Neuroendocrine carcinoma of the breast with Merkel cell carcinoma-like features

A case of neuroendocrine carcinoma of the breast with Merkel cell carcinoma-like features occurring in a 38-year-old female is described. A 2.5 x 2.5 x 1.3 cm tumor in the upper-outer quadrant of the right breast was ill-defined, yellowish white and rubbery. Microscopically, the tumor was characterized by a diffuse proliferation of non-cohesive round cells. The tumor cells had round nuclei with finely dispersed chromatin and inconspicuous nucleoli. The cytoplasm was generally scant, pale to eosinophilic, and argyrophilic. There were scattered foci of small nests or intraductal proliferations. Immunohistochemically, many tumor cells showed positive reaction for CAM5.2, vimentin, chromogranin A, and synaptophysin. The tumor was also positive for neuron-specific enolase (NSE), Leu-7, calcitonin, S-100 protein, and glial fibrillary acidic protein. Flow cytometric analysis showed that the tumor was DNA diploid. An axillary lymph node was positive for metastasis. The patient was well, without disease, for 6 years after mastectomy. It is suggested that the tumor might originate from ductal cells and show Merkel cell carcinoma-like neuroendocrine differentiation.