@bobgolf The explanation as I understand it is that different organs have different tolerances to radiation. Some tissue can withstand large doses, while others will readily be killed. My understanding is that Protons are good for cancers within or in front of highly sensitive organs. It is used in childhood cancers where every cell in the body is vulnerable and protons can be limited to depth and reduce exposure to tissue behind the treatment area. It's used in Brest Cancer treatment to minimized radiation exposure to the heart below the breastbone. For Prostate Cancer, it depends on where the cells are located, has it moved into the prostate bed, has it metastasized to other critical areas

For me, I'm lucky to have caught my Pc early and it is totally contained in my prostate. In my situation, particularly where my lesion is located, Protons was not the best solution. No exit dose is an important feature, but not the only consideration in choosing a treatment. It falls on your own treatment goals. Mine was to preserve my long-term QOL, 10-20 years from now, and considering all risk factors and after much research, I felt that other treatments supported my goals better.