Real-world insights into bone marrow carcinomatosis in metastatic breast cancer: a retrospective analysis from two large university breast cancer centers

PURPOSE: Bone marrow carcinomatosis (BMC) is a rare but clinically relevant manifestation of metastatic breast cancer (mBC), characterized by diffuse marrow infiltration and hematologic dysfunction. Evidence on epidemiology, clinical presentation, prognosis, and optimal treatment is limited. We performed a bicentric retrospective real-world analysis of patients diagnosed with BMC, representing the largest reported European cohort to date. METHODS: We retrospectively analyzed 37 patients diagnosed with BMC at two German university breast cancer centers between 2010 and 2023. Clinical characteristics, hematologic abnormalities, metastatic patterns, treatments, and overall survival (OS) were evaluated. Incidence estimates were calculated based on institutional breast cancer and mBC populations. A systematic literature review contextualized the findings. RESULTS: The estimated incidence of BMC was 2.6% among patients with mBC and 0.5% among all breast cancer cases. Most patients had HR-positive/HER2-negative disease (89%). Hematologic abnormalities were frequent, including anemia (84%), thrombocytopenia (76%), and pancytopenia (57%). Median OS after BMC diagnosis was 12 months (95% CI 6.0-38.0). Patients with bone-only metastases did not reach median OS, whereas those with additional metastatic sites had a median OS of 9 months (95% CI 5.0-12.0). Patients receiving endocrine therapy plus CDK4/6 inhibition showed numerically longer OS (34 months, 95% CI 5.0-NA) compared with chemotherapy alone (12 months, 95% CI 5.0-38.0). CONCLUSIONS: BMC remains a rare but challenging complication of mBC. Endocrine-based targeted therapy may improve outcomes in selected HR-positive/HER2-negative patients. Larger multicenter studies are needed to optimize diagnosis and treatment strategies.