The presentation of brain metastases by breast cancer subtype and potential implications for surveillance brain MRIs.

e13101 Background: As systemic therapy improves, the prevalence of breast cancer brain metastases(BCBM) is increasing. Current National Comprehensive Cancer Network(NCCN) guidelines do not recommend breast cancer brain MRI surveillance unless suspicious central nervous system(CNS) symptoms are present. We sought to understand the presentation and outcomes of BCBM by subtype. Methods: Breast cancer patients who developed brain metastases(BM) at our institution between 2015 and 2025 were identified. Details of initial presentation, treatments delivered and overall survival(OS) were assessed. Kruskal-Wallis and Pearsons's chi-square tests were used to test differences between subtypes. OS was calculated from dates of initial BM diagnosis using the Kaplan-Meier method. The Cox proportional hazards model was used for multivariate analysis (MVA) to identify variables prognostic for OS. Results: A total of 426 patients were identified including 176(41%) hormone receptor (HR)+/HER2-, 133(31%) HER2+, and 117(27%) triple negative (TN). HER2+ patients were younger at initial presentation (HER2+ median age: 55, TN: 56, HR+/HER2-: 57, p = 0.03). TN patients had the shortest interval from stage IV diagnosis to BM diagnosis (TN median: 10.4 months, HER2+:12.6 months, and HR+/HER2-: 16 months, p = 0.0005) and cancer diagnosis to BM diagnosis (TN median: 26.8 months, HER2+: 27.8 months, and HR+/HER2-: 54.7 months, p 20 BM at diagnosis (HER2+:21%, TN:11%, HR+/HER2-:10%, p = 0.04). Median OS following BM diagnosis was 38.1, 11.6 and 9.1 months, p 20 BM(p = 0.0013), > 2 lines of systemic therapy (p 20 brain metastases, leptomeningeal disease, and symptomatic presentation predicted for worse OS. These findings potentially support early brain MRI surveillance to facilitate detection.