Acinic Cell Carcinoma of the Breast: A Case Report and Review of Literature.

Acinic cell carcinoma (ACC) is an exceedingly rare type of triple-negative breast cancer (TNBC). We are reporting a case of a 46-year-old female patient who presented with a palpable lump in her left breast not associated with pain, pruritis, or change of skin color. An open biopsy revealed a mass of about 20 x 25 mm of fleshy, white tan with a lobular configuration and necrosis. The histopathological examination revealed cells with cytoplasmic granularity arranged in a microglandular pattern and a solid pattern, and the case was initially reported as ACC. The most remarkable feature was the presence of small and large, brightly eosinophilic cytoplasmic granules, and some cells are clear or multivacuolated, resembling lipoblasts. Cellular pleomorphism and anaplasia are very mild, and the mitotic activity was very low. The tumor showed a scant and vascularized stroma in the area of hyalinization. Small clusters of lymphoid infiltration in the stroma were seen. Histochemical stains revealed that the acinar cells in ACC contain abundant diastase-resistant, periodic acid Schiff (PAS)-positive cytoplasmic granules. Mucicarmine and Alcian blue were negative. The immunohistochemistry workup revealed that the case was positive for discovered on gastrointestinal stromal tumors-1 (DOG-1) and the positivity pattern ranged from apical membranous, cytoplasmic, and complete membranous. In addition, the tumor cells were positive for low-molecular-weight cytokeratin, carcinoembryonic antigen (CEA), and epithelial membrane antigen (EMA). The FISH workup for the ETV6-NTRK3 fusion was negative, arguing against secretory carcinoma (SC). A diagnosis of acinar cell carcinoma of the breast is very rare, and the presence of cytoplasmic granules is helpful for its diagnosis. In the absence of these granules, the diagnosis is very difficult, and other diagnoses will be put in the differential diagnosis, particularly SC, lactating adenosis, and microglandular adenosis. Immunohistochemical and histochemical stains and genetic workups will support the diagnosis of ACC.