Joseph Pierson, MD

The Diagnosis: Cutaneous B-cell Lymphoma  

Histopathology was suggestive of cutaneous B-cell lymphoma (Figure). Further immunohistochemical studies including Bcl-6 positivity and Bcl-2 negativity in the large atypical cells supported a diagnosis of primary cutaneous follicle center lymphoma (PCFCL). The designation of primary cutaneous B-cell lymphoma includes several different types of lymphoma, including marginal zone lymphoma, diffuse large B-cell lymphoma, and intravascular lymphoma. To be considered a primary cutaneous lymphoma, there must be evidence of the lymphoma in the skin without concomitant evidence of systemic involvement, as determined through a full staging workup. Primary cutaneous follicle center lymphoma is an indolent lymphoma that most commonly presents as solitary or grouped, pink to plum-colored papules, plaques, nodules, and tumors on the scalp, forehead, or back.¹ The lesions often are biopsied as suspected basal cell carcinomas or Merkel cell carcinomas (MCCs). Lesions on the face or scalp may easily evade diagnosis, as they initially may mimic rosacea or insect bites. Less common presentations include infiltrative lesions that cause rhinophymatous changes or scarring alopecia. Multifocal or disseminated lesions rarely can be observed. This case presentation is unique in its patchy appearance that clinically resembled angiosarcoma.² When identified and treated, the disease-specific 5-year survival rate for PCFCL is greater than 95%.³  

Merkel cell carcinoma was first described in 1972 and has been diagnosed with increasing frequency each year.⁴ It generally presents as an erythematous or violaceous, tender, indurated nodule on sun-exposed skin of the head or neck in elderly White men. However, other presentations have been reported, including papules, plaques, cystlike structures, pruritic tumors, pedunculated lesions, subcutaneous masses, and telangiectatic papules.⁵ Histopathologically, MCC is characterized by dermal nests and sheets of basaloid cells with finely granular salt and pepper-like chromatin. The histologic features can resemble other small blue cell tumors; therefore, the differential diagnosis can be broad.⁵ Immunohistochemistry that can confirm the diagnosis of MCC generally will be positive for cytokeratin 20 and neuroendocrine markers but negative for cytokeratin ⁷ and thyroid transcription factor ¹. Merkel cell carcinoma is an aggressive tumor with a high risk for local recurrence and distant metastasis that carries a generally poor prognosis, especially when there is evidence of metastatic disease at presentation.^5,6  

Rosacea can appear as telangiectatic patches, though generally not as one discrete patch limited to the forehead, as in our patient. Histologic features vary based on the age of the lesion and clinical variant. In early lesions there is a mild perivascular lymphoplasmacytic infiltrate within the dermis, while older lesions can have a mixed infiltrate crowded around vessels and adnexal structures. Granulomas often are seen near hair follicles and interspersed throughout the dermis with ectatic vessels and dermal edema.⁷  

Angiosarcoma is divided into 3 clinicopathological subtypes: idiopathic angiosarcoma of the head and neck, angiosarcoma in the setting of lymphedema, and postirradiation angiosarcoma.⁷ Idiopathic angiosarcoma most closely mimics PCFCL, as it can present as single or multifocal nodules, plaques, or patches. Histologically, the 3 groups appear similar with poorly circumscribed, infiltrative, dermal tumors. The neoplastic endothelial cells have large hyperchromatic nuclei that protrude into vascular lumens. The prognosis for idiopathic angiosarcoma of the head and neck is poor, with a 5-year survival rate of 15% to 34%, which often is due to delayed diagnosis.⁷ 

