[A Case of Primary Acinic Cell Carcinoma of the Breast].

Acinic cell carcinoma(ACC)is an invasive malignancy primarily characterized by proliferation of tumor cells that resemble acinar cells of the salivary glands and pancreas. ACC of the mammary glands is rare. We report a case of primary ACC of the breast. Two masses were revealed in the left mammary gland of a 57-year-old woman who visited our hospital through screening mammography. The lesions were identified as synchronous multiple breast carcinoma of 2 different histological types; ACC and tubulolobular carcinoma. For treatment, left mastectomy and sentinel lymph node biopsy were performed, followed by postoperative chemotherapy and endocrine therapy. Hematoxylin-eosin staining of ACC revealed abundant acinar- like structures formed by tumor cells with prominent eosinophilic granules in the cytoplasm. Immunostaining was positive for S-100 protein, α1-antichymotrypsin, α1-antitrypsin, and lysozyme. The tumor cells were negative for estrogen, progesterone, and HER2 receptors, which indicated that they had a triple-negative phenotype. Although primary ACC of the breast is regarded as low-grade triple-negative breast carcinoma with a favorable prognosis, further accumulation of cases may be needed to elucidate the biological features of ACC and investigate appropriate therapeutic strategies.