P11.26.A IS BREAST CANCER HISTOLOGICAL SUBTYPE CORRELATED WITH OUTCOMES IN STEREOTACTIC RADIOSURGERY FOR BRAIN METASTASES?

Abstract BACKGROUND Breast cancer is the most common form of cancer worldwide with an estimated incidence of 2.26 million per year. Brain metastasis affects 10-16 % of these. Stereotactic radiosurgery (SRS) is frequently used to provide local control of brain metastases. It is important to review this in the context of tumour biology and evolving systemic therapies which may have CNS activity. MATERIAL AND METHODS Retrospective single centre observational study of breast cancer patients with brain metastases treated with SRS between January 2020 and June 2024. Overall survival (OS) was calculated from the start of SRS to death. Progression free survival was calculated from the start of each individual treatment to CNS progression on imaging (PFS-CNS). Survival analysis was conducted using Kaplan Meier plots and multivariate analysis (MVA) in the “R” statistical language. RESULTS 85 patients were treated of which 72 were included for analysis due to missing data. There were 81 episodes of SRS treatment. KPS in all patients was >70. Stratification by Luminal A (n=11), Luminal B (n=13), HER2 positive (n=29), and Basal (n=19). Median overall survival was 26.0 months (95%CI: 13.7-32.6). PFS-CNS was 16.2 months (95%CI: 11.3-33.0) in all patients. There was a statistically significant difference in OS dependent on tumour histology with basal type performing the worst (p= 0.047; Basal type 13.7 months, 95%CI: 4.1-n/a; Her2 33.5, 95%CI: 25.1-n/a) There was no difference in CNS-PFS between histological groups (p=0.88). MVA showed that KPS (HR0.60, p=0.002) and receiving systemic therapy (SACT) after SRS (HR0.25, p<0.001) are associated with improved OS. In addition, receiving SACT after SRS is associated with improved PFS-CNS on MVA (HR 0.21, p<0.001) CONCLUSION SRS is an effective intervention for local disease control regardless of breast cancer histological subtype. Receiving SACT after SRS is associated with both improved local control and survival. OS differences between subgroups are likely due to factors such as biological variations, patient selection, and the availability of CNS-penetrant/active SACT. This data is consistent with other published work. Further work into the scheduling of SRS in the context of CNS penetrant/effective SACT is required for optimal patient care.