Thyroid transcription factor-1 and "basal marker"--expressing small cell carcinoma of the breast.

Small cell carcinoma of the breast is a very rare entity that is histologically indistinguishable from small cell carcinomas of other organs. The presence of an in situ component is the most important feature that indicates the primary nature of the breast tumor. Thyroid transcription factor-1 (TTF-1) is a marker specific to lung and thyroid but is also expressed in small cell carcinoma of pulmonary and extrapulmonary origin. TTF-1 expression in breast small cell carcinoma has been reported only rarely. This reported case is unique because of the characteristic morphological features and immunohistochemical profile. The invasive tumor demonstrated neuroendocrine differentiation morphologically and immunohistochemically, expressed by TTF-1, EGFR, and basal-type cytokeratins. An unequivocal in situ component was identified intimately admixed with the invasive carcinoma. To the authors' knowledge, this is the first report of breast small cell carcinoma showing the expression of "basal markers."