time of analysis there were 185 DFS events; 99 in the C arm and 86 in the H arm. The estimated 7-year DFS is 75.6% (95% confidence interval [CI] 70.3-80.1) in the C arm and 81.8% (95% CI 77.5-85.3) in the H arm. The DFS HR (C/H) is 0.85 (95% CI 0.64-1.14). Comparison of biochemical recurrence (HR Z 0.77, 95% CI 0.51-1.17) and overall survival (HR Z 0.95, 95% CI 0.65-1.41) also met protocol noninferiority criteria. Grade 3 gastrointestinal toxicity is 3.0% (C) versus 4.6% (H), relative risk (RR) for H versus C 1.53, (95% CI 0.86-2.83); grade 3 genitourinary toxicity is 4.5% (C) versus 6.4% (H), RR Z 1.43 (95% CI 0.86-2.37). Conclusion: In men with low-risk prostate cancer, 70 Gy in 28 fractions over 5.6 weeks is noninferior to 73.8 Gy in 41 fractions over 8.2 weeks. (ClinicalTrials.gov identifier: NCT00331773.) Acknowledgment(s): This project was supported by grants U10CA21661 (RTOG-Ops-Stat), U10CA37422 (CCOP), CA81647 (ATC), U10CA 180868 (NRG Oncology Operations), and U10CA180822 (NRG Oncology SDMC) from the National Cancer Institute (NCI).