Synchronous Multicentric Breast Cancer With Opposite Molecular Phenotypes: A Case Report and Therapeutic Considerations.

BACKGROUND: Multifocal and multicentric breast cancers are increasingly diagnosed due to advances in imaging. Although some degree of molecular heterogeneity between tumor foci is recognized, most multicentric tumors share similar biological profiles. The presence of synchronous multicentric breast cancer with completely opposing molecular phenotypes remains exceptional. CASE PRESENTATION: We report the case of a 67-year-old woman diagnosed with synchronous multicentric breast cancer of the left breast. One tumor focus showed a HER2-enriched phenotype (ER-negative, PR-negative, HER2-amplified, and Ki67 60%), whereas a second spatially distinct focus (both confirmed on the surgical specimen) exhibited a luminal A phenotype (ER 100%, PR 1%, HER2-negative, and Ki67 15%). Both lesions were confirmed by magnetic resonance imaging and independently biopsied. The patient underwent modified radical mastectomy and received adjuvant anti-HER2 targeted therapy and endocrine therapy. At 22 months of follow-up, she remains disease-free. CONCLUSION: This case highlights the importance of comprehensive imaging assessment and independent molecular characterization of all suspicious tumor foci in multicentric breast cancer, as molecular discordance may have significant prognostic and therapeutic implications.