Intertumoral Heterogeneity in Multifocal Breast Cancer Mimicking a Collision Tumor on Imaging: A Case Report.

Breast cancer exhibits heterogeneity characterized by intertumor heterogeneity, where distinct lesions present different subtypes, and intratumor heterogeneity, where a single tumor evolves over time. We present a unique case of synchronous, adjacent multifocal breast cancers demonstrating histological findings of distinct molecular subtypes. A 72-year-old woman with a history of ovarian cancer and a family history of breast cancer presented with a right breast lump. Imaging showed two contiguous but distinct lesions. Core needle biopsy identified invasive ductal carcinoma. Immunohistochemistry revealed luminal B and luminal B/human epidermal growth factor receptor 2 (HER2)-positive subtypes. Total mastectomy and sentinel lymph node biopsy were performed, and pathology confirmed two partially fused tumors separated by fibrous stroma. The HER2-positive component demonstrated higher proliferative activity and nuclear grade. Postoperatively, the patient received chemotherapy, anti-HER2 therapy, and remains disease-free on endocrine therapy. This case highlights a rare breast cancer presentation with identical histology but different molecular subtypes mimicking a collision tumor on imaging. It underscores the clinical relevance of tumor heterogeneity and the importance of combining imaging, pathology, and molecular profiling for accurate diagnosis and tailored treatment.