Several studies have investigated the use of SBRT boost similar to brachytherapy boost paired with pelvic radiation for intermediate risk and high risk prostate cancer. PROMETHEUS was a phase 2 study of 135 patients with intermediate risk (76%) or high risk (24%) prostate cancer received 19-20 Gy in 2 fractions given one week apart followed 46 Gy in 23 fractions using IMRT. The study mandated MRI fusion for RT planning, rectal displacement (SpaceOAR in 59%, Rectafix in 41%), and intrafraction image guidance. There were no acute grade 3 events, the rate of acute grade 2 GI AEs was 4.4%, and the rate of acute grade 2 GU AEs was 26.6%. The highest prevalence of late grade 2+ GI AEs was 3.7% at 12 months, and the highest prevalence of late grade 2+ GU AEs was 12% at 18 months. A notable finding was a transient second rise in urinary events at 18 months that subsequently resolved. The median PSA dropped to 0.2 at 36 months post treatment. | Chan, Radiother Oncol 2026
