The dose-depth curve of protons and photons can be seen in the image below:
Now, what I've heard is, that in some cases, proton therapy is advantageous compared to photons, and of course the other way around in some other cases. I can see, that the Bragg peak of the proton curve is very sharp, which means, that in order to hit a tumour, with some size, you have to make a spread Bragg peak resulting in some higher dose in the plateau region, than then ~20% it is now. But I'm not sure that even that would result in a larger dose in that area, than if you were to use photons ?
So why is it, that in some cases protons are better than photons, and the other way around ? As far as I understand, the proton radiation borders, when it hits the desired target, is very sharp, and therefore very precise – but still not very good if you have to hit a tumour that is VERY close to tissue of high risk. And therefore you often use photons for that. But it just seems that photons are much more spread out, so I can't really see why they should be less of a concern to tissue very close to the irradiated area ?
Best Answer
Some practical information first - the charged particle beam passing through the matter suffers from energy (and also angular) straggling. That means that even if an ideally monoenergetic beam is used, there will be always a finite volume with Bragg peak losses. The bigger initial energy, the bigger is the volume. Protons stopping at 40 mm have straggling less than 1 mm, at 125mm have 1.5 mm.
1st question - Compare the dose at Bragg peak : in your picture - you need multiply the gamma curve by two to have the similar dose for protons and gamma at 150 mm. To compensate the unwanted irradiation in plateau, you can irradiate the patient from several angles, keeping the Bragg peak always in the same place. Which also can/should be done for gamma irradiation.
2nd question - charged particles are definitely less damaging the surrounding tissue than gamma. Brain and near-eye tumors are best candidates for this kind of strictly localized treatment. However gamma is much less expensive and it is said that studies on treated patients show, that statistical results are similar for gamma and protons.