Rare acinic cell breast cancer treated with mastectomy

What happened, in summary

This 34-year-old woman found a painless lump in the upper outer quadrant of her left breast. On examination, the hard mass measured about 30 mm, with normal overlying skin and nipple and no palpable axillary or supraclavicular nodes. Ultrasound showed an irregular hypoechoic mass classified as BI-RADS 4B, and enhanced breast MRI was BI-RADS 6. Core biopsy suggested a malignant small round cell tumor, described as undifferentiated or poorly differentiated carcinoma. Surgery was performed, and the detailed case text describes modified radical mastectomy with axillary lymph node excision; intraoperative sentinel nodes and surgical margins were negative. Final pathology showed a 27 mm breast acinic cell carcinoma with 2 components: a classical acinic component and a high-grade solid component with necrosis, pleomorphic nuclei, prominent nucleoli, and frequent mitoses. The tumor was ER-negative, PR-negative, HER2-negative, and had no mismatch repair protein deficiency. The high-grade component had Ki-67 of 95%, p53 staining around 80%, diffuse cyclin D1 and vimentin, and loss of E-cadherin membrane expression. Sequencing found shared alterations between the classical and high-grade components, including TP53, LMO1, MDC1, MSH3, KMT2D, and CCND3 mutations, plus copy-number gains including CCND1, FGFR2, MYC, and IDH1. No gene fusions or rearrangements were detected. She received 8 cycles of adjuvant epirubicin plus cyclophosphamide and radiotherapy. As of 43 months of follow-up, she was alive with no metastasis or recurrence. Because breast acinic cell carcinoma is rare and can resemble poorly differentiated carcinoma on biopsy, final diagnosis depended on detailed morphology, immunohistochemistry, and genomic comparison of the classical and high-grade tumor areas.