Why are protons used in cancer therapy ? Is there any advantages compared to classics X-rays treatment or electron treatment ?
The goal of such a treatment is to induce damages in the cells of the tumor by mean of ionizing radiation.
These radiations can be X-rays (photons), electron, proton or things like carbon ions.
The problem is: if you try to irradiate a tumor, you first have to go through normal tissues and the risk is to damage them also.
Photons will transfer energy probabilistically at each collision and the X-ray beam will follow a law like exp[-x] where x is the penetration depth. You then give more energy around the tumor than to the tumor itself.
The main advantage of using protons is the "Bragg peak": the way protons interacts with matter, they will give much more energy at a certain depth (th so called Bragg peak), if you tune the energy of the beam, and other parameters, so that the Bragg peak coincide with the tumor, then you have a big advantage.