’s painting “The Starry Night” and may be a marker for Cowden disease (Figure 1).³ Solitary fibrous tumors are distinguished by more hypercellular areas, patternless pattern, and staghorn-shaped blood vessels (Figure 2).⁴ Spindle cell lipoma classically demonstrates a mixture of mature adipocytes and bland spindle cells in a mucinous or fibrous background with thick collagen bundles with no storiform pattern (Figure 3). Some variants of spindle cell lipoma have minimal or no fat.⁵ All of these conditions have positive immunohistochemical staining for CD34.

However, dermatofibroma is CD34‒. Dermatofibroma is characterized by an interstitial spindle cell proliferation with a loose storiform pattern, collagen trapping at the outer edges of the tumor, overlying platelike acanthosis, and sometimes follicular induction (Figure 4).

Nuchal-type fibroma also can resemble scleredema. Both lesions can show increased and thickened collagen bundles without notable fibroblast proliferation; the difference is the occurrence of mucin in scleredema. However, incases of late-stage scleredema, mucin is not always demonstrated. Therefore, one can conclude that histologically NTF is closely associated with late-stage scleredema.⁶

Nuchal-Type Fibroma

Nuchal-type fibroma (NTF) is a rare benign proliferation of the dermis and subcutis associated with diabetes mellitus and Gardner syndrome.^1,2 Forty-four percent of patients with NTF have diabetes mellitus.² The posterior aspect of the neck is the most frequently affected site, but lesions also may present on the upper back, lumbosacral area, buttocks, and face. Physical examination generally reveals an indurated, asymptomatic, ill-defined, 3-cm or smaller nodule that is hard and white, unencapsulated, and poorly circumscribed.

Histopathologic examination of NTF typically reveals a nodular paucicellular proliferation of thick collagen bundles with inconspicuous fibroblasts, radiation of collagenous septa into the subcutaneous fat, and entrapment of mature adipose tissue and small nerves (quiz image A). Collagen bundles are thickened with entrapment of adipose tissue without increased cellularity (quiz image B). S-100 staining can show the entrapped nerves.

Similar to NTF, sclerotic fibroma is a firm dermal nodule with histologic examination usually demonstrating a paucicellular collagenous tumor. In sclerotic fibromas, the collagen pattern resembles Vincent van Gogh’s painting “The Starry Night” and may be a marker for Cowden disease (Figure 1).³ Solitary fibrous tumors are distinguished by more hypercellular areas, patternless pattern, and staghorn-shaped blood vessels (Figure 2).⁴ Spindle cell lipoma classically demonstrates a mixture of mature adipocytes and bland spindle cells in a mucinous or fibrous background with thick collagen bundles with no storiform pattern (Figure 3). Some variants of spindle cell lipoma have minimal or no fat.⁵ All of these conditions have positive immunohistochemical staining for CD34.

However, dermatofibroma is CD34‒. Dermatofibroma is characterized by an interstitial spindle cell proliferation with a loose storiform pattern, collagen trapping at the outer edges of the tumor, overlying platelike acanthosis, and sometimes follicular induction (Figure 4).

Nuchal-type fibroma also can resemble scleredema. Both lesions can show increased and thickened collagen bundles without notable fibroblast proliferation; the difference is the occurrence of mucin in scleredema. However, incases of late-stage scleredema, mucin is not always demonstrated. Therefore, one can conclude that histologically NTF is closely associated with late-stage scleredema.⁶

The best diagnosis is:

a. bronchogenic cyst
b. dermoid cyst
c. epidermal inclusion cyst
d. hidrocystoma
e. steatocystoma

H&E, original magnification ×40.

H&E, original magnification ×100.

Continue to the next page for the diagnosis >>

 

Dermoid Cyst

Dermoid cysts often present clinically as firm subcutaneous nodules on the head or neck in young children. They tend to arise along the lateral aspect of the eyebrow but also can occur on the nose, forehead, neck, chest, or scalp.¹ Dermoid cysts are thought to arise from the sequestration of ectodermal tissues along the embryonic fusion planes during development.² As such, they represent congenital defects and often are identified at birth; however, some are not noticed until much later when they enlarge or become inflamed or infected. Midline dermoid cysts may be associated with underlying dysraphism or intracranial extension.^3,4 Thus, any midline lesion warrants evaluation that incorporates imaging with computed tomography or magnetic resonance imaging.^4,5 Histologically, dermoid cysts are lined by a keratinizing stratified squamous epithelium (quiz image A), but the lining may be brightly eosinophilic and wavy resembling shark teeth.^1,3 The wall of a dermoid cyst commonly contains mature adnexal structures such as terminal hair follicles, sebaceous glands, apocrine glands, and/or eccrine glands (quiz image B).¹ Smooth muscle also may be seen within the lining; however, bone and cartilage are not commonly reported in dermoid cysts.² Lamellar keratin is typical of the cyst contents, and terminal hair shafts also are sometimes noted within the cystic space (quiz image B).^1,2 Treatment options include excision at the time of diagnosis or close clinical monitoring with subsequent excision if the lesion grows or becomes symptomatic.^4,5 Many practitioners opt to excise these cysts at diagnosis, as untreated lesions are at risk for infection and/or inflammation or may be cosmetically deforming.^6,7 Surgical resection, including removal of the wall of the cyst, is curative and reoccurrence is rare.⁵

Figure 1. Bronchogenic cyst demonstrating a ciliated pseudostratified epithelial lining encircled by smooth muscle (H&E, original magnification ×200).
Figure 2. Epidermal inclusion cyst containing loose lamellar keratin and a lining that closely resembles the surface epidermis (H&E, original magnification ×40).

Bronchogenic cysts demonstrate an epithelial lining that often is pseudostratified cuboidal or columnar as well as ciliated (Figure 1). Goblet cells are present in the lining in approximately 50% of cases. Smooth muscle may be seen circumferentially surrounding the cyst lining, and rare cases also contain cartilage.¹ In contrast to dermoid cysts, other types of adnexal structures are not found within the lining. Bronchogenic cysts that arise in the skin are extremely rare.² These cysts are thought to arise from respiratory epithelium that has been sequestered during embryologic formation of the tracheobronchial tree. They often are seen overlying the suprasternal notch and occasionally are found on the anterior aspect of the neck or chin. These cysts also are present at birth, similar to dermoid cysts.³

Epidermal inclusion cysts have a lining that histologically bears close resemblance to the surface epidermis. These cysts contain loose lamellar keratin, similar to a dermoid cyst. In contrast, the lining of an epidermal inclusion cyst will lack adnexal structures (Figure 2).¹ Clinically, epidermal inclusion cysts often present as smooth, dome-shaped papules and nodules with a central punctum. They are classically found on the face, neck, and trunk. These cysts are thought to arise after a traumatic insult to the pilosebaceous unit.²

Hidrocystomas can be apocrine or eccrine.³ Eccrine hidrocystomas are unilocular cysts that are lined by 2 layers of flattened to cuboidal epithelial cells (Figure 3). The cysts are filled with clear fluid and often are found adjacent to normal eccrine glands.¹ Apocrine hidrocystomas are unilocular or multilocular cysts that are lined by 1 to several layers of epithelial cells. The lining of an apocrine hidrocystoma will often exhibit luminal decapitation secretion.³ Apocrine and eccrine hidrocystomas are clinically identical and appear as blue translucent papules on the cheeks or eyelids of adults.^1-3 They usually occur periorbitally but also can be seen on the trunk, popliteal fossa, external ears, or vulva. Eccrine hidrocystomas can wax and wane in accordance with the amount of sweat produced; thus, they often expand in size during the summer months.²

