Triple-Negative Accessory Breast Cancer Associated with a Germline BRCA1 Mutation: A Case Report

Abstract

Background: Accessory breast carcinoma is a rare malignancy arising from ectopic mammary tissue along the embryologic milk line, most frequently located in the axillary region. Due to its atypical anatomical presentation, diagnosis may be delayed or mistaken for benign axillary lesions or metastatic lymphadenopathy. However, the molecular characteristics and hereditary predisposition patterns of accessory breast cancer remain poorly understood.

Case Presentation: A 36-year-old postpartum woman presented with a progressively enlarging right axillary mass. Imaging demonstrated a lesion arising from accessory breast tissue without evidence of malignancy in the orthotopic breasts. Core needle biopsy revealed invasive ductal carcinoma of no special type with a triple-negative phenotype (estrogen receptor-negative, progesterone receptor-negative, and HER2-negative) and a Ki-67 index of 70%. The patient received neoadjuvant dose-dense anthracycline-based chemotherapy followed by paclitaxel and carboplatin, achieving a near-pathologic complete response after surgical excision and sentinel lymph node biopsy. Adjuvant radiotherapy was subsequently administered. Germline genetic testing identified a pathogenic BRCA1 mutation. At 24 months of follow-up, the patient remains clinically and radiologically disease-free.

Conclusion: Triple-negative carcinoma arising from accessory breast tissue is a rare presentation that may be associated with hereditary breast cancer predisposition. This case highlights the potential value of germline evaluation in young patients with triple-negative accessory breast carcinoma and emphasizes the role of molecular characterization in individualized clinical management.