In photon SRS, rates of radiation necrosis are approximately 5-10%, and grade 3 or greater toxicity can be seen in up to 40%1. The purpose of this study is to report a single institution’s experience using proton SRS. It was expected that clinical outcomes would be comparable to photon SRS. Between the years 2006 to 2017, 76 patients and 151 intact brain lesions were treated with 20-25 Gy in 1-2 fractions of radiosurgery using proton beam at one institution. A retrospective analysis was conducted of patient and treatment characteristics as well as failure patterns, toxicity and overall survival. Statistical analysis included Kaplan-Meier estimates of freedom from local failure, freedom from distant brain failure, and overall survival. The individual impact of patient and treatment factors on mortality and progression was determined using Cox proportional hazards regressions. The median follow-up from proton SRS was 6 months (interquartile range 2.9-14.4 months). Freedom from local failure, freedom from distant brain failure, and overall survival at 6 and 12 months was 88.3% (95% confidence interval [CI] 82.1%-94.9%) and 75.5% (95% CI 65.9%-86.7%), 76.7% (95% CI 66.5%-88.5%) and 67.9% (95% CI 55.5%-83.0%), and 54.8% (95% CI 44.7%-67.3%) and 33.7% (95% CI 24.4%-46.5%), respectively. Median overall survival was 6.4 months. There were no acute CTCAE grade 4 or 5 toxicities observed. Any acute toxicity occurred in 15.9% of SRS courses (grade 2 in 7.3% and grade 3 in 2.4%). There were eight cases of radiation necrosis (5.2%), three diagnosed pathologically and five based on imaging review. Factors found to predict for mortality were RPA class (p = 0.044), extent of extracranial disease (p = 0.009), and the presence of neurologic symptoms (p = 0.017). Factors found to predict for local failure were number of fractions (p = 0.0018), GTV diameter (p = 0.005), maximum dose (p = 0.0007), and conformity index (p = 0.035). Thus far one institutional series showing proton SRS to have comparable clinical outcomes to photon SRS for the treatment of brain metastases has been published2. Our study adds to the breadth of data on the issue. This technique can be performed safely, with similar failure patterns and survival rates as photon-based radiosurgery and with low rates of toxicity, including radiation necrosis. Due to proton beam resulting in lower integral dose to the brain, this technology may confer a clinical benefit in selected patients requiring a high number of SRS treatments and merits further study.