Pigmented purpuric dermatoses (PPDs) are chronic skin disorders characterized by purpura due to extravasation of blood from capillaries; the resulting hemosiderin deposition leads to pigmentation.⁷ There are various forms of PPD, which are classified into groups based on clinical appearance including Schamberg disease, purpura annularis telangiectodes of Majocchi, pigmented purpuric lichenoid dermatosis of Gougerot and Blum, lichen aureus, and others including eczematid and itching variants, which some consider to be distinct entities. Purpura annularis telangiectodes of Majocchi is the specific PPD that should be included in the clinical differential for PCFCL because it presents as annular patches with telangiectasias. Histologically, PPDs are characterized by a CD4⁺ lymphocytic infiltrate in the upper dermis with extravasated red blood cells and the presence of hemosiderin mostly within macrophages and a lack of true vasculitis. Clonality of the T cells has been shown, and there is some evidence that PPD may overlap with mycosis fungoides. However, this overlap mainly has been seen in patients with widespread lesions and would not apply to this case. In general, patients with PPD can be reassured of the benign process. In cases of widespread PPD, patients should be followed clinically to assess for progression to mycosis fungoides, though the likelihood is low.⁷  

Our patient underwent a full staging workup, which confirmed the diagnosis of PCFCL. He was treated with radiation to the forehead that resulted in clearance of the lesion. Approximately 2 years after the initial diagnosis, the patient was alive and well with no evidence of recurrence of PCFCL. 

In conclusion, it is imperative to identify unusual, macular, vascular-appearing patches, especially on the head and neck in older individuals. Because the clinical presentations of PCFCL, angiosarcoma, rosacea, MCC, and PPD can overlap with one another as well as with other entities, it is necessary to have a high level of suspicion and low threshold to biopsy these types of lesions, as outcomes can be drastically different. 

The Diagnosis: Cutaneous B-cell Lymphoma  

Histopathology was suggestive of cutaneous B-cell lymphoma (Figure). Further immunohistochemical studies including Bcl-6 positivity and Bcl-2 negativity in the large atypical cells supported a diagnosis of primary cutaneous follicle center lymphoma (PCFCL). The designation of primary cutaneous B-cell lymphoma includes several different types of lymphoma, including marginal zone lymphoma, diffuse large B-cell lymphoma, and intravascular lymphoma. To be considered a primary cutaneous lymphoma, there must be evidence of the lymphoma in the skin without concomitant evidence of systemic involvement, as determined through a full staging workup. Primary cutaneous follicle center lymphoma is an indolent lymphoma that most commonly presents as solitary or grouped, pink to plum-colored papules, plaques, nodules, and tumors on the scalp, forehead, or back.¹ The lesions often are biopsied as suspected basal cell carcinomas or Merkel cell carcinomas (MCCs). Lesions on the face or scalp may easily evade diagnosis, as they initially may mimic rosacea or insect bites. Less common presentations include infiltrative lesions that cause rhinophymatous changes or scarring alopecia. Multifocal or disseminated lesions rarely can be observed. This case presentation is unique in its patchy appearance that clinically resembled angiosarcoma.² When identified and treated, the disease-specific 5-year survival rate for PCFCL is greater than 95%.³  

Merkel cell carcinoma was first described in 1972 and has been diagnosed with increasing frequency each year.⁴ It generally presents as an erythematous or violaceous, tender, indurated nodule on sun-exposed skin of the head or neck in elderly White men. However, other presentations have been reported, including papules, plaques, cystlike structures, pruritic tumors, pedunculated lesions, subcutaneous masses, and telangiectatic papules.⁵ Histopathologically, MCC is characterized by dermal nests and sheets of basaloid cells with finely granular salt and pepper-like chromatin. The histologic features can resemble other small blue cell tumors; therefore, the differential diagnosis can be broad.⁵ Immunohistochemistry that can confirm the diagnosis of MCC generally will be positive for cytokeratin 20 and neuroendocrine markers but negative for cytokeratin ⁷ and thyroid transcription factor ¹. Merkel cell carcinoma is an aggressive tumor with a high risk for local recurrence and distant metastasis that carries a generally poor prognosis, especially when there is evidence of metastatic disease at presentation.^5,6  

Rosacea can appear as telangiectatic patches, though generally not as one discrete patch limited to the forehead, as in our patient. Histologic features vary based on the age of the lesion and clinical variant. In early lesions there is a mild perivascular lymphoplasmacytic infiltrate within the dermis, while older lesions can have a mixed infiltrate crowded around vessels and adnexal structures. Granulomas often are seen near hair follicles and interspersed throughout the dermis with ectatic vessels and dermal edema.⁷  

