Supplementary Material for: A Rare Case of Adenoid Cystic Breast Cancer: A Case Report and Literature Review.

Introduction: Adenoid cystic carcinoma of the breast (AdCC) is an exceptionally rare malignancy, representing less than 0.1% of all breast cancers. Despite its triple-negative receptor profile, it typically follows an indolent course with excellent prognosis, unlike conventional triple-negative breast cancer (TNBC). Case Presentation: We report the case of a 48-year-old woman with a significant familial history of cancer who was diagnosed with breast AdCC. Imaging revealed a suspicious lesion in the left breast, and biopsy confirmed AdCC. Surgical resection showed a 58 mm, pT3, pN0(sn) triple-negative tumor with a low proliferative index. Genetic testing revealed a BRIP1 variant of uncertain significance (VUS). No adjuvant chemotherapy was given, and the patient remains in remission under surveillance. Discussion: This case highlights the diagnostic challenges of AdCC, including its resemblance to benign lesions on imaging and its overlap with TNBC histologically. While most AdCC cases are sporadic, the presence of a BRIP1 VUS in a patient with strong family history raises questions about possible hereditary implications. Management was based on current best practices emphasizing surgical resection and selective use of radiotherapy. Conclusion: Breast AdCC, though rare, requires heightened clinical awareness for accurate diagnosis and management. This case underscores the potential role of genetic evaluation in rare breast cancer subtypes and the importance of continued case reporting to guide future recommendations.