Abstract

41-year-old male with Fitzpatrick skin type V with history of dissecting cellulitis of the scalp and hidradenitis suppurativa reported a lesion on his right lateral thigh that was solid, painful and would “burst” a few times per year. Punch biopsy of this 3.2 x 2.5 cm shiny brown tumor with overlying central scale/crust revealed an atypical spindle cell neoplasm consistent with atypical fibroxanthoma (AFX) / pleomorphic dermal sarcoma (PDS). Incisional biopsy, which narrowly excised the tumor, showed an intradermal and subcutaneous histiocytic proliferation with pleomorphic cells similar to those seen in atypical fibrous histiocytoma (AFH; dermatofibroma with “monster cells”). However, given significant degree of nuclear pleomorphism, large size, and mitotic activity, PDS remained difficult to exclude. Tumor cells were CD68+, CD163+, CKCKT-, SOX-10-, Desmin-, and CD34-. Diagnostic criteria of these rare entities is an ongoing debate. There are no distinguishing specific immunohistochemical stains or molecular tests. Other spindled neoplasms must be excluded. AFX/PDS lie on a spectrum, and some argue AFH should be re-categorized under PDS given 2/21 cases in the largest study to date had metastasis. Clinically, AFX/PDS is often on sun-damaged areas of the head and neck in elderly, presenting as rapidly enlarging dome-shaped, skin-colored or ulcerated nodules. In contrast, AFH presents on extremities of young to middle-aged adults as a solitary, firm, cystic cutaneous nodule. In our case, the tumor was slow-growing, not on sun-damaged skin, and consistent with AFH, or less likely, an atypical case of AFX/UPS. Regardless, margin control and close monitoring is paramount.