Acinic cell carcinoma of the breast is an uncommon neoplasm. Since the first case of this rare variant of breast carcinoma was reported in 1996, only 10 cases have been reported in the English-language literature. Reported herein is the first case of primary acinic cell carcinoma of the breast in a Japanese woman. To the naked eye, the tumor appeared well circumscribed and the cut surface was grayish-pink and hemorrhaging. Microscopically, the tumor was predominantly made up of a monotonous proliferation of cells with a finely granular cytoplasm, resembling acinic cells of the parotid gland. Some neoplastic cells had a clear cytoplasm. In spite of extensive sampling, no common histological patterns of breast carcinoma such as in situ and invasive ductal carcinoma were recognized in the present case, indicating that the present case was pure acinic cell carcinoma. In addition, the immunohistochemical profile of this tumor was identical to that of the acinic cell carcinoma of the salivary gland: estrogen receptor, progesterone receptor, HER2 and cytokeratin (CK)20 were negative and amylase and CK7 were positive. The patient has been well for 22 months since the wide local excision of the tumor and no signs of salivary neoplasm are evident to date.

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