6 Background: Stereotactic body radiotherapy (SBRT) for the treatment of prostate cancer is rapidly increasing. Reported regimens differ in time, dose, and fractionation. We report the results of a multicentre, Canadian randomized phase II study to investigate the impact of overall treatment time on quality of life (QOL) and toxicity. Methods: Men with cT1-2b, Gleason <=7, and PSA <= 20 ng/mL prostate cancer were randomly assigned to 40 Gy in 5 fractions delivered every other day (11 days) vs. once per week (29 days) using gantry-based SBRT. QOL was assessed using the Expanded Prostate Cancer Index Composite (EPIC) at baseline, weeks 2, 4, 6, 12, months 6 and 12, then annually. Toxicity was graded according to RTOG Criteria. The primary end point was the proportion of patients with a minimum clinically important change (MCIC) in bowel QOL at any time during the acute (<=12 week) period and was analyzed by Fisher’s exact test with a two-sided p < 0.05 considered significant. MCIC was defined as a decrease in EPIC score of >0.5 standard deviation of the baseline value. ClinicalTrials.gov NCT01423474. Results: 152 men from 3 centres were randomized with a median follow-up of 13.1 months. Baseline characteristics were similar in both arms except for the International Prostate Symptom Score with medians of 4 vs. 7 in the 11 and 29 day groups, respectively (p=0.02). There were significant differences between the 11 and 29 day groups in the proportion of patients with acute MCIC in bowel (90.0% vs. 69.6%, p<0.01) and urinary (95.7% vs. 74.6%, p<0.01) scores, respectively. No differences were found in acute sexual (p=0.38) or hormonal (p=0.48) QOL. Worst acute grade 1, 2, 3 toxicities were 64, 18, 0% vs. 41, 11, 0% (p<0.01) for GI and 38, 32, 1% vs. 30, 34, 3% (p=0.69) for GU in the 11 and 29 day groups, respectively. There were no late grade 3+ GI toxicities. Late grade 3 GU toxicity occurred in 1 vs. 0 patients in the 11 and 29 day arms (p=0.32). Conclusions: Prostate SBRT delivered over 29 days was associated with superior bowel and urinary QOL compared to 11 days in the first 3 months of treatment. There were few severe (grade 3+) toxicities in either group. Follow-up is continuing to compare long-term outcomes. Clinical trial information: NCT01423474.