Angiosarcoma is divided into 3 clinicopathological subtypes: idiopathic angiosarcoma of the head and neck, angiosarcoma in the setting of lymphedema, and postirradiation angiosarcoma.⁷ Idiopathic angiosarcoma most closely mimics PCFCL, as it can present as single or multifocal nodules, plaques, or patches. Histologically, the 3 groups appear similar with poorly circumscribed, infiltrative, dermal tumors. The neoplastic endothelial cells have large hyperchromatic nuclei that protrude into vascular lumens. The prognosis for idiopathic angiosarcoma of the head and neck is poor, with a 5-year survival rate of 15% to 34%, which often is due to delayed diagnosis.⁷ 

Pigmented purpuric dermatoses (PPDs) are chronic skin disorders characterized by purpura due to extravasation of blood from capillaries; the resulting hemosiderin deposition leads to pigmentation.⁷ There are various forms of PPD, which are classified into groups based on clinical appearance including Schamberg disease, purpura annularis telangiectodes of Majocchi, pigmented purpuric lichenoid dermatosis of Gougerot and Blum, lichen aureus, and others including eczematid and itching variants, which some consider to be distinct entities. Purpura annularis telangiectodes of Majocchi is the specific PPD that should be included in the clinical differential for PCFCL because it presents as annular patches with telangiectasias. Histologically, PPDs are characterized by a CD4⁺ lymphocytic infiltrate in the upper dermis with extravasated red blood cells and the presence of hemosiderin mostly within macrophages and a lack of true vasculitis. Clonality of the T cells has been shown, and there is some evidence that PPD may overlap with mycosis fungoides. However, this overlap mainly has been seen in patients with widespread lesions and would not apply to this case. In general, patients with PPD can be reassured of the benign process. In cases of widespread PPD, patients should be followed clinically to assess for progression to mycosis fungoides, though the likelihood is low.⁷  

Our patient underwent a full staging workup, which confirmed the diagnosis of PCFCL. He was treated with radiation to the forehead that resulted in clearance of the lesion. Approximately 2 years after the initial diagnosis, the patient was alive and well with no evidence of recurrence of PCFCL. 

In conclusion, it is imperative to identify unusual, macular, vascular-appearing patches, especially on the head and neck in older individuals. Because the clinical presentations of PCFCL, angiosarcoma, rosacea, MCC, and PPD can overlap with one another as well as with other entities, it is necessary to have a high level of suspicion and low threshold to biopsy these types of lesions, as outcomes can be drastically different. 

The Diagnosis: Cutaneous B-cell Lymphoma  

Histopathology was suggestive of cutaneous B-cell lymphoma (Figure). Further immunohistochemical studies including Bcl-6 positivity and Bcl-2 negativity in the large atypical cells supported a diagnosis of primary cutaneous follicle center lymphoma (PCFCL). The designation of primary cutaneous B-cell lymphoma includes several different types of lymphoma, including marginal zone lymphoma, diffuse large B-cell lymphoma, and intravascular lymphoma. To be considered a primary cutaneous lymphoma, there must be evidence of the lymphoma in the skin without concomitant evidence of systemic involvement, as determined through a full staging workup. Primary cutaneous follicle center lymphoma is an indolent lymphoma that most commonly presents as solitary or grouped, pink to plum-colored papules, plaques, nodules, and tumors on the scalp, forehead, or back.¹ The lesions often are biopsied as suspected basal cell carcinomas or Merkel cell carcinomas (MCCs). Lesions on the face or scalp may easily evade diagnosis, as they initially may mimic rosacea or insect bites. Less common presentations include infiltrative lesions that cause rhinophymatous changes or scarring alopecia. Multifocal or disseminated lesions rarely can be observed. This case presentation is unique in its patchy appearance that clinically resembled angiosarcoma.² When identified and treated, the disease-specific 5-year survival rate for PCFCL is greater than 95%.³  

