HMB‐45 (gp103) and MART‐1 expression within giant cells in an atypical fibroxanthoma: a case report

Background: Atypical fibroxanthoma (AFX) is a cutaneous tumor that primarily occurs in the sun‐damaged skin of the head and neck of adults. It is often a rapidly growing, solitary lesion that may clinically resemble squamous cell carcinoma, malignant melanoma, or lobular hemangioma. The histologic differential diagnosis primarily includes spindle cell squamous carcinoma and spindle cell melanoma, and immunohistochemical studies are often needed to establish the diagnosis. Case report: We report an unusual case of an AFX with aberrant HMB‐45 and MART‐1 (melanoma antigen recognized by T cells‐1) immunohistochemical expression. Clinical information was obtained. Histologic examination and immunohistochemical studies were performed. Results: A 54‐year‐old woman presented with a 1.5 cm posterior scalp lesion, which was excised. Microscopic examination revealed a dermal tumor composed of pleomorphic and spindled cells with numerous giant cells. The tumor cells expressed CD68 but did not express either keratin or S‐100. In addition, there was focal gp100 (with HMB‐45) and MART‐1 expression limited to the large, multinucleated cells with vacuolated cytoplasm. A diagnosis of AFX was subsequently made. Conclusions: This is the first reported case of an AFX with HMB‐45 and MART‐1 reactivity.