Synchronous Bilateral Breast Invasive Ductal Carcinoma With Osteoclast-Like Stromal Giant Cells in a 44-Year-Old Woman: A Case Report.

Breast carcinoma with osteoclast-like stromal giant cells (OCGC) is a rare histological variant of invasive breast carcinoma. While it typically presents as a unilateral disease, its clinicopathological significance and the mechanisms underlying its formation remain incompletely understood. To our knowledge, a case of synchronous bilateral primary invasive ductal carcinoma with OCGCs has not been previously reported. A 44-year-old premenopausal woman was diagnosed with synchronous bilateral invasive ductal carcinoma with OCGCs. Imaging revealed small masses in both breasts, and core needle biopsies demonstrated invasive carcinoma with associated non-invasive components and numerous OCGCs in the tumor stroma. Both tumors were hormone receptor-positive and human epidermal growth factor receptor 2 (HER2)-negative (luminal A-like). The patient underwent bilateral mastectomy and sentinel lymph node biopsy, which showed no lymph node metastasis. The postoperative course was uneventful, and the patient has remained recurrence-free for six months after surgery. Notably, final pathological examination confirmed independent ductal carcinoma in situ (DCIS) components with OCGCs in both breasts, supporting the diagnosis of bilateral primary tumors rather than metastatic disease. Immunohistochemically, the OCGCs were CD68-positive, confirming their macrophage lineage. Genetic testing showed no pathogenic BRCA1/2 variants. This report highlights an extremely rare, pathologically confirmed case of synchronous bilateral primary breast carcinoma with OCGCs. The presence of OCGCs in both invasive and in situ components suggests that their formation can be induced at early stages of tumor development, likely reflecting a distinct immune-reactive tumor microenvironment driven by host-related factors.