Steatocystomas, or simple sebaceous duct cysts, histologically demonstrate a characteristically wavy and eosinophilic cuticle resembling shark teeth (Figure 4) similar to the lining of the sebaceous duct where it enters the follicle.¹ Sebaceous glands are an almost invariable feature, either present within the lining of the cyst (Figure 4) or in the adjacent tissue.² In comparison, dermoid cysts may have a red wavy cuticle but also will usually have terminal hair follicles or eccrine or apocrine glands within the wall of the cyst. Steatocystomas typically are collapsed and empty or only contain sebaceous debris (Figure 4) rather than the lamellar keratin seen in dermoid and epidermoid inclusion cysts. Steatocystomas can occur as solitary (steatocystoma simplex) or multiple (steatocystoma multiplex) lesions.^1,3 They are clinically comprised of small dome-shaped papules that often are translucent and yellow. These cysts are commonly found on the sternum of males and the axillae or groin of females.²

Figure 3. Eccrine hidrocystoma with clear contents and lined by 2 layers of cuboidal epithelial cells (H&E, original magnification ×100).		Figure 4. Steatocystoma with a red wavy cuticle, sparse sebaceous contents, and sebaceous glands within the lining (H&E, original magnification ×100).

The best diagnosis is:

a. bronchogenic cyst
b. dermoid cyst
c. epidermal inclusion cyst
d. hidrocystoma
e. steatocystoma

H&E, original magnification ×40.

H&E, original magnification ×100.

Continue to the next page for the diagnosis >>

 

Dermoid Cyst

Dermoid cysts often present clinically as firm subcutaneous nodules on the head or neck in young children. They tend to arise along the lateral aspect of the eyebrow but also can occur on the nose, forehead, neck, chest, or scalp.¹ Dermoid cysts are thought to arise from the sequestration of ectodermal tissues along the embryonic fusion planes during development.² As such, they represent congenital defects and often are identified at birth; however, some are not noticed until much later when they enlarge or become inflamed or infected. Midline dermoid cysts may be associated with underlying dysraphism or intracranial extension.^3,4 Thus, any midline lesion warrants evaluation that incorporates imaging with computed tomography or magnetic resonance imaging.^4,5 Histologically, dermoid cysts are lined by a keratinizing stratified squamous epithelium (quiz image A), but the lining may be brightly eosinophilic and wavy resembling shark teeth.^1,3 The wall of a dermoid cyst commonly contains mature adnexal structures such as terminal hair follicles, sebaceous glands, apocrine glands, and/or eccrine glands (quiz image B).¹ Smooth muscle also may be seen within the lining; however, bone and cartilage are not commonly reported in dermoid cysts.² Lamellar keratin is typical of the cyst contents, and terminal hair shafts also are sometimes noted within the cystic space (quiz image B).^1,2 Treatment options include excision at the time of diagnosis or close clinical monitoring with subsequent excision if the lesion grows or becomes symptomatic.^4,5 Many practitioners opt to excise these cysts at diagnosis, as untreated lesions are at risk for infection and/or inflammation or may be cosmetically deforming.^6,7 Surgical resection, including removal of the wall of the cyst, is curative and reoccurrence is rare.⁵

Figure 1. Bronchogenic cyst demonstrating a ciliated pseudostratified epithelial lining encircled by smooth muscle (H&E, original magnification ×200).
Figure 2. Epidermal inclusion cyst containing loose lamellar keratin and a lining that closely resembles the surface epidermis (H&E, original magnification ×40).

Bronchogenic cysts demonstrate an epithelial lining that often is pseudostratified cuboidal or columnar as well as ciliated (Figure 1). Goblet cells are present in the lining in approximately 50% of cases. Smooth muscle may be seen circumferentially surrounding the cyst lining, and rare cases also contain cartilage.¹ In contrast to dermoid cysts, other types of adnexal structures are not found within the lining. Bronchogenic cysts that arise in the skin are extremely rare.² These cysts are thought to arise from respiratory epithelium that has been sequestered during embryologic formation of the tracheobronchial tree. They often are seen overlying the suprasternal notch and occasionally are found on the anterior aspect of the neck or chin. These cysts also are present at birth, similar to dermoid cysts.³

Epidermal inclusion cysts have a lining that histologically bears close resemblance to the surface epidermis. These cysts contain loose lamellar keratin, similar to a dermoid cyst. In contrast, the lining of an epidermal inclusion cyst will lack adnexal structures (Figure 2).¹ Clinically, epidermal inclusion cysts often present as smooth, dome-shaped papules and nodules with a central punctum. They are classically found on the face, neck, and trunk. These cysts are thought to arise after a traumatic insult to the pilosebaceous unit.²

Hidrocystomas can be apocrine or eccrine.³ Eccrine hidrocystomas are unilocular cysts that are lined by 2 layers of flattened to cuboidal epithelial cells (Figure 3). The cysts are filled with clear fluid and often are found adjacent to normal eccrine glands.¹ Apocrine hidrocystomas are unilocular or multilocular cysts that are lined by 1 to several layers of epithelial cells. The lining of an apocrine hidrocystoma will often exhibit luminal decapitation secretion.³ Apocrine and eccrine hidrocystomas are clinically identical and appear as blue translucent papules on the cheeks or eyelids of adults.^1-3 They usually occur periorbitally but also can be seen on the trunk, popliteal fossa, external ears, or vulva. Eccrine hidrocystomas can wax and wane in accordance with the amount of sweat produced; thus, they often expand in size during the summer months.²

Steatocystomas, or simple sebaceous duct cysts, histologically demonstrate a characteristically wavy and eosinophilic cuticle resembling shark teeth (Figure 4) similar to the lining of the sebaceous duct where it enters the follicle.¹ Sebaceous glands are an almost invariable feature, either present within the lining of the cyst (Figure 4) or in the adjacent tissue.² In comparison, dermoid cysts may have a red wavy cuticle but also will usually have terminal hair follicles or eccrine or apocrine glands within the wall of the cyst. Steatocystomas typically are collapsed and empty or only contain sebaceous debris (Figure 4) rather than the lamellar keratin seen in dermoid and epidermoid inclusion cysts. Steatocystomas can occur as solitary (steatocystoma simplex) or multiple (steatocystoma multiplex) lesions.^1,3 They are clinically comprised of small dome-shaped papules that often are translucent and yellow. These cysts are commonly found on the sternum of males and the axillae or groin of females.²

Figure 3. Eccrine hidrocystoma with clear contents and lined by 2 layers of cuboidal epithelial cells (H&E, original magnification ×100).		Figure 4. Steatocystoma with a red wavy cuticle, sparse sebaceous contents, and sebaceous glands within the lining (H&E, original magnification ×100).

The best diagnosis is:

a. bronchogenic cyst
b. dermoid cyst
c. epidermal inclusion cyst
d. hidrocystoma
e. steatocystoma

H&E, original magnification ×40.

H&E, original magnification ×100.

