I can understand your concern. Mayo has published a report on their research comparing urinary and bowel issues from Hypofractionated proton beam therapy with 5 administrations to a larger number of administrations of typical strength proton beams. They found no differences in effects. They followed up for 18 to 25 months.

I have to mention that the choice is not between 5 and 39. My Gleason score was 4+4 or 8. It was at advanced stage. My treatment included 3 shots of Antigen Deprivation Therapy and 20 proton beam sessions. I was told that the number of radiation sessions was dependent on the containment of the cancer and health of patient. I had some urinary issues, pain and frequency. With medications the problem was controlled.

The guidelines for treating PCa also recommends that ADT is used concurrently with the radiation treatment, especially for patients with more advanced cancer like mine. I met other patients with lower Gleason scores who didn't need ADT. ADT has its own side effects that were manageable. Your oncologist should be able to tell you what treatment is best for you. I learned that one size doesn't fit all. Mayo doctors are really on top of the issues that affect treatments. In my case they had 13 doctors consult on the case.