A Case of Multifocal Adenoid Cystic Carcinoma of the Breast: Using Immunohistochemical Diagnosis to Guide Conservative Management.

Adenoid cystic carcinoma (ACC) of the breast is exceptionally rare and can be misdiagnosed as a more aggressive triple-negative subtype. A 56-year-old woman presented with a painless lesion in the right breast. Imaging (Breast Imaging Reporting and Data System (BI-RADS) 5 mammography and ultrasound) identified four multifocal nodules in the upper outer quadrant at the 10 o'clock position, and core needle biopsies confirmed ACC via immunohistochemistry (negative estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2)/neu; positive CD117, cytokeratin 7 (CK7), calponin). She underwent bracket‑guided partial mastectomy with clear margins, followed by adjuvant radiotherapy. Histopathology revealed a 2.5 cm Nottingham Grade I ACC (pT2 pN0 M0) and a 6 mm focus of ductal carcinoma in situ. Despite receptor negativity, endocrine therapy was recommended. Precise immunoprofiling differentiates ACC from basal-like cancers, enabling conservative management. Our case highlights a rare multifocal presentation of ACC managed conservatively with a favorable short-term outcome. This case adds to growing evidence suggesting that, with accurate immunohistochemical diagnosis and multidisciplinary care, breast-conserving surgery may be a safe option in select patients with multifocal ACC. Further multicenter studies are needed to establish management guidelines.