Continue to the next page for the diagnosis >>

 

Dermoid Cyst

Dermoid cysts often present clinically as firm subcutaneous nodules on the head or neck in young children. They tend to arise along the lateral aspect of the eyebrow but also can occur on the nose, forehead, neck, chest, or scalp.¹ Dermoid cysts are thought to arise from the sequestration of ectodermal tissues along the embryonic fusion planes during development.² As such, they represent congenital defects and often are identified at birth; however, some are not noticed until much later when they enlarge or become inflamed or infected. Midline dermoid cysts may be associated with underlying dysraphism or intracranial extension.^3,4 Thus, any midline lesion warrants evaluation that incorporates imaging with computed tomography or magnetic resonance imaging.^4,5 Histologically, dermoid cysts are lined by a keratinizing stratified squamous epithelium (quiz image A), but the lining may be brightly eosinophilic and wavy resembling shark teeth.^1,3 The wall of a dermoid cyst commonly contains mature adnexal structures such as terminal hair follicles, sebaceous glands, apocrine glands, and/or eccrine glands (quiz image B).¹ Smooth muscle also may be seen within the lining; however, bone and cartilage are not commonly reported in dermoid cysts.² Lamellar keratin is typical of the cyst contents, and terminal hair shafts also are sometimes noted within the cystic space (quiz image B).^1,2 Treatment options include excision at the time of diagnosis or close clinical monitoring with subsequent excision if the lesion grows or becomes symptomatic.^4,5 Many practitioners opt to excise these cysts at diagnosis, as untreated lesions are at risk for infection and/or inflammation or may be cosmetically deforming.^6,7 Surgical resection, including removal of the wall of the cyst, is curative and reoccurrence is rare.⁵

Figure 1. Bronchogenic cyst demonstrating a ciliated pseudostratified epithelial lining encircled by smooth muscle (H&E, original magnification ×200).
Figure 2. Epidermal inclusion cyst containing loose lamellar keratin and a lining that closely resembles the surface epidermis (H&E, original magnification ×40).

Bronchogenic cysts demonstrate an epithelial lining that often is pseudostratified cuboidal or columnar as well as ciliated (Figure 1). Goblet cells are present in the lining in approximately 50% of cases. Smooth muscle may be seen circumferentially surrounding the cyst lining, and rare cases also contain cartilage.¹ In contrast to dermoid cysts, other types of adnexal structures are not found within the lining. Bronchogenic cysts that arise in the skin are extremely rare.² These cysts are thought to arise from respiratory epithelium that has been sequestered during embryologic formation of the tracheobronchial tree. They often are seen overlying the suprasternal notch and occasionally are found on the anterior aspect of the neck or chin. These cysts also are present at birth, similar to dermoid cysts.³

Epidermal inclusion cysts have a lining that histologically bears close resemblance to the surface epidermis. These cysts contain loose lamellar keratin, similar to a dermoid cyst. In contrast, the lining of an epidermal inclusion cyst will lack adnexal structures (Figure 2).¹ Clinically, epidermal inclusion cysts often present as smooth, dome-shaped papules and nodules with a central punctum. They are classically found on the face, neck, and trunk. These cysts are thought to arise after a traumatic insult to the pilosebaceous unit.²

Hidrocystomas can be apocrine or eccrine.³ Eccrine hidrocystomas are unilocular cysts that are lined by 2 layers of flattened to cuboidal epithelial cells (Figure 3). The cysts are filled with clear fluid and often are found adjacent to normal eccrine glands.¹ Apocrine hidrocystomas are unilocular or multilocular cysts that are lined by 1 to several layers of epithelial cells. The lining of an apocrine hidrocystoma will often exhibit luminal decapitation secretion.³ Apocrine and eccrine hidrocystomas are clinically identical and appear as blue translucent papules on the cheeks or eyelids of adults.^1-3 They usually occur periorbitally but also can be seen on the trunk, popliteal fossa, external ears, or vulva. Eccrine hidrocystomas can wax and wane in accordance with the amount of sweat produced; thus, they often expand in size during the summer months.²

Steatocystomas, or simple sebaceous duct cysts, histologically demonstrate a characteristically wavy and eosinophilic cuticle resembling shark teeth (Figure 4) similar to the lining of the sebaceous duct where it enters the follicle.¹ Sebaceous glands are an almost invariable feature, either present within the lining of the cyst (Figure 4) or in the adjacent tissue.² In comparison, dermoid cysts may have a red wavy cuticle but also will usually have terminal hair follicles or eccrine or apocrine glands within the wall of the cyst. Steatocystomas typically are collapsed and empty or only contain sebaceous debris (Figure 4) rather than the lamellar keratin seen in dermoid and epidermoid inclusion cysts. Steatocystomas can occur as solitary (steatocystoma simplex) or multiple (steatocystoma multiplex) lesions.^1,3 They are clinically comprised of small dome-shaped papules that often are translucent and yellow. These cysts are commonly found on the sternum of males and the axillae or groin of females.²

Figure 3. Eccrine hidrocystoma with clear contents and lined by 2 layers of cuboidal epithelial cells (H&E, original magnification ×100).		Figure 4. Steatocystoma with a red wavy cuticle, sparse sebaceous contents, and sebaceous glands within the lining (H&E, original magnification ×100).

The best diagnosis is:

a. granular cell tumor
b. intradermal nevus
c. Langerhans cell disease
d. mastocytosis
e. multicentric reticulohistiocytosis

Monomorphic cell infiltrate in the upper dermis (H&E, original magnification ×100).

A closer view reveals cuboidal or spindle cells with basal hyperpigmentation (H&E, original magnification ×200).

Continue to the next page for the diagnosis >>

 

Mastocytosis

Mastocytosis is a clonal proliferation of mast cells in the skin and various systems of the body including the bone marrow, liver, lymph nodes, and gastrointestinal tract.^1,2 Mast cell proliferation is closely associated with germline and acquired activating KIT mutations.^3-5 Adult-onset mastocytosis is likely to involve several organs, whereas pediatric mastocytosis usually affects only the skin and is self-limiting. Patients with profound mast cell infiltration in the skin or other organs are likely to have attacks of flushing, palpitation, or diarrhea resulting from the degranulation of mast cells and release of histamine.^6,7 In a majority of patients with advanced systemic mastocytosis, mast cells are positive for the Ki-1 antigen (CD30), whereas in most patients with indolent systemic mastocytosis, only a few mast cells are positive for CD30.⁸ Recently, CD30 was reported as a new drug target in patients with CD30⁺ advanced systemic mastocytosis.⁹ Because the skin frequently is involved and easily accessible in comparison with other organs, skin biopsy often is performed to establish a diagnosis of mastocytosis. Cutaneous mastocytosis comprises urticaria pigmentosa, solitary mastocytoma, diffuse cutaneous mastocytosis, and telangiectasia macularis eruptiva perstans; approximately 80% of all cases have urticaria pigmentosa.^10-12 In cutaneous mastocytosis, skin biopsy typically shows monomorphous mast cell infiltrate mostly in the upper third of the dermis. The density of mast cells varies according to the clinical variant. For example, a lesion of telangiectasia macularis eruptiva perstans has only a perivascular mast cell infiltrate, whereas a solitary mastocytoma has sheets of mast cells in the dermis, sometimes extending into the subcutis. A skin biopsy of the brown macule on the waist showed a number of cuboidal or spindle mast cells in the upper dermis with occasional eosinophils. These mast cells are monomorphous, and no mitotic figures, necrotic cells, or atypical cells are seen. Mast cells have metachromatic granules in the cytoplasm, which can be seen with toluidine blue or Giemsa stain. CD117 (c-kit) also is positive. Mast cells in urticaria pigmentosa easily may be mistaken for nevus cells. Hyperpigmentation of the basal layer, a characteristic feature seen in urticaria pigmentosa, also may erroneously suggest a diagnosis of a melanocytic nevus.

