Purpose/Objective(s) Treatment of high-risk (HR) prostate cancer (PC) usually involves pelvic conventional fractionation (CF)-EBRT, followed by CF-EBRT boost to prostate in combination with androgen deprivation therapy (ADT). PBS (NCT03380806) is a randomized phase II trial that evaluated stereotactic body radiotherapy (SBRT) boost after pelvic CF-EBRT in patients with unfavorable intermediate (UI) and HR prostate cancer. Materials/Methods Patients with localized unfavorable intermediate (UI) and HR PC, from two Ontario radiotherapy facilities, were randomized (1:1) to CF-EBRT boost (32-34Gy in 15-17 daily fractions) or SBRT boost (19.5-21Gy delivered in 3 weekly fractions) following pelvic CF-EBRT (45-46Gy over 23-25 daily fractions). The primary objective was to assess early changes (3 months after radiotherapy) in gastrointestinal (GI), genitourinary (GU) and global patient-reported QoL using the expanded prostate index composite (EPIC). Secondary outcomes included GU/GI QoL, IPSS scores, GU/GI CTCAE v.5.0 toxicity and biochemical control up to 30 months. Differences between the treatment arms were compared using linear regression and linear mixed effects regression models as well as the Fisher exact test accordingly. Results Between September 10, 2019, and July 26, 2022, 53 patients were randomly assigned to CF-EBRT and 47 to SBRT boost radiotherapy. Median follow up time was 24 (12-24) months. Comparing CF-EBRT with SBRT three months after the treatment, there were no significant differences in the mean urinary 11.5 vs 8.6, P = 0.233; bowel 5.2 vs 6.4, P = 0.573; and global 8.3 vs 7.5, P = 0.612, EPIC scores. Additionally, the changes in the IPSS scores were similar in both groups, with -7.0 vs -4.6, P = 0.105. CTCAE v.5.0 gastrointestinal and urinary grade 2-4 toxicity rates after 3 months follow-up were comparable between the two groups, with an odds ratio of 0.90 (95% CI = 0.22, 3.71), and p > 0.999. Rates of biochemical failure in both groups were <5%. Conclusion In UI and HR PC, SBRT boost treatment of the prostate after CF-EBRT treatment of the pelvis is well tolerated and demonstrates similar QOL outcomes and toxicity events with CF-EBRT boost. Early biochemical control rates appear similar. Long-term QOL, toxicity and disease control results are awaited.