Proton therapy planning and image-guidance strategies within a randomized controlled trial for high-risk prostate cancer

The Danish Prostate Cancer Group is launching the randomized trial, PROstate PROTON Trial 1 (NCT05350475), that compares photons and protons to the prostate and pelvic lymph nodes in treatment of high-risk prostate cancer. The aim of the work described in this paper was, in preparation of this trial, to establish a strategy for conventionally fractionated proton therapy of prostate and elective pelvic lymph nodes that is feasible and robust. Proton treatments are image-guided based on gold fiducial markers and on-board imaging systems in line with current practice. Our established proton beam configuration consists of four coplanar fields; two posterior oblique fields and two lateral oblique fields, chosen to minimize range uncertainties associated with penetrating a varying amount of material from both treatment couch and patient body. Proton plans are robustly optimized to ensure target coverage while keeping normal tissue doses as low as is reasonably achievable throughout the course of treatment. Specific focus is on dose to the bowel as a reduction in gastrointestinal toxicity is the primary endpoint of the trial. Strategies have been established using previously treated patients and will be further investigated and evaluated through the ongoing pilot phase of the trial.