Merkel cell carcinoma was first described in 1972 and has been diagnosed with increasing frequency each year.⁴ It generally presents as an erythematous or violaceous, tender, indurated nodule on sun-exposed skin of the head or neck in elderly White men. However, other presentations have been reported, including papules, plaques, cystlike structures, pruritic tumors, pedunculated lesions, subcutaneous masses, and telangiectatic papules.⁵ Histopathologically, MCC is characterized by dermal nests and sheets of basaloid cells with finely granular salt and pepper-like chromatin. The histologic features can resemble other small blue cell tumors; therefore, the differential diagnosis can be broad.⁵ Immunohistochemistry that can confirm the diagnosis of MCC generally will be positive for cytokeratin 20 and neuroendocrine markers but negative for cytokeratin ⁷ and thyroid transcription factor ¹. Merkel cell carcinoma is an aggressive tumor with a high risk for local recurrence and distant metastasis that carries a generally poor prognosis, especially when there is evidence of metastatic disease at presentation.^5,6  

Rosacea can appear as telangiectatic patches, though generally not as one discrete patch limited to the forehead, as in our patient. Histologic features vary based on the age of the lesion and clinical variant. In early lesions there is a mild perivascular lymphoplasmacytic infiltrate within the dermis, while older lesions can have a mixed infiltrate crowded around vessels and adnexal structures. Granulomas often are seen near hair follicles and interspersed throughout the dermis with ectatic vessels and dermal edema.⁷  

Angiosarcoma is divided into 3 clinicopathological subtypes: idiopathic angiosarcoma of the head and neck, angiosarcoma in the setting of lymphedema, and postirradiation angiosarcoma.⁷ Idiopathic angiosarcoma most closely mimics PCFCL, as it can present as single or multifocal nodules, plaques, or patches. Histologically, the 3 groups appear similar with poorly circumscribed, infiltrative, dermal tumors. The neoplastic endothelial cells have large hyperchromatic nuclei that protrude into vascular lumens. The prognosis for idiopathic angiosarcoma of the head and neck is poor, with a 5-year survival rate of 15% to 34%, which often is due to delayed diagnosis.⁷ 

Pigmented purpuric dermatoses (PPDs) are chronic skin disorders characterized by purpura due to extravasation of blood from capillaries; the resulting hemosiderin deposition leads to pigmentation.⁷ There are various forms of PPD, which are classified into groups based on clinical appearance including Schamberg disease, purpura annularis telangiectodes of Majocchi, pigmented purpuric lichenoid dermatosis of Gougerot and Blum, lichen aureus, and others including eczematid and itching variants, which some consider to be distinct entities. Purpura annularis telangiectodes of Majocchi is the specific PPD that should be included in the clinical differential for PCFCL because it presents as annular patches with telangiectasias. Histologically, PPDs are characterized by a CD4⁺ lymphocytic infiltrate in the upper dermis with extravasated red blood cells and the presence of hemosiderin mostly within macrophages and a lack of true vasculitis. Clonality of the T cells has been shown, and there is some evidence that PPD may overlap with mycosis fungoides. However, this overlap mainly has been seen in patients with widespread lesions and would not apply to this case. In general, patients with PPD can be reassured of the benign process. In cases of widespread PPD, patients should be followed clinically to assess for progression to mycosis fungoides, though the likelihood is low.⁷  

Our patient underwent a full staging workup, which confirmed the diagnosis of PCFCL. He was treated with radiation to the forehead that resulted in clearance of the lesion. Approximately 2 years after the initial diagnosis, the patient was alive and well with no evidence of recurrence of PCFCL. 

In conclusion, it is imperative to identify unusual, macular, vascular-appearing patches, especially on the head and neck in older individuals. Because the clinical presentations of PCFCL, angiosarcoma, rosacea, MCC, and PPD can overlap with one another as well as with other entities, it is necessary to have a high level of suspicion and low threshold to biopsy these types of lesions, as outcomes can be drastically different. 

The Diagnosis:  Primary Cutaneous Perivascular Epithelioid Cell Tumor  

Perivascular epithelioid cell tumors (PEComas) were first described in 1996.¹ They comprise a family of rare mesenchymal neoplasms that have a unique characteristic of staining positive for melanocytic and smooth muscle markers on immunohistochemistry.² These neoplasms have been described in many areas of the body including the uterus, bladder, heart, pancreas, and prostate. The majority of PEComas are extracutaneous, with only 8% of reported cases originating on the skin.³ A case of primary cutaneous PEComa (pcPEComa) was described in 2003.⁴ The primary cutaneous form is extremely rare.^3,5-7  

A broad deep shave biopsy was performed in our patient in an attempt to sample the entire lesion. Histopathologic examination of the nodule demonstrated a dermal neoplasm comprised of a diffuse proliferation of large polygonal cells with abundant clear cytoplasm, fine chromatin, and prominent nucleoli (Figure 1A). Higher-power magnification showed moderate nuclear pleomorphism and only rare mitotic figures (Figure 1B).

Immunohistochemical staining revealed positivity for myomelanocytic markers with positivity for human melanoma black 45 (HMB-45)(Figure 2) and desmin (not shown). Additionally, the tumor was positive for CD163 and negative for smooth muscle actin, cytokeratin, and S-100 protein.  

Perivascular epithelioid cell tumors are characterized histologically as mesenchymal neoplasms containing large epithelioid to spindled cells with a slightly granular, vacuolated cytoplasm. These cells often are found in close proximity to vascular structures.^3,5,8 The hallmark of PEComas is the expression of both melanocytic and muscle markers.^3,8 A review of staining patterns of pcPEComas emphasized that immunophenotypes between visceral and primary cutaneous forms may vary considerably.^3,5,8 The most consistent and sensitive melanocytic marker is HMB-45 (88%-92% positive).^3,8 Positive Melan-A staining varies in the literature from 0% to 50% of cases.³ Our patient's neoplasm expressed the characteristic myomelanocytic immunophenotype with both HMB-45 and desmin positivity. 

Given the histologic characteristics, these lesions can be mistaken for melanocytic and other nonmelanocytic tumors with a clear cell morphology such as balloon cell nevus, hypomelanotic blue nevus, and melanoma.^2,3 A pigmented case of pcPEComa was reported in 2015 and was originally diagnosed as metastatic melanoma.⁶ Unlike pcPEComa, melanoma usually stains positive with S-100 protein in up to 99% of cases⁸ and is negative for muscle markers; however, a case series reported S-100 protein positivity in 38% of pcPEComas.³ Nonmelanocytic neoplasms in the histologic differential diagnosis include clear cell sarcoma and clear cell renal cell carcinoma, both of which show immunoreactivity for cytokeratin.⁹  

Histologic criteria exist for establishing malignancy potential for visceral PEComas but not for pcPEComas, though it has been suggested that the same malignancy criteria should be applied to pcPEComas.^3,9 Features associated with malignancy include size greater than 8 cm, mitotic activity greater than 1 mitosis per 50 high-power fields, infiltrative growth pattern, high nuclear grade, necrosis, and vascular invasion. Based on these criteria, fulfilling 2 or more features technically classifies the lesion as malignant, 1 feature classifies it as uncertain malignant potential, and a lack of these features renders the lesion benign.⁹  

The overwhelming majority of pcPEComas are considered benign. One case of pcPEComa was considered malignant with a high mitotic rate (5 mitoses per 10 high-power fields) and nuclear atypia.¹⁰ Further workup with thoracic computed tomography and positron emission tomography-computed tomography was negative for metastasis. Treatment with wide excision and radiotherapy was performed with no sign of recurrence at 24-month follow-up.¹⁰  

Although pcPEComas arising from the dermis seem to be benign overall, PEComas originating from the subcutaneous tissue may have greater malignancy potential. Two cases of subcutaneous PEComas presenting as nodules resulted in metastasis; one case had local nodal metastasis and another developed metastasis to the lungs months later.^10,11 

