Background/Purpose ATRT is a rare and aggressive CNS tumor usually presenting in very young children (age less than 5 years). Aggressive treatments have improved outcomes. Such strategies have included radiation therapy as studies have demonstrated the benefit of instituting radiation early in the treatment course. However, at such a young age, short and long term radiation toxicities can be devastating. We prospectively enrolled pediatric CNS ATRT patients onto the Proton Collaborative Group registry protocol to evaluate the efficacy and toxicities of proton radiation therapy in this population. Prospective Study on Pediatric Patients with Atypical Teratoid Rhabdoid Tumors (ATRT) of the Central Nervous System (CNS) John Han-Chih Chang 1, Michael Confer 2, Andrew Chang 2, Stewart Goldman 3, Megan Dunn 1, William Hartsell 1. 1 CDH Proton Center, 2 Procure Proton Therapy Center – Oklahoma City, 3 Anne and Robert H. Lurie Children’s Hospital of Chicago. Materials and Methods Twelve consecutive pediatric ATRT patients were treated with at the Central DuPage Hospital Proton Center (n = 7) and the Procure Proton Therapy Center – Oklahoma City (n = 5) between March 2010 – December 2013 utilizing 3D Conformal Proton Therapy. Eight patients were treated per the Dana- Farber Cancer Institute protocol approach, while 4 were treated either on or per Children’s Oncology Group Protocol ACNS Survival data was calculated from the date of diagnosis, while follow up interval was analyzed from last day of radiation therapy. Results All twelve patients were evaluable. They were all 3 years of age or younger (range 4.4 – 37.7 months). Seven patients had gross total resections, while 4 had subtotal resections along with another 1 not documented. All patients received multiagent intensive chemotherapy. Radiation was to local fields for 9 patients, while 3 had craniospinal irradiation. The median follow up was 14.2 months at our last update (range of 1 – 43 months). At last follow up 10 patients were alive without evidence of disease. The other 2 children died of progressive disease at 9.3 and 24.7 months from diagnosis. Median overall survival was 21 months (range of 4.9 – 49.2 months). All patients received the intended course of therapy to 50 – 54 Cobalt Gray Equivalent (CGE). Proton therapy was able to reduce the dose to the cochlea, optic chiasm, hippocampus, and integral whole brain dose relative to historic controls. Only 4 children had grade 3 toxicities during radiation therapy (all were acute nausea, vomiting and anorexia that responded to steroids). Conclusion The initial results on the largest prospective series of CNS ATRT patients treated with proton therapy seem to be favorable. The aggressive treatment regimens utilizing proton beam therapy yield proven efficacy and improved acute toxicity profiles, which is critically important in this young patient population with such an aggressive disease. We look forward to continuing patient treatments and follow up on long term quality of life measures. Figures A & B: Proton Treatment Plan for a 2 year old with Left Frontal ATRT Figures C & D: Proton Treatment Plan for a 2 year old with 4 th Ventricular ATRT A D C B Table 1: Doses to Target Volume and Critical OAR