Photon radiation will hit the targeted area in the prostate but will have an entry and exit, thus passing through healthy tissue. Proton radiation (as I was told at Mayo) releases the radiation when it hits the target therfore the radiation does not have an entry or exit. Theoretically proton should have less side affects, but no random clinical trials have been conducted at this time to prove this. Brachytherapy (HDR & LDR/seeds) places the radiation inside the gland and therefore has no entry or exit point. The mapping process and technolgy does help reduce radiation to healthy tissue compared to radiation treatment several years ago when it was more of a shot gun approach.

I can tell you that the margins used by proton or photon machines vary but the larger the margin number the greater the amount of healthy tissue is affected which impacts side effects. The MRIdian uses 2 mm margins while other photon and proton machines use 4-6 mm margins.