Silent but Distinct: Rare Adenoid Cystic Carcinoma of the Breast.

Adenoid cystic carcinoma (ACC) of the breast is an exceptionally rare malignancy, representing less than 0.1% of all breast cancers. Although it often demonstrates a triple-negative immunophenotype, its clinical course is typically more indolent than conventional triple-negative breast carcinomas. We report the case of a 64-year-old woman who presented with spontaneous nipple discharge from the left breast. Imaging revealed a suspicious lesion (Breast Imaging-Reporting and Data System (BI-RADS 5), leading to a core needle biopsy and tumorectomy, which identified a 2 cm invasive carcinoma of no special type. Completion mastectomy with axillary lymph node dissection revealed an 18 mm residual grade 1 tumor with clear margins and no nodal involvement. Immunohistochemistry confirmed a triple-negative profile, and staging CT scans showed no evidence of distant metastasis. The patient subsequently underwent adjuvant radiotherapy to the left chest wall. Given these findings, including complete surgical resection, absence of nodal involvement, and lack of distant metastasis, the clinical course is consistent with the favorable prognosis typically observed in breast ACC. This case highlights the diagnostic challenges, distinctive pathological features, and favorable outcomes associated with this rare entity, emphasizing the importance of accurate histopathologic evaluation and appropriate surgical management in guiding long-term prognosis.