A case report of synchronous primary bilateral breast cancer with different molecular types

Background: The incidence of synchronous bilateral breast cancer is extremely low, and the majority of patients present with identical immunohistochemical (IHC) subtypes in both breasts. Cases with discordant IHC subtypes are relatively rare, posing significant diagnostic and therapeutic challenges. It is useful to report every case in order to establish treatment algorithms. Case presentation: In this report, we presented a74-year-old female with a history of asthma and endometrial carcinoma who diagnosed with T1N0M0 ER+, PR+, HER2- invasive ductal carcinoma in left breast and T1N3M0 TNBC invasive lobular carcinoma, for which she underwent left breast-conserving surgery with sentinel lymph node biopsy and right modified radical mastectomy with completion axillary lymph node dissection. A curative-intent treatment plan has been formulated, consisting of four cycles of doxorubicin/cyclophosphamide followed by paclitaxel (A: 80 mg/m², C: 600 mg/m² - T: 100 mg/m²) chemotherapy. Subsequently, the patient will be subjected to bilateral surgical site radiotherapy, followed by one year of low-dose capecitabine consolidation therapy and at least five years of anastrozole endocrine therapy. Conclusions: The diagnosis of synchronous primary bilateral breast cancer must be based on definitive pathological findings. Molecular subtype discordance in synchronous primary bilateral breast cancer poses unique therapeutic challenges, and no unified treatment standard currently exists. Therefore, treatment decisions require individualized consideration.