RESUMO
Primary small cell carcinoma (SmCC) of the breast is very rare and may be difficult to distinguish from metastatic small cell carcinoma. Confident histopathological diagnosis of a primary breast SmCC requires the demonstration of an in situ component. We report a case of primary small cell carcinoma of the breast with coexisting carcinoma in situ in which the invasive carcinoma and in situ component both expressed neuroendocrine markers and Thyroid transcription factor-1 (TTF-1) by immunohistochemistry. Expression of neuroendocrine markers and TTF-1 in the in situ component allowed a highly confident diagnosis of primary small cell carcinoma of the breast. To our knowledge there is only one previous report of TTF-1 expressing in situ carcinoma associated with a primary SmCC of the breast.