Melanoma masquerading as xanthogranuloma: a diagnostic pitfall.

Melanoma can present with protean combinations and permutations of histologic features mimicking a plethora of nonmelanocytic benign and malignant proliferations. Anecdotal cases of melanoma closely simulating fibrohistiocytic proliferations have been reported. At times, the reliable differentiation between melanoma and histiocytic proliferations could be vexing histopathologically. We report an unusual presentation of melanoma in an 87-year-old man strikingly resembling xanthogranuloma both clinically and histopathologically. Histologic sections revealed a diffuse proliferation of pleomorphic cells some with foamy cytoplasm and occasional Touton-like giant cells in the dermis accompanied by inflammatory cells. Rare single-cell pagetoid scatter was evident within the epidermis. The infiltrate had patchy staining on CD163, interpreted as part of the inflammatory component but the atypical cells stained heavily with Melan A and tyrosinase confirming the diagnosis of malignant melanoma. Our case demonstrates yet another face of malignant melanoma and the critical but judicious use of immunohistochemistry in reliably distinguishing between melanoma and histiocytic tumors.