A case report of synchronous bilateral breast cancer with distinct histological subtypes and favorable long-term survival.

Bilateral synchronous breast cancer accounts for a small proportion of all breast cancer cases, and those with markedly different pathological and molecular subtypes are extremely rare. Such cases pose substantial challenges in accurate diagnosis, treatment decision-making, and prognostic evaluation. Here, we present a 51-year-old postmenopausal female patient who presented with an incidental painless left breast mass. Imaging examinations revealed multiple bilateral breast lesions classified as BI-RADS 4A, 4B, and 4C. All clinical and imaging data were reviewed by the multidisciplinary tumor board to develop personalized treatment regimens. Intraoperative frozen pathology verified bilateral breast malignancy, and the patient received bilateral mastectomy. Postoperative pathological examination confirmed two independent primary breast cancers: the right breast lesion was invasive breast carcinoma with apocrine differentiation showing a triple-negative immunophenotype, while the left breast contained multifocal mucinous carcinoma with a luminal-like immunophenotype. Subsequent adjuvant therapy included six cycles of TAC chemotherapy (liposomal doxorubicin, liposomal paclitaxel, and cyclophosphamide), followed by eight cycles of capecitabine maintenance therapy and long-term adjuvant endocrine therapy with exemestane. At 31 months postoperatively, the patient remained disease-free with no signs of local recurrence or distant metastasis. This case highlights the importance of comprehensive imaging evaluation, thorough pathological examination, accurate molecular subtyping, and individualized multidisciplinary treatment for patients with synchronous bilateral breast cancer of heterogeneous subtypes.