Aberrant Melan-A expression in atypical fibroxanthoma and undifferentiated pleomorphic sarcoma of the skin

Background: Atypical fibroxanthoma (AFX) is a distinctive clinicopathologic entity presenting on sun‐damaged skin of the elderly. Its behavior is benign if strict diagnostic criteria are applied. Tumors showing invasion of deeper structures or perineural/lymphovascular invasion are best regarded as undifferentiated pleomorphic sarcoma of the skin. The diagnosis requires immunohistochemical studies to exclude melanoma, squamous cell carcinoma, angiosarcoma and leiomyosarcoma. Methods: Two AFX and one undifferentiated pleomorphic sarcoma showing aberrant expression of Melan‐A were identified. Clinical data were obtained and histopathological features, immunohistochemical profile and electron microscopy were assessed. Results: All tumors arose on sun‐damaged skin of elderly males. Two AFX showed pushing growth into superficial subcutis only. The undifferentiated pleomorphic sarcoma was characterized by infiltrative growth into galea as well as perineural invasion. Multifocal expression of Melan‐A and MART‐1 was largely limited to tumor giant cells in the absence of S100 or HMB‐45 labeling. No melanosomes or premelanosomes were identified by electron microscopy. Conclusions: Aberrant expression of Melan‐A and MART‐1 in AFX and undifferentiated pleomorphic sarcoma of the skin represents an important diagnostic pitfall with potential for misdiagnosis as melanoma. Melan‐A expression limited to tumor giant cells, in the absence of S‐100 positivity, is a helpful diagnostic feature. Thum C, Hollowood K, Birch J, Goodlad JR, Brenn T. Aberrant Melan‐A expression in atypical fibroxanthoma and undifferentiated pleomorphic sarcoma of the skin.