Signet-ring cell melanoma: a potential diagnostic pitfall.

Malignant melanoma is commonly known as the great mimicker and can present in various clinical manifestations and with diverse morphological variants. One of the rare histological variants is the signet-ring cell type. The signet-ring morphology has been reported in numerous other neoplasms including adenocarcinoma, lymphoma, liposarcoma, squamous cell carcinoma, and basal cell carcinoma. We report a rare case of primary signet-ring cell malignant melanoma in a 62-year-old man. He initially presented with an enlarging nevus on his right flank with surrounding erythema. A biopsy showed atypical epithelioid and signet cells with prominent nucleoli and occasional mitoses. Initial diagnosis favored metastatic signet-cell carcinoma of gastrointestinal origin. Review of the biopsy and immunohistochemical analysis revealed the malignant signet-ring cells stained with S100, vimentin, and melanoma cocktail, in keeping with melanoma. The signet-cell morphology can be found in a variety of other malignancies. To prevent potential misdiagnoses, thorough histological examination should be aided by an appropriate immunohistochemical panel to confirm melanoma and exclude erroneous differentials.