Merkel cell carcinoma, melanoma, metastatic mimics of breast cancer

Merkel cell carcinoma and melanoma can each occur primarily in breast skin, or metastasize to the breast. The breast is a rare site of metastasis of essentially any and every type of tumor, including carcinomas, sarcomas, and hematolymphoid neoplasms, and 10–30% of breast metastases may represent the initial presentation of disease. Although metastases generally recapitulate histologic features of the primary tumor, they are diagnostically challenging given their rarity and morphologic overlap with breast carcinoma, including special types of breast cancer. Histologic clues may include lack of carcinoma in situ, lack of central elastosis, pattern of infiltration around normal breast structures, yet none of these are specific. Careful correlation with clinical history and judicious use of immunostain panels is essential in approaching these cases.