Granular cell tumors predominantly affect the oral cavity, but the skin also can be involved. It comprises a fascicular infiltrate of large and polygonal cells with characteristic eosinophilic granular cytoplasm in the dermis (Figure 1).¹³ Cell membranes are not always distinct. Although the nuclei usually are small and centrally located, irregular and plump nuclei with distinct nucleoli also may be seen. The overlying epidermis tends to be hyperplastic. Granular cell tumor is considered a group of lesions of varying histogenesis. Cases in which tumors originated from a neural crest–derived peripheral nerve–related cell as well as a Schwann cell have been reported.^14,15 The origin of granular cell tumors is controversial.

Figure 1. Granular cell tumor showing fascicles of large and polygonal cells with characteristic eosinophilic granular cytoplasm in the dermis (H&E, original magnification ×200).

Figure 2. Intradermal nevus showing nests with melanin in the uppermost area of the lesion and neurotized nevus cells in the lower part (H&E, original magnification ×100). Pseudovascular spaces are seen on the right side.

Intradermal nevus usually has nests and cords of nevus cells in the upper dermis. The uppermost melanocytes often contain a moderate amount of melanin, whereas nevus cells in the mid and lower dermis usually do not contain melanin (Figure 2). Shrinkage during tissue processing maycause clefts between nevus cells, resulting in pseudovascular spaces.¹⁶ The deeper dermis may have a neuroid appearance with spindle-shaped cells and Meissner corpuscle–like structures.¹⁷

Although Langerhans cell disease was formerly known as Langerhans cell histiocytosis and subdivided into several clinical subtypes, including Letterer-Siwe disease, Hand-Schüller-Christian disease, and eosinophilic granuloma, these clinical subtypes commonly overlapped. Langerhans cell disease is now used as a terminology that encompasses all subtypes.^18,19 Langerhans cell disease is characterized by a proliferation of Langerhans cells with a variable mixture of other inflammatory cells. The constituent cells are large and ovoid with a distinct folded or lobulated, often kidney-shaped nucleus.²⁰ Langerhans cells usually infiltrate the upper dermis and occasionally the epidermis (Figure 3). CD1a, HLA-DR, S-100 protein, and langerin are positive in Langerhans cells.²¹

Figure 3. Langerhans cell disease showing an infiltrate of large and ovoid Langerhans cells with a distinct folded or lobulated, often kidney-shaped nucleus in the upper dermis and epidermis (H&E, original magnification ×200).

Figure 4. Multicentric reticulohistiocytosis showing a mixture of mononuclear and multinucleate histiocytes with abundant eosinophilic and finely granular cytoplasm (H&E, original magnification ×200).

Multicentric reticulohistiocytosis is characterized by a combination of papulonodular cutaneous lesions and severe arthropathy.²² An irregular mixture of mononuclear and multinucleate histiocytes showing abundant eosinophilic and finely granular cytoplasm, often with a ground-glass appearance, is seen along with lymphocytic infiltration (Figure 4).²³ A few giant cells may be seen in early lesions; older lesions more commonly have giant cells and fibrosis.

The best diagnosis is:

a. granular cell tumor
b. intradermal nevus
c. Langerhans cell disease
d. mastocytosis
e. multicentric reticulohistiocytosis

Monomorphic cell infiltrate in the upper dermis (H&E, original magnification ×100).

A closer view reveals cuboidal or spindle cells with basal hyperpigmentation (H&E, original magnification ×200).

Continue to the next page for the diagnosis >>

 

Mastocytosis

Mastocytosis is a clonal proliferation of mast cells in the skin and various systems of the body including the bone marrow, liver, lymph nodes, and gastrointestinal tract.^1,2 Mast cell proliferation is closely associated with germline and acquired activating KIT mutations.^3-5 Adult-onset mastocytosis is likely to involve several organs, whereas pediatric mastocytosis usually affects only the skin and is self-limiting. Patients with profound mast cell infiltration in the skin or other organs are likely to have attacks of flushing, palpitation, or diarrhea resulting from the degranulation of mast cells and release of histamine.^6,7 In a majority of patients with advanced systemic mastocytosis, mast cells are positive for the Ki-1 antigen (CD30), whereas in most patients with indolent systemic mastocytosis, only a few mast cells are positive for CD30.⁸ Recently, CD30 was reported as a new drug target in patients with CD30⁺ advanced systemic mastocytosis.⁹ Because the skin frequently is involved and easily accessible in comparison with other organs, skin biopsy often is performed to establish a diagnosis of mastocytosis. Cutaneous mastocytosis comprises urticaria pigmentosa, solitary mastocytoma, diffuse cutaneous mastocytosis, and telangiectasia macularis eruptiva perstans; approximately 80% of all cases have urticaria pigmentosa.^10-12 In cutaneous mastocytosis, skin biopsy typically shows monomorphous mast cell infiltrate mostly in the upper third of the dermis. The density of mast cells varies according to the clinical variant. For example, a lesion of telangiectasia macularis eruptiva perstans has only a perivascular mast cell infiltrate, whereas a solitary mastocytoma has sheets of mast cells in the dermis, sometimes extending into the subcutis. A skin biopsy of the brown macule on the waist showed a number of cuboidal or spindle mast cells in the upper dermis with occasional eosinophils. These mast cells are monomorphous, and no mitotic figures, necrotic cells, or atypical cells are seen. Mast cells have metachromatic granules in the cytoplasm, which can be seen with toluidine blue or Giemsa stain. CD117 (c-kit) also is positive. Mast cells in urticaria pigmentosa easily may be mistaken for nevus cells. Hyperpigmentation of the basal layer, a characteristic feature seen in urticaria pigmentosa, also may erroneously suggest a diagnosis of a melanocytic nevus.

Granular cell tumors predominantly affect the oral cavity, but the skin also can be involved. It comprises a fascicular infiltrate of large and polygonal cells with characteristic eosinophilic granular cytoplasm in the dermis (Figure 1).¹³ Cell membranes are not always distinct. Although the nuclei usually are small and centrally located, irregular and plump nuclei with distinct nucleoli also may be seen. The overlying epidermis tends to be hyperplastic. Granular cell tumor is considered a group of lesions of varying histogenesis. Cases in which tumors originated from a neural crest–derived peripheral nerve–related cell as well as a Schwann cell have been reported.^14,15 The origin of granular cell tumors is controversial.

Figure 1. Granular cell tumor showing fascicles of large and polygonal cells with characteristic eosinophilic granular cytoplasm in the dermis (H&E, original magnification ×200).

Figure 2. Intradermal nevus showing nests with melanin in the uppermost area of the lesion and neurotized nevus cells in the lower part (H&E, original magnification ×100). Pseudovascular spaces are seen on the right side.

Intradermal nevus usually has nests and cords of nevus cells in the upper dermis. The uppermost melanocytes often contain a moderate amount of melanin, whereas nevus cells in the mid and lower dermis usually do not contain melanin (Figure 2). Shrinkage during tissue processing maycause clefts between nevus cells, resulting in pseudovascular spaces.¹⁶ The deeper dermis may have a neuroid appearance with spindle-shaped cells and Meissner corpuscle–like structures.¹⁷

Although Langerhans cell disease was formerly known as Langerhans cell histiocytosis and subdivided into several clinical subtypes, including Letterer-Siwe disease, Hand-Schüller-Christian disease, and eosinophilic granuloma, these clinical subtypes commonly overlapped. Langerhans cell disease is now used as a terminology that encompasses all subtypes.^18,19 Langerhans cell disease is characterized by a proliferation of Langerhans cells with a variable mixture of other inflammatory cells. The constituent cells are large and ovoid with a distinct folded or lobulated, often kidney-shaped nucleus.²⁰ Langerhans cells usually infiltrate the upper dermis and occasionally the epidermis (Figure 3). CD1a, HLA-DR, S-100 protein, and langerin are positive in Langerhans cells.²¹

Figure 3. Langerhans cell disease showing an infiltrate of large and ovoid Langerhans cells with a distinct folded or lobulated, often kidney-shaped nucleus in the upper dermis and epidermis (H&E, original magnification ×200).