The Diagnosis:  Primary Cutaneous Perivascular Epithelioid Cell Tumor  

Perivascular epithelioid cell tumors (PEComas) were first described in 1996.¹ They comprise a family of rare mesenchymal neoplasms that have a unique characteristic of staining positive for melanocytic and smooth muscle markers on immunohistochemistry.² These neoplasms have been described in many areas of the body including the uterus, bladder, heart, pancreas, and prostate. The majority of PEComas are extracutaneous, with only 8% of reported cases originating on the skin.³ A case of primary cutaneous PEComa (pcPEComa) was described in 2003.⁴ The primary cutaneous form is extremely rare.^3,5-7  

A broad deep shave biopsy was performed in our patient in an attempt to sample the entire lesion. Histopathologic examination of the nodule demonstrated a dermal neoplasm comprised of a diffuse proliferation of large polygonal cells with abundant clear cytoplasm, fine chromatin, and prominent nucleoli (Figure 1A). Higher-power magnification showed moderate nuclear pleomorphism and only rare mitotic figures (Figure 1B).

Immunohistochemical staining revealed positivity for myomelanocytic markers with positivity for human melanoma black 45 (HMB-45)(Figure 2) and desmin (not shown). Additionally, the tumor was positive for CD163 and negative for smooth muscle actin, cytokeratin, and S-100 protein.  

Perivascular epithelioid cell tumors are characterized histologically as mesenchymal neoplasms containing large epithelioid to spindled cells with a slightly granular, vacuolated cytoplasm. These cells often are found in close proximity to vascular structures.^3,5,8 The hallmark of PEComas is the expression of both melanocytic and muscle markers.^3,8 A review of staining patterns of pcPEComas emphasized that immunophenotypes between visceral and primary cutaneous forms may vary considerably.^3,5,8 The most consistent and sensitive melanocytic marker is HMB-45 (88%-92% positive).^3,8 Positive Melan-A staining varies in the literature from 0% to 50% of cases.³ Our patient's neoplasm expressed the characteristic myomelanocytic immunophenotype with both HMB-45 and desmin positivity. 

Given the histologic characteristics, these lesions can be mistaken for melanocytic and other nonmelanocytic tumors with a clear cell morphology such as balloon cell nevus, hypomelanotic blue nevus, and melanoma.^2,3 A pigmented case of pcPEComa was reported in 2015 and was originally diagnosed as metastatic melanoma.⁶ Unlike pcPEComa, melanoma usually stains positive with S-100 protein in up to 99% of cases⁸ and is negative for muscle markers; however, a case series reported S-100 protein positivity in 38% of pcPEComas.³ Nonmelanocytic neoplasms in the histologic differential diagnosis include clear cell sarcoma and clear cell renal cell carcinoma, both of which show immunoreactivity for cytokeratin.⁹  

Histologic criteria exist for establishing malignancy potential for visceral PEComas but not for pcPEComas, though it has been suggested that the same malignancy criteria should be applied to pcPEComas.^3,9 Features associated with malignancy include size greater than 8 cm, mitotic activity greater than 1 mitosis per 50 high-power fields, infiltrative growth pattern, high nuclear grade, necrosis, and vascular invasion. Based on these criteria, fulfilling 2 or more features technically classifies the lesion as malignant, 1 feature classifies it as uncertain malignant potential, and a lack of these features renders the lesion benign.⁹  

The overwhelming majority of pcPEComas are considered benign. One case of pcPEComa was considered malignant with a high mitotic rate (5 mitoses per 10 high-power fields) and nuclear atypia.¹⁰ Further workup with thoracic computed tomography and positron emission tomography-computed tomography was negative for metastasis. Treatment with wide excision and radiotherapy was performed with no sign of recurrence at 24-month follow-up.¹⁰  

Although pcPEComas arising from the dermis seem to be benign overall, PEComas originating from the subcutaneous tissue may have greater malignancy potential. Two cases of subcutaneous PEComas presenting as nodules resulted in metastasis; one case had local nodal metastasis and another developed metastasis to the lungs months later.^10,11 

The Diagnosis:  Primary Cutaneous Perivascular Epithelioid Cell Tumor  

Perivascular epithelioid cell tumors (PEComas) were first described in 1996.¹ They comprise a family of rare mesenchymal neoplasms that have a unique characteristic of staining positive for melanocytic and smooth muscle markers on immunohistochemistry.² These neoplasms have been described in many areas of the body including the uterus, bladder, heart, pancreas, and prostate. The majority of PEComas are extracutaneous, with only 8% of reported cases originating on the skin.³ A case of primary cutaneous PEComa (pcPEComa) was described in 2003.⁴ The primary cutaneous form is extremely rare.^3,5-7  

A broad deep shave biopsy was performed in our patient in an attempt to sample the entire lesion. Histopathologic examination of the nodule demonstrated a dermal neoplasm comprised of a diffuse proliferation of large polygonal cells with abundant clear cytoplasm, fine chromatin, and prominent nucleoli (Figure 1A). Higher-power magnification showed moderate nuclear pleomorphism and only rare mitotic figures (Figure 1B).

Immunohistochemical staining revealed positivity for myomelanocytic markers with positivity for human melanoma black 45 (HMB-45)(Figure 2) and desmin (not shown). Additionally, the tumor was positive for CD163 and negative for smooth muscle actin, cytokeratin, and S-100 protein.  

Perivascular epithelioid cell tumors are characterized histologically as mesenchymal neoplasms containing large epithelioid to spindled cells with a slightly granular, vacuolated cytoplasm. These cells often are found in close proximity to vascular structures.^3,5,8 The hallmark of PEComas is the expression of both melanocytic and muscle markers.^3,8 A review of staining patterns of pcPEComas emphasized that immunophenotypes between visceral and primary cutaneous forms may vary considerably.^3,5,8 The most consistent and sensitive melanocytic marker is HMB-45 (88%-92% positive).^3,8 Positive Melan-A staining varies in the literature from 0% to 50% of cases.³ Our patient's neoplasm expressed the characteristic myomelanocytic immunophenotype with both HMB-45 and desmin positivity. 

Given the histologic characteristics, these lesions can be mistaken for melanocytic and other nonmelanocytic tumors with a clear cell morphology such as balloon cell nevus, hypomelanotic blue nevus, and melanoma.^2,3 A pigmented case of pcPEComa was reported in 2015 and was originally diagnosed as metastatic melanoma.⁶ Unlike pcPEComa, melanoma usually stains positive with S-100 protein in up to 99% of cases⁸ and is negative for muscle markers; however, a case series reported S-100 protein positivity in 38% of pcPEComas.³ Nonmelanocytic neoplasms in the histologic differential diagnosis include clear cell sarcoma and clear cell renal cell carcinoma, both of which show immunoreactivity for cytokeratin.⁹  

Histologic criteria exist for establishing malignancy potential for visceral PEComas but not for pcPEComas, though it has been suggested that the same malignancy criteria should be applied to pcPEComas.^3,9 Features associated with malignancy include size greater than 8 cm, mitotic activity greater than 1 mitosis per 50 high-power fields, infiltrative growth pattern, high nuclear grade, necrosis, and vascular invasion. Based on these criteria, fulfilling 2 or more features technically classifies the lesion as malignant, 1 feature classifies it as uncertain malignant potential, and a lack of these features renders the lesion benign.⁹  

The overwhelming majority of pcPEComas are considered benign. One case of pcPEComa was considered malignant with a high mitotic rate (5 mitoses per 10 high-power fields) and nuclear atypia.¹⁰ Further workup with thoracic computed tomography and positron emission tomography-computed tomography was negative for metastasis. Treatment with wide excision and radiotherapy was performed with no sign of recurrence at 24-month follow-up.¹⁰  

Although pcPEComas arising from the dermis seem to be benign overall, PEComas originating from the subcutaneous tissue may have greater malignancy potential. Two cases of subcutaneous PEComas presenting as nodules resulted in metastasis; one case had local nodal metastasis and another developed metastasis to the lungs months later.^10,11