This pathology lecture, led by Dr. Jared Gardner and Dr. Ed Fulton, focuses on the rare entity of primary cutaneous PEComa (Perivascular Epithelioid Cell Tumor). The cutaneous variant is histologically distinct from its visceral counterparts. On shave biopsy, the tumor presents as a nodular dermal lesion composed of pale, clear cells with vacuolated (but non-lipid) cytoplasm and characteristic granular pink material, arranged in vague nests or trabeculae between dermal collagen bundles. The critical diagnostic feature is the unique immunohistochemical profile: co-expression of melanocytic markers (HMB-45, Mart-1) and muscle markers (Desmin/Actin) in roughly half of cases, coupled with negativity for S100 and SOX10, which serves to exclude common melanocytic lesions (Nevus, Melanoma) and highly malignant differentials like Clear Cell Sarcoma. The video cautions against using non-specific markers like CD10 and CD68, which are positive in cutaneous PEComas but also in many differential diagnoses (e.g., metastatic Renal Cell Carcinoma). The prognosis for the vast majority of the approximately 30 reported cutaneous cases is benign and indolent, typically requiring conservative excision.
Primary Cutaneous PEComa: Diagnosis and Differential
0:00 Defining a Rare Entity: Primary cutaneous PEComa (Perivascular Epithelioid Cell Tumor) is a rare dermal lesion, distinct from internal PEComa subtypes.
0:44 Histological Appearance (Low Power): Presents as a nodular, relatively circumscribed dermal lesion filled with sheets of very pale or clear cells, often retaining pink wisps of background dermal collagen and a delicate vascular network.
1:46 Cytology (High Power): Tumor cells are clear/pale with a vacuolated cytoplasm (due possibly to glycogen, not sharp lipid vacuoles) and contain characteristic granular pink material. Nuclei are oval/round with fine chromatin and small nucleoli. Mitotic activity is usually low (<1 per 10 HPF).
4:44 Key Immunophenotype: PEComas exhibit co-expression of melanocytic markers (HMB-45, Mart-1/Melan-A) and muscle markers (smooth muscle actin and/or Desmin). This co-expression is unique and crucial for diagnosis.
5:28 Differential Diagnosis (Melanocytic): Must rule out balloon cell change in Nevus or Melanoma. Melanocytic lesions typically show strong, diffuse expression of S100 protein and SOX10.
7:56 Crucial Exclusion Criteria: In cutaneous PEComas, S100 expression is rare, weak, and may be a non-specific artifact of glycogen; SOX10 is essentially always negative. SOX10 negativity strongly argues against a melanocytic process.
Clear Cell Sarcoma (CCS): Highly malignant and shares HMB-45/Mart-1 positivity, but is distinguished by strong S100 and SOX10 expression and higher mitotic activity.
Metastatic Renal Cell Carcinoma (RCC): Important differential, as both RCC and cutaneous PEComa stain strongly and diffusely for CD10. CD10 is considered unreliable for distinction in dermpath.
15:54 Non-Specific Stains: CD68 is often positive (stains lysosomes) but is non-specific and should not be misinterpreted as indicating a histiocytic process (e.g., xanthoma).
18:37 Prognosis: The lesion is extremely rare (around 30 cases reported). The vast majority behave in a benign and indolent fashion. Conservative, complete excision is recommended.
19:24 Muscle Marker Caveat: A significant difference from deep internal PEComas is that only about half of cutaneous PEComas express muscle markers. A case fitting all other criteria (histology, HMB-45/Mart-1 positive, S100/SOX10 negative) is still consistent with PEComa even if muscle markers are negative.