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 ?