Figure 4. Multicentric reticulohistiocytosis showing a mixture of mononuclear and multinucleate histiocytes with abundant eosinophilic and finely granular cytoplasm (H&E, original magnification ×200).

Multicentric reticulohistiocytosis is characterized by a combination of papulonodular cutaneous lesions and severe arthropathy.²² An irregular mixture of mononuclear and multinucleate histiocytes showing abundant eosinophilic and finely granular cytoplasm, often with a ground-glass appearance, is seen along with lymphocytic infiltration (Figure 4).²³ A few giant cells may be seen in early lesions; older lesions more commonly have giant cells and fibrosis.

The best diagnosis is:

a. granular cell tumor
b. intradermal nevus
c. Langerhans cell disease
d. mastocytosis
e. multicentric reticulohistiocytosis

Monomorphic cell infiltrate in the upper dermis (H&E, original magnification ×100).

A closer view reveals cuboidal or spindle cells with basal hyperpigmentation (H&E, original magnification ×200).

Continue to the next page for the diagnosis >>

 

Mastocytosis

Mastocytosis is a clonal proliferation of mast cells in the skin and various systems of the body including the bone marrow, liver, lymph nodes, and gastrointestinal tract.^1,2 Mast cell proliferation is closely associated with germline and acquired activating KIT mutations.^3-5 Adult-onset mastocytosis is likely to involve several organs, whereas pediatric mastocytosis usually affects only the skin and is self-limiting. Patients with profound mast cell infiltration in the skin or other organs are likely to have attacks of flushing, palpitation, or diarrhea resulting from the degranulation of mast cells and release of histamine.^6,7 In a majority of patients with advanced systemic mastocytosis, mast cells are positive for the Ki-1 antigen (CD30), whereas in most patients with indolent systemic mastocytosis, only a few mast cells are positive for CD30.⁸ Recently, CD30 was reported as a new drug target in patients with CD30⁺ advanced systemic mastocytosis.⁹ Because the skin frequently is involved and easily accessible in comparison with other organs, skin biopsy often is performed to establish a diagnosis of mastocytosis. Cutaneous mastocytosis comprises urticaria pigmentosa, solitary mastocytoma, diffuse cutaneous mastocytosis, and telangiectasia macularis eruptiva perstans; approximately 80% of all cases have urticaria pigmentosa.^10-12 In cutaneous mastocytosis, skin biopsy typically shows monomorphous mast cell infiltrate mostly in the upper third of the dermis. The density of mast cells varies according to the clinical variant. For example, a lesion of telangiectasia macularis eruptiva perstans has only a perivascular mast cell infiltrate, whereas a solitary mastocytoma has sheets of mast cells in the dermis, sometimes extending into the subcutis. A skin biopsy of the brown macule on the waist showed a number of cuboidal or spindle mast cells in the upper dermis with occasional eosinophils. These mast cells are monomorphous, and no mitotic figures, necrotic cells, or atypical cells are seen. Mast cells have metachromatic granules in the cytoplasm, which can be seen with toluidine blue or Giemsa stain. CD117 (c-kit) also is positive. Mast cells in urticaria pigmentosa easily may be mistaken for nevus cells. Hyperpigmentation of the basal layer, a characteristic feature seen in urticaria pigmentosa, also may erroneously suggest a diagnosis of a melanocytic nevus.

Granular cell tumors predominantly affect the oral cavity, but the skin also can be involved. It comprises a fascicular infiltrate of large and polygonal cells with characteristic eosinophilic granular cytoplasm in the dermis (Figure 1).¹³ Cell membranes are not always distinct. Although the nuclei usually are small and centrally located, irregular and plump nuclei with distinct nucleoli also may be seen. The overlying epidermis tends to be hyperplastic. Granular cell tumor is considered a group of lesions of varying histogenesis. Cases in which tumors originated from a neural crest–derived peripheral nerve–related cell as well as a Schwann cell have been reported.^14,15 The origin of granular cell tumors is controversial.

Figure 1. Granular cell tumor showing fascicles of large and polygonal cells with characteristic eosinophilic granular cytoplasm in the dermis (H&E, original magnification ×200).

Figure 2. Intradermal nevus showing nests with melanin in the uppermost area of the lesion and neurotized nevus cells in the lower part (H&E, original magnification ×100). Pseudovascular spaces are seen on the right side.

Intradermal nevus usually has nests and cords of nevus cells in the upper dermis. The uppermost melanocytes often contain a moderate amount of melanin, whereas nevus cells in the mid and lower dermis usually do not contain melanin (Figure 2). Shrinkage during tissue processing maycause clefts between nevus cells, resulting in pseudovascular spaces.¹⁶ The deeper dermis may have a neuroid appearance with spindle-shaped cells and Meissner corpuscle–like structures.¹⁷

Although Langerhans cell disease was formerly known as Langerhans cell histiocytosis and subdivided into several clinical subtypes, including Letterer-Siwe disease, Hand-Schüller-Christian disease, and eosinophilic granuloma, these clinical subtypes commonly overlapped. Langerhans cell disease is now used as a terminology that encompasses all subtypes.^18,19 Langerhans cell disease is characterized by a proliferation of Langerhans cells with a variable mixture of other inflammatory cells. The constituent cells are large and ovoid with a distinct folded or lobulated, often kidney-shaped nucleus.²⁰ Langerhans cells usually infiltrate the upper dermis and occasionally the epidermis (Figure 3). CD1a, HLA-DR, S-100 protein, and langerin are positive in Langerhans cells.²¹

Figure 3. Langerhans cell disease showing an infiltrate of large and ovoid Langerhans cells with a distinct folded or lobulated, often kidney-shaped nucleus in the upper dermis and epidermis (H&E, original magnification ×200).

Figure 4. Multicentric reticulohistiocytosis showing a mixture of mononuclear and multinucleate histiocytes with abundant eosinophilic and finely granular cytoplasm (H&E, original magnification ×200).

Multicentric reticulohistiocytosis is characterized by a combination of papulonodular cutaneous lesions and severe arthropathy.²² An irregular mixture of mononuclear and multinucleate histiocytes showing abundant eosinophilic and finely granular cytoplasm, often with a ground-glass appearance, is seen along with lymphocytic infiltration (Figure 4).²³ A few giant cells may be seen in early lesions; older lesions more commonly have giant cells and fibrosis.

