Synchronous bilateral breast cancer with discordant immunohistochemical profiles: A case series and therapeutic implications

Background: Bilateral breast cancer (BBC) with discordant immunohistochemical (IHC) profiles is uncommon but presents significant challenges in diagnosis and treatment. When tumors in each breast exhibit distinct receptor status—such as triple-negative in one and hormone receptor-positive or HER2-positive in the other—treatment decisions become complex. Methods: We retrospectively analyzed five patients diagnosed between 2020 and 2024 with synchronous bilateral breast cancer exhibiting discordant IHC profiles. Clinicopathological characteristics, treatment regimens, and outcomes were reviewed. Results: All five patients presented with distinct molecular subtypes between the two breasts. Combinations included triple-negative and luminal A, HER2-positive and luminal B, and other discordant patterns. All were managed with personalized treatment plans addressing each tumor's biology. Treatment included combinations of chemotherapy, endocrine therapy, HER2-targeted therapy, and surgery. All patients completed treatment with no evidence of distant recurrence at the time of last follow-up. Conclusion: Discordant BBC highlights the importance of individualized treatment strategies and tumor board-based decision-making. Therapeutic planning should be guided by the most aggressive tumor phenotype, while ensuring comprehensive care for both malignancies. Keywords