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e14008 Background: We analyzed the treatment outcome of breast cancer patients with brain metastases (BM) in Korea to identify the prognostic factors and the role of whole brain radiation therapy (WBRT). Methods: Seven hundred thirty patients of breast cancer with BM treated at 17 institutions in Korea from 2000 to 2014 were analyzed. The median follow-up duration was 12 months. The analysis consisted of three cohorts: in cohort A, a total of 730 patients were included; in cohort B, 538 patients with available follow-up imaging after initial brain-directed treatment; and in cohort C, 54 patients receiving salvage WBRT due to recurrent BM after initial Stereotactic radiosurgery or WBRT. Overall survival (OS) was calculated from BM diagnosis in cohort A or from the last day of salvage WBRT in cohort C. Results: Median OS of cohort A was 15 months. In multivariate analysis, histologic grade 3, extracranial metastasis, number of BM > 4, hormone receptor (HR) or HER2 negativity, and shorter time interval to diagnosis of BM were associated with inferior OS. Among 538 patients in cohort B, 201 showed subsequent development of new BM at a median of 11 months after stereotactic radiosurgery or WBRT for the management of initial BM (at 1 year, HR+/HER2- 51.9%, HER2+ 44.0%, and TNBC 69.6%, respectively; p = 0.008). Upfront WBRT reduced subsequent development of new BM, which showed the significant difference among molecular subtypes (HR+/HER2-, 42% reduction at 1 year, p < 0.001; HER2+, 18.5%, p = 0.004; TNBC, 16.9%, p = 0.071). Multivariate analysis showed that shorter time interval to BM, TNBC subtype, extracranial systemic disease, number of BM > 4, and involvement of both tentoria increased subsequent development of new BM. Anti-HER2 therapy for HER2+ patients and upfront WBRT significantly reduced risk of new BM. In cohort C, upfront WBRT prolonged the salvage WBRT-free duration (median 6.9 vs. 8.7 months, p = 0.058). Median OS was 6.8 months after salvage WBRT. Longer interval to salvage WBRT, controlled primary tumor, high dose of salvage WBRT (BED10 > 37.5 Gy), and systemic treatment after salvage WBRT showed better OS. Uncontrolled extracranial systemic disease and salvage WBRT due to local progression without distant intracranial failure showed worse OS. Conclusions: The rates of new BM showed the significant differences among molecular subtypes. Upfront WBRT decreased subsequent development of new BM and this effect was dependent on the molecular subtype as well. Anti-HER2 therapy for HER2+ patients significantly decreased the subsequent development of new BM. On salvage WBRT setting, the patients having high dose of salvage WBRT, stable extracranial systemic disease and subsequent systemic therapy showed better OS.