Primary acinic cell carcinoma of the breast: insights from a nine-case series and comprehensive literature review.

PURPOSE: To better understand primary acinic cell carcinoma (AcCC) of the breast, this study investigated its clinical and pathological features. METHODS: Nine cases of primary acinic cell carcinoma were thoroughly evaluated using routine pathological and immunohistochemical methods. The clinical features, pathological morphological features, and prognosis of each case were collected and analyzed. RESULTS: The tumors were located in the left breast in four cases (44.4%) and the right breast in five (55.6%). Most lesions (88.9%, 8/9) were in the upper outer quadrant, with one (11.1%) in the lower outer quadrant. One case (11.1%) was categorized as BI-RADS 6, and eight (88.9%) as BI-RADS 4. Histologically, the cases exhibited diverse growth patterns. The predominant patterns were microglandular and acinar. Less common patterns included microcystic, cystic, follicular, clear cell, and solid structures. Eosinophilic secretions were visible within the lumens.The tumor cells showed cytoplasmic variations (granular, clear, or basophilic) and prominent nucleoli. Immunohistochemically, all tumors were negative for estrogen receptor (ER) and androgen receptor (AR). Progesterone receptor (PR) was weakly positive in two cases and negative in the remaining seven. Human Epidermal Growth Factor Receptor 2 (HER2) immunohistochemistry was negative in five cases and scored 1 + in four. Alpha-1-antichymotrypsin (AACT) was positive in four cases, while lysozyme was positive in eight. Collagen IV was negative in all cases, whereas epithelial membrane antigen (EMA) was diffusely positive. S100 protein expression was detected in seven cases. Notably, all cases exhibited an mutant-type p53 expression pattern, and the Ki-67 proliferation index ranged from 5% to 80%. CONCLUSION: AcCC of the breast is a rare, triple-negative malignancy that typically presents as a BI-RADS 4 lesion in the upper outer quadrant. Its diagnosis is based on characteristic morphological features-primarily microglands and acinar patterns, along with less common cystic, follicular, clear cell, and solid architectures. This is supported by a distinctive immunohistochemical profile, which consistently includes mutant-type expression of p53 protein. A comprehensive understanding of the clinicopathological and immunophenotypic spectrum of AcCC is crucial for accurate diagnosis and differential diagnosis. Enhanced recognition of this entity, particularly its rare morphological variants, will help reduce diagnostic errors and guide more precise clinical management, ultimately improving prognostic assessment for affected patients.