Lichen striatus (LS) is a benign, uncommon, self-limited, linear inflammatory skin disorder that primarily affects children up to 15 years of age, most commonly around 2 to 3 years of age, and is seen more frequently in girls.¹ It presents with a sudden eruption of asymptomatic small, flat-topped, lichenoid, scaly papules in a linear array on a single extremity. The lesions may be erythematous, flesh colored, or hypopigmented.^1,2 Multiple lesions appear over days to weeks and coalesce into linear plaques in a continuous or interrupted pattern along the lines of Blaschko, indicating possible somatic mosaicism.¹ Although typically asymptomatic, it may be pruritic. Most cases spontaneously resolve within 1 year.³ Recurrences are unusual. Digital involvement may result in onycholysis, longitudinal ridging, splitting, and nail loss.¹ The underlying cause of LS may be an abnormal immunologic reaction or genetic predisposition that is precipitated by some trigger such as a viral infection, trauma, hypersensitivity reaction, vaccine, seasonal variation, medication, or pregnancy.^1,2 An association with atopy has been described. Treatment is not necessary but options include topical steroids, topical retinoids, and topical calcineurin inhibitors.²

Histologically, findings in LS are somewhat variable but typically show a combination of spongiotic and lichenoid interface dermatitis with a perivascular and periadnexal lymphocytic infiltrate (Figure 1). Epidermal changes include intercellular and intracellular edema, focal spongiosis, lymphocytic exocytosis, parakeratosis, patchy hyperkeratosis, and keratinocyte necrosis (Figure 2A).^1,3 The epidermis is normal or slightly acanthotic, and dyskeratotic keratinocytes can be found in the granular and horny layers or at the dermoepidermal junction.² The lymphohistiocytic infiltrate in the superficial and deep dermis surrounds vascular plexuses and cutaneous adnexa such as eccrine glands and hair follicles.¹ Perivascular lymphoid aggregates and eccrine coil involvement are particularly distinctive of LS (Figure 2B).⁴ Pigment incontinence also may be seen.

Another condition that distributes linearly along the lines of Blaschko is linear epidermolytic hyperkeratosis (EHK). Similar to LS, histology shows hyperkeratosis, focal parakeratosis, and acanthosis of the epidermis.⁵ However, EHK shows epidermolysis, acantholysis, and perinuclear vacuolization in spinous and granular layers (Figure 3).⁵ The lack of perivascular and periadnexal inflammation also can help differentiate EHK from LS.

Linear lichen planus (LLP), similar to LS, histologically shows a lichenoid lymphocytic bandlike infiltrate obscuring the dermoepidermal junction, vacuolization of the basal cell layer, and pigment incontinence.^1,2 Although LS and LLP can have histologic overlap, the absence of adnexal or perieccrine lymphocytic inflammation can help distinguish the two.³ The histopathologic changes of intercellular edema or mild spongiosis, exocytosis, and parakeratosis present in LS also are typically absent in LLP. Linear lichen planus characteristically consists of wedge-shaped hypergranulosis and irregular acanthosis with saw-toothed rete ridges (Figure 4).² In addition, lobular eosinophilic deposits known as cytoid or Civatte bodies representing degenerated keratinocytes can be visualized at the dermoepidermal junction in LLP.² Immunofluorescence will highlight Civatte bodies with IgM, IgG, and C3, also helping to differentiate these 2 conditions.¹

Linear porokeratosis can be mistaken for the linear lesion of LS. Both entities may reveal perivascular lymphocytes in the dermis, and porokeratosis can be lichenoid in the central portion of the lesion.⁶ However, porokeratosis is unique in that it contains a cornoid lamella, characterized by a thin column of tightly packed parakeratotic cells extending from an invagination of the epidermis through the adjacent stratum corneum (Figure 5).⁶ Beneath the cornoid lamella, the granular layer is either absent or markedly attenuated, and pyknotic keratinocytes with perinuclear edema are present in the spinous layer.⁶ The epidermis in the central portion of the porokeratotic lesion may be normal, hyperplastic, or atrophic with effacement of rete ridges.

Similar to LS, linear psoriasis follows lines of Blaschko clinically. However, it is distinguished by its characteristic psoriatic epidermal changes as well as its lack of lichenoid or perieccrine inflammation.³ Typical findings in linear psoriasis include hyperkeratosis, confluent parakeratosis with entrapped neutrophilic microabscesses, acanthosis with regular elongation of rete ridges, intraepidermal neutrophils, thinned suprapapillary plates, dilated capillaries in the tips of the dermal papillae, and a chronic dermal inflammatory infiltrate (Figure 6).⁴

Lichen striatus (LS) is a benign, uncommon, self-limited, linear inflammatory skin disorder that primarily affects children up to 15 years of age, most commonly around 2 to 3 years of age, and is seen more frequently in girls.¹ It presents with a sudden eruption of asymptomatic small, flat-topped, lichenoid, scaly papules in a linear array on a single extremity. The lesions may be erythematous, flesh colored, or hypopigmented.^1,2 Multiple lesions appear over days to weeks and coalesce into linear plaques in a continuous or interrupted pattern along the lines of Blaschko, indicating possible somatic mosaicism.¹ Although typically asymptomatic, it may be pruritic. Most cases spontaneously resolve within 1 year.³ Recurrences are unusual. Digital involvement may result in onycholysis, longitudinal ridging, splitting, and nail loss.¹ The underlying cause of LS may be an abnormal immunologic reaction or genetic predisposition that is precipitated by some trigger such as a viral infection, trauma, hypersensitivity reaction, vaccine, seasonal variation, medication, or pregnancy.^1,2 An association with atopy has been described. Treatment is not necessary but options include topical steroids, topical retinoids, and topical calcineurin inhibitors.²

Histologically, findings in LS are somewhat variable but typically show a combination of spongiotic and lichenoid interface dermatitis with a perivascular and periadnexal lymphocytic infiltrate (Figure 1). Epidermal changes include intercellular and intracellular edema, focal spongiosis, lymphocytic exocytosis, parakeratosis, patchy hyperkeratosis, and keratinocyte necrosis (Figure 2A).^1,3 The epidermis is normal or slightly acanthotic, and dyskeratotic keratinocytes can be found in the granular and horny layers or at the dermoepidermal junction.² The lymphohistiocytic infiltrate in the superficial and deep dermis surrounds vascular plexuses and cutaneous adnexa such as eccrine glands and hair follicles.¹ Perivascular lymphoid aggregates and eccrine coil involvement are particularly distinctive of LS (Figure 2B).⁴ Pigment incontinence also may be seen.

Another condition that distributes linearly along the lines of Blaschko is linear epidermolytic hyperkeratosis (EHK). Similar to LS, histology shows hyperkeratosis, focal parakeratosis, and acanthosis of the epidermis.⁵ However, EHK shows epidermolysis, acantholysis, and perinuclear vacuolization in spinous and granular layers (Figure 3).⁵ The lack of perivascular and periadnexal inflammation also can help differentiate EHK from LS.

Linear lichen planus (LLP), similar to LS, histologically shows a lichenoid lymphocytic bandlike infiltrate obscuring the dermoepidermal junction, vacuolization of the basal cell layer, and pigment incontinence.^1,2 Although LS and LLP can have histologic overlap, the absence of adnexal or perieccrine lymphocytic inflammation can help distinguish the two.³ The histopathologic changes of intercellular edema or mild spongiosis, exocytosis, and parakeratosis present in LS also are typically absent in LLP. Linear lichen planus characteristically consists of wedge-shaped hypergranulosis and irregular acanthosis with saw-toothed rete ridges (Figure 4).² In addition, lobular eosinophilic deposits known as cytoid or Civatte bodies representing degenerated keratinocytes can be visualized at the dermoepidermal junction in LLP.² Immunofluorescence will highlight Civatte bodies with IgM, IgG, and C3, also helping to differentiate these 2 conditions.¹

Linear porokeratosis can be mistaken for the linear lesion of LS. Both entities may reveal perivascular lymphocytes in the dermis, and porokeratosis can be lichenoid in the central portion of the lesion.⁶ However, porokeratosis is unique in that it contains a cornoid lamella, characterized by a thin column of tightly packed parakeratotic cells extending from an invagination of the epidermis through the adjacent stratum corneum (Figure 5).⁶ Beneath the cornoid lamella, the granular layer is either absent or markedly attenuated, and pyknotic keratinocytes with perinuclear edema are present in the spinous layer.⁶ The epidermis in the central portion of the porokeratotic lesion may be normal, hyperplastic, or atrophic with effacement of rete ridges.

