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

1104 Background: Bone marrow carcinomatosis (BMC) is a rare but severe manifestation of metastatic breast cancer (mBC) characterized by diffuse bone marrow infiltration and hematologic dysfunction. Data on incidence, clinical presentation, and treatment outcomes remain limited. Methods: This bicentric retrospective real-world study included breast cancer patients diagnosed with BMC between 2010 and 2023 at two German university centers. BMC was diagnosed using bone marrow biopsy and/or leukoerythroblastic blood smear in the presence of cytopenia. Clinical characteristics, systemic treatments, and overall survival (OS) were analyzed descriptively. Survival was estimated using Kaplan–Meier method. Results: During the study period, 1,399 patients presented with mBC at one of two participating centers. Among these, 37 were diagnosed with BMC, corresponding to an incidence of 2.6% (0.5% of all breast cancer cases). Median age was 58 years (range 30 – 76). Most patients had hormone receptor (HR)–positive HER2-negative disease (89%). At BMC diagnosis, anemia occurred in 84%, thrombocytopenia in 76%, and pancytopenia in 57% of patients. Median OS after BMC diagnosis was 12.0 months (95% CI: 6.0, 38.0). Patients with bone-only metastases showed longer survival compared with those with additional visceral involvement (median OS not reached vs. 9 months). In HR-positive HER2-negative disease, patients receiving endocrine-based therapy including CDK4/6 inhibitors demonstrated numerically longer survival compared with chemotherapy-based approaches; however, subgroup sizes were small and analyses exploratory. Conclusions: BMC is an uncommon but clinically relevant manifestation of mBC associated with substantial morbidity and limited prognosis. This largest European real-world cohort highlights heterogeneous treatment approaches and outcomes. Endocrine-based strategies combined with targeted agents appear feasible in selected patients. Multicenter registries and prospective studies are needed to optimize diagnostic and therapeutic strategies in this patient group. Real-world data of the analyzed cohort of patients with BMC. No of patients IHC subtype n (%) Metastatic status at initial BC presentation n (%) Metastatic site at BMC diagnosisn (%) Diagnostic method n (%) Median time to BMC in months (range) Initial treatmentfor BMCn (%) a) HR pos/HER2negb) HER2 posc) TNBC a) M0b) M1 a) visceralb) brainc) bone +/- otherd) bone only a) BM biopsy / smearb) blood smear a) after mBC diagnosisb) after initial BC diagnosis a) combined CTb) mono CTc) ET+CDK4/6id) ET onlye) Anti HER2f) BSC 37 a) 33 (89)b) 4 (11)c) 0 (0) a) 16 (43)b) 21 (57) a) 22 (60)b) 1 (3)c) 37 (100)d) 11 (30) a) 26 (70)b) 11 (30) a) 28 (0-107)b) 58 (0-249) a) 3 (8)b) 20 (54)c) 6 (16)d) 1 (3)e) 1 (3)f) 1 (3)Other: 5 (14)