Is there any evidence that demonstrates an increase in rectal or bladder toxicity with protons for prostate cancer compared to IMRT?

This is a great and controversial question. I recently contributed to a review that tried to summarize the current studies to date looking at this question. As you know, there are no completed phase 3 trials comparing protons to photons in this space, and we eagerly await results from the PARTIQoL trial (ClinicalTrials.gov identifier: NCT01617161) which is a randomized trial (protons vs. IMRT) for low- to intermediate-risk localized prostate cancer as well as COMPPARE (ClinicalTrials.gov identifier: NCT03561220), a prospective registry that will evaluate quality of life, toxicity, and disease control outcomes in patients who receive proton beam therapy as well as patients who receive IMRT. Therefore, the current data on organs-at-risk come ...

It is unclear if proton therapy increases or reduces the risk for rectal or bladder toxicity when compared to IMRT. There are two prospective clinical trials including the aforementioned PARTiQol and COMPPARE trial that may help to answer this question in the future. The dosimetric benefit of proton therapy when compared to IMRT is in the low to moderate radiation dose range. Published dosimetric comparisons between proton therapy and IMRT have shown that proton therapy usually reduces the V10-V50 delivered to the rectum and bladder when delivering standard fractionation to 70 to 80 Gy total. These dosimetric improvements may reduce the risk for toxicities including rectal urgency and frequency which is thought to be related to radiation do...