tAim: Evaluation of energy deposition of protons in human brain and calculation of thesecondary neutrons and photons produced by protons in proton therapy.Background: Radiation therapy is one of the main methods of treating localized cancertumors. The use of high energy proton beam in radiotherapy was proposed almost 60 yearsago. In recent years, there has been a revival of interest in this subject in the context of radi-ation therapy. High energy protons suffer little angular deflection and have a well-definedpenetration range, with a sharp increase in the energy loss at the end of their trajectories,namely the Bragg peak.Materials and methods: A slab head phantom was used for the purpose of simulating protontherapy in brain tissue. