Nowadays, proton therapy is becoming one of the most powerful technic for cancer treatment. It consists in irradiating tumors with high-energy particles beam, in our case protons in the range of 65-230 MeV. The higher the energy is, the larger the penetration depth is. In human tissues, the penetration depth is about 3 cm for 65 MeV protons, and about 32 cm at 230 MeV. The unique feature with protons is their steep stopping profile in matter with the occurrence of a Bragg peak. As a result, it allows using higher doses (or at least more concentrate) than in conventional X-ray radiotherapy while strongly limiting radiation damages nearby healthy tissues. Moreover, by tuning the Bragg peak position with attenuator filter it is possible to precisely adjust the in depth shape of the proton beam. Proton therapy is consequently considered as the most accurate approach for tumor irradiation and it is becoming essential when the tumor to be irradiated is close to vital and sensitive organs.
A typical example of the superiority and benefits of the proton therapy approach is the eye tumor. Indeed, the irradiation must be limited to the tumor in the eye while sparing whenever possible macula and the optic nerve. In order to accurately reach the tumor, the penetration depth is tailored by interposing matter into the beam (in general PMMA), while lateral definition is obtained by metal masks. To illustrate the advantage of using a proton beam instead of conventional X-ray photons a sketch of the dose reparation for eye tumor treatment is shown in right panel of the figure below. No words are needed to describe how much concentrate the irradiation is, and thus the toxicity reduction for neighboring healthy tissues, cells and organs.
