Mammary small cell carcinoma (SmCC) is a very rare neoplasm with a poor prognosis compared with other invasive carcinomas. We studied the histological and immunohistochemical profiles of two cases of mammary SmCC, and compared them with those of five cases of carcinoma with endocrine features (CEF) and five cases of invasive ductal carcinoma (IDC), to elucidate the correct diagnosis of mammary SmCC. Immunohistochemical analysis was performed with antibodies against cytokeratins (CKAE1/AE3, CK34βE12, CKCAM5.2, CK7, CK8, CK19, CK20), epithelial membrane antigen (EMA), vimentin, CD10, neural cell adhesion molecule (NCAM; CD56), neuron-specific enolase (NSE), chromogranin A, S-100 protein, carcino-embryonic antigen (CEA), E-cadherin, N-cadherin, thyroid transcription factor-1 (TTF-1), p53, estrogen (ER), progesterone (PR), HER2/neu, bcl-2, synaptophysin, calcitonin and Leu7. SmCCs were diffusely and strongly positive for NCAM in comparison with CEFs and IDCs. SmCCs were negative for vimentin, whereas CEFs and IDCs were positive. Neuro-endocrine carcinomas, including SmCCs and CEFs, were diffusely and strongly positive for NSE, compared with IDCs. Moreover, neuroendocrine carcinomas were negative for CK34βE12, CK20 and CD10, whereas IDCs were positive. Our study suggests that NCAM and vimentin are useful markers for the diagnosis of mammary SmCC. CK34βE12, NSE, CD10, CK20 and chromogranin A appear to be useful for differentiating neuroendocrine carcinoma from IDCs.

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