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Why are protons used in cancer therapy ? Is there any advantages compared to classics X-rays treatment or electron treatment ?

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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.

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A nice plot to illustrate this concept. –  Andrew Mar 9 '11 at 22:14
    
Bragg peak is the key but it might be worth noting a major part is that you stop the beam which is more important then exit dose. –  Kortuk Jun 25 '12 at 15:43
    
On that note, the fact that you list things such as carbon ions tells me you are keeping this clear and concise. –  Kortuk Jun 25 '12 at 15:49

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