A Case Report of Synchronous Multicentric Breast Carcinoma With Biologically Discordant Phenotypes: Luminal A and Triple-Negative Subtypes.

We present the case of a 45-year-old Caucasian woman diagnosed with synchronous bicentric breast cancer of differing molecular phenotypes in the same breast. The first tumor, an invasive ductal carcinoma (G1), was estrogen and progesterone receptor-positive and HER2-negative, with a low proliferative index (Ki67 10%). A second lesion, located in a different quadrant and appearing within weeks after biopsy, exhibited a triple-negative phenotype and a higher proliferative index (Ki67 30%). Both tumors were classified as stage IA. Due to the multicentric nature of disease, the patient underwent mastectomy with sentinel node biopsy and immediate reconstruction. Genetic testing was negative for BRCA1/2, CHEK2, and PALB2 mutations, though familial cancer history suggested a possible hereditary syndrome (FCC suspicion). This case underscores the complexity of managing multicentric breast cancer with discordant receptor status and raises questions about the role of biopsy sites in tumor development.