Primary cutaneous PEComa: Distinctive clear cell lesions of skin

PEComas arising in somatic soft tissue or skin are rare. To further characterize the clinicopathologic spectrum, we herein report a series of 10 primary cutaneous PEComas. Eight patients were female and 2 patients were male. The age at presentation ranged from 15 to 81 years (median, 52y). None had tuberous sclerosis. Tumor size ranged from 0.7 to 2cm (median size, 1.5cm). Eight tumors were located on the limbs and 2 on the back. The tumors involved the dermis and commonly showed infiltration into the subcutaneous fat. Architecturally, a nested, focally trabecular growth pattern with prominent vasculature composed of delicate thin-walled capillaries was observed. Six tumors were composed of epithelioid cells and 4 showed mixed epithelioid and spindle cell morphology. Tumor cells had clear, palely eosinophilic or granular cytoplasm. Multinucleate tumor giant cells were observed in 3 cases. Mitotic activity ranged from 0 to 2 mitoses per 30 high power fields (mean < 1/10 HPF). Pleomorphism or necrosis was absent. All cases showed at least focal positivity for HMB-45. Melan A was positive in 5/7. In cases where HMB-45 was positive in fewer than 5% of tumor cells (5110), microphthalmia transcription factor was positive in the majority of the tumor cell nuclei. Desmin positivity was observed in 5 and smooth muscle actin in 1 case, respectively. The other muscle markers (caldesmon, calponin) and also pan-keratin and epithelial membrane antigen were negative. Follow-up data were available in 6 cases and ranged between 3 and 108 months (median, 47 mo). None has recurred to date. Primary cutaneous PEComas are rare and thus far apparently benign cutaneous tumors. Accurate recognition of this entity is essential because of potential misdiagnosis as malignant tumors, especially malignant melanoma.