Similar to LS, linear psoriasis follows lines of Blaschko clinically. However, it is distinguished by its characteristic psoriatic epidermal changes as well as its lack of lichenoid or perieccrine inflammation.³ Typical findings in linear psoriasis include hyperkeratosis, confluent parakeratosis with entrapped neutrophilic microabscesses, acanthosis with regular elongation of rete ridges, intraepidermal neutrophils, thinned suprapapillary plates, dilated capillaries in the tips of the dermal papillae, and a chronic dermal inflammatory infiltrate (Figure 6).⁴

Lichen striatus (LS) is a benign, uncommon, self-limited, linear inflammatory skin disorder that primarily affects children up to 15 years of age, most commonly around 2 to 3 years of age, and is seen more frequently in girls.¹ It presents with a sudden eruption of asymptomatic small, flat-topped, lichenoid, scaly papules in a linear array on a single extremity. The lesions may be erythematous, flesh colored, or hypopigmented.^1,2 Multiple lesions appear over days to weeks and coalesce into linear plaques in a continuous or interrupted pattern along the lines of Blaschko, indicating possible somatic mosaicism.¹ Although typically asymptomatic, it may be pruritic. Most cases spontaneously resolve within 1 year.³ Recurrences are unusual. Digital involvement may result in onycholysis, longitudinal ridging, splitting, and nail loss.¹ The underlying cause of LS may be an abnormal immunologic reaction or genetic predisposition that is precipitated by some trigger such as a viral infection, trauma, hypersensitivity reaction, vaccine, seasonal variation, medication, or pregnancy.^1,2 An association with atopy has been described. Treatment is not necessary but options include topical steroids, topical retinoids, and topical calcineurin inhibitors.²

Histologically, findings in LS are somewhat variable but typically show a combination of spongiotic and lichenoid interface dermatitis with a perivascular and periadnexal lymphocytic infiltrate (Figure 1). Epidermal changes include intercellular and intracellular edema, focal spongiosis, lymphocytic exocytosis, parakeratosis, patchy hyperkeratosis, and keratinocyte necrosis (Figure 2A).^1,3 The epidermis is normal or slightly acanthotic, and dyskeratotic keratinocytes can be found in the granular and horny layers or at the dermoepidermal junction.² The lymphohistiocytic infiltrate in the superficial and deep dermis surrounds vascular plexuses and cutaneous adnexa such as eccrine glands and hair follicles.¹ Perivascular lymphoid aggregates and eccrine coil involvement are particularly distinctive of LS (Figure 2B).⁴ Pigment incontinence also may be seen.

Another condition that distributes linearly along the lines of Blaschko is linear epidermolytic hyperkeratosis (EHK). Similar to LS, histology shows hyperkeratosis, focal parakeratosis, and acanthosis of the epidermis.⁵ However, EHK shows epidermolysis, acantholysis, and perinuclear vacuolization in spinous and granular layers (Figure 3).⁵ The lack of perivascular and periadnexal inflammation also can help differentiate EHK from LS.

Linear lichen planus (LLP), similar to LS, histologically shows a lichenoid lymphocytic bandlike infiltrate obscuring the dermoepidermal junction, vacuolization of the basal cell layer, and pigment incontinence.^1,2 Although LS and LLP can have histologic overlap, the absence of adnexal or perieccrine lymphocytic inflammation can help distinguish the two.³ The histopathologic changes of intercellular edema or mild spongiosis, exocytosis, and parakeratosis present in LS also are typically absent in LLP. Linear lichen planus characteristically consists of wedge-shaped hypergranulosis and irregular acanthosis with saw-toothed rete ridges (Figure 4).² In addition, lobular eosinophilic deposits known as cytoid or Civatte bodies representing degenerated keratinocytes can be visualized at the dermoepidermal junction in LLP.² Immunofluorescence will highlight Civatte bodies with IgM, IgG, and C3, also helping to differentiate these 2 conditions.¹

Linear porokeratosis can be mistaken for the linear lesion of LS. Both entities may reveal perivascular lymphocytes in the dermis, and porokeratosis can be lichenoid in the central portion of the lesion.⁶ However, porokeratosis is unique in that it contains a cornoid lamella, characterized by a thin column of tightly packed parakeratotic cells extending from an invagination of the epidermis through the adjacent stratum corneum (Figure 5).⁶ Beneath the cornoid lamella, the granular layer is either absent or markedly attenuated, and pyknotic keratinocytes with perinuclear edema are present in the spinous layer.⁶ The epidermis in the central portion of the porokeratotic lesion may be normal, hyperplastic, or atrophic with effacement of rete ridges.

Similar to LS, linear psoriasis follows lines of Blaschko clinically. However, it is distinguished by its characteristic psoriatic epidermal changes as well as its lack of lichenoid or perieccrine inflammation.³ Typical findings in linear psoriasis include hyperkeratosis, confluent parakeratosis with entrapped neutrophilic microabscesses, acanthosis with regular elongation of rete ridges, intraepidermal neutrophils, thinned suprapapillary plates, dilated capillaries in the tips of the dermal papillae, and a chronic dermal inflammatory infiltrate (Figure 6).⁴

Syphilis often is referred to as the “great imitator” due to the protean presentations of secondary-stage disease, the most common of which are skin manifestations.¹ Secondary syphilis typically begins 3 to 10 weeks after initial exposure due to systemic dissemination of Treponema pallidum, and although presentations can vary widely, the classic presentation includes nonspecific generalized symptoms (eg, fever, malaise, lymphadenopathy), variable skin findings (eg, nonpruritic papulosquamous eruption), and mucosal ulcerations or plaques.¹ Early and accurate diagnosis of syphilis is critical to avoid the morbidity associated with advanced disease.

The classic histopathologic appearance of secondary syphilis is characterized by psoriasiform epidermal changes; a dermal inflammatory infiltrate of lymphocytes, histiocytes, and plasma cells in a lichenoid and/or superficial and deep perivascular distribution (Figure 1); and endothelial swelling of dermal blood vessels.¹ The presence of plasma cells in the infiltrate (Figure 2) is particularly useful for differentiating secondary syphilis from other clinicopathological mimickers, but this finding is not always present. Silver-based histochemical stains (eg, Warthin-Starry silver stain) can be used to high-light T pallidum organisms; however, histochemical staining is plagued by low diagnostic sensitivity for identifying the causative organism, making immunohistochemical and/or serologic testing the preferred method for confirming the diagnosis.¹

Arthropod assault is characterized by a superficial and deep perivascular lymphocytic inflammatory infiltrate with a variable number of polymorphonuclear cells.² Overlying spongiosis or focal epidermal necrosis and increased eosinophils are typical of arthropod assault (Figure 3).² The infiltrate seen following insect bites is classically described as wedge-shaped, although recent literature has disputed the sensitivity of this finding, identifying adnexal structure involvement as an alternative sensitive marker for identifying insect bites.²

 

Chronic cutaneous lupus erythematosus demonstrates a spectrum of histopathologic changes depending on the age of the lesion biopsied; however, characteristic histopathologic features typically include variable epidermal atrophy or acanthosis with basal layer vacuolar degeneration, basement membrane thickening, follicular plugging, superficial and deep perivascular and periappendageal lymphocytic inflammation, and dermal mucin deposition (Figure 4).⁴

Fixed drug eruption histopathologically presents as an interface tissue reaction–associated single-cell necrosis to broader areas of epidermal necrosis, as well as superficial to mid-dermal lymphocytic infiltrate. Unlike secondary syphilis, a fixed drug eruption is characterized by prominent melanin pigment incontinence and eosinophils (Figure 5).⁵

Similar to secondary syphilis, pityriasis lichenoides et varioliformis acuta (PLEVA) demonstrates variable psoriasiform epidermal hyperplasia with a lichenoid and perivascular lymphocytic infiltrate. Other findings in PLEVA include parakeratosis, variable epidermal necrosis, and prominent exocytosis of lymphocytes. Unlike typical secondary syphilis, PLEVA often is associated with lymphocytic vasculitis, consisting of the invasion of vessel walls by lymphocytes with extravasation of erythrocytes and an absence of conspicuous plasma cells (Figure 6).⁶

Syphilis often is referred to as the “great imitator” due to the protean presentations of secondary-stage disease, the most common of which are skin manifestations.¹ Secondary syphilis typically begins 3 to 10 weeks after initial exposure due to systemic dissemination of Treponema pallidum, and although presentations can vary widely, the classic presentation includes nonspecific generalized symptoms (eg, fever, malaise, lymphadenopathy), variable skin findings (eg, nonpruritic papulosquamous eruption), and mucosal ulcerations or plaques.¹ Early and accurate diagnosis of syphilis is critical to avoid the morbidity associated with advanced disease.

The classic histopathologic appearance of secondary syphilis is characterized by psoriasiform epidermal changes; a dermal inflammatory infiltrate of lymphocytes, histiocytes, and plasma cells in a lichenoid and/or superficial and deep perivascular distribution (Figure 1); and endothelial swelling of dermal blood vessels.¹ The presence of plasma cells in the infiltrate (Figure 2) is particularly useful for differentiating secondary syphilis from other clinicopathological mimickers, but this finding is not always present. Silver-based histochemical stains (eg, Warthin-Starry silver stain) can be used to high-light T pallidum organisms; however, histochemical staining is plagued by low diagnostic sensitivity for identifying the causative organism, making immunohistochemical and/or serologic testing the preferred method for confirming the diagnosis.¹

Arthropod assault is characterized by a superficial and deep perivascular lymphocytic inflammatory infiltrate with a variable number of polymorphonuclear cells.² Overlying spongiosis or focal epidermal necrosis and increased eosinophils are typical of arthropod assault (Figure 3).² The infiltrate seen following insect bites is classically described as wedge-shaped, although recent literature has disputed the sensitivity of this finding, identifying adnexal structure involvement as an alternative sensitive marker for identifying insect bites.²

 

Chronic cutaneous lupus erythematosus demonstrates a spectrum of histopathologic changes depending on the age of the lesion biopsied; however, characteristic histopathologic features typically include variable epidermal atrophy or acanthosis with basal layer vacuolar degeneration, basement membrane thickening, follicular plugging, superficial and deep perivascular and periappendageal lymphocytic inflammation, and dermal mucin deposition (Figure 4).⁴

Fixed drug eruption histopathologically presents as an interface tissue reaction–associated single-cell necrosis to broader areas of epidermal necrosis, as well as superficial to mid-dermal lymphocytic infiltrate. Unlike secondary syphilis, a fixed drug eruption is characterized by prominent melanin pigment incontinence and eosinophils (Figure 5).⁵

Similar to secondary syphilis, pityriasis lichenoides et varioliformis acuta (PLEVA) demonstrates variable psoriasiform epidermal hyperplasia with a lichenoid and perivascular lymphocytic infiltrate. Other findings in PLEVA include parakeratosis, variable epidermal necrosis, and prominent exocytosis of lymphocytes. Unlike typical secondary syphilis, PLEVA often is associated with lymphocytic vasculitis, consisting of the invasion of vessel walls by lymphocytes with extravasation of erythrocytes and an absence of conspicuous plasma cells (Figure 6).⁶

Syphilis often is referred to as the “great imitator” due to the protean presentations of secondary-stage disease, the most common of which are skin manifestations.¹ Secondary syphilis typically begins 3 to 10 weeks after initial exposure due to systemic dissemination of Treponema pallidum, and although presentations can vary widely, the classic presentation includes nonspecific generalized symptoms (eg, fever, malaise, lymphadenopathy), variable skin findings (eg, nonpruritic papulosquamous eruption), and mucosal ulcerations or plaques.¹ Early and accurate diagnosis of syphilis is critical to avoid the morbidity associated with advanced disease.

The classic histopathologic appearance of secondary syphilis is characterized by psoriasiform epidermal changes; a dermal inflammatory infiltrate of lymphocytes, histiocytes, and plasma cells in a lichenoid and/or superficial and deep perivascular distribution (Figure 1); and endothelial swelling of dermal blood vessels.¹ The presence of plasma cells in the infiltrate (Figure 2) is particularly useful for differentiating secondary syphilis from other clinicopathological mimickers, but this finding is not always present. Silver-based histochemical stains (eg, Warthin-Starry silver stain) can be used to high-light T pallidum organisms; however, histochemical staining is plagued by low diagnostic sensitivity for identifying the causative organism, making immunohistochemical and/or serologic testing the preferred method for confirming the diagnosis.¹

Arthropod assault is characterized by a superficial and deep perivascular lymphocytic inflammatory infiltrate with a variable number of polymorphonuclear cells.² Overlying spongiosis or focal epidermal necrosis and increased eosinophils are typical of arthropod assault (Figure 3).² The infiltrate seen following insect bites is classically described as wedge-shaped, although recent literature has disputed the sensitivity of this finding, identifying adnexal structure involvement as an alternative sensitive marker for identifying insect bites.²

 

Chronic cutaneous lupus erythematosus demonstrates a spectrum of histopathologic changes depending on the age of the lesion biopsied; however, characteristic histopathologic features typically include variable epidermal atrophy or acanthosis with basal layer vacuolar degeneration, basement membrane thickening, follicular plugging, superficial and deep perivascular and periappendageal lymphocytic inflammation, and dermal mucin deposition (Figure 4).⁴

Fixed drug eruption histopathologically presents as an interface tissue reaction–associated single-cell necrosis to broader areas of epidermal necrosis, as well as superficial to mid-dermal lymphocytic infiltrate. Unlike secondary syphilis, a fixed drug eruption is characterized by prominent melanin pigment incontinence and eosinophils (Figure 5).⁵

Similar to secondary syphilis, pityriasis lichenoides et varioliformis acuta (PLEVA) demonstrates variable psoriasiform epidermal hyperplasia with a lichenoid and perivascular lymphocytic infiltrate. Other findings in PLEVA include parakeratosis, variable epidermal necrosis, and prominent exocytosis of lymphocytes. Unlike typical secondary syphilis, PLEVA often is associated with lymphocytic vasculitis, consisting of the invasion of vessel walls by lymphocytes with extravasation of erythrocytes and an absence of conspicuous plasma cells (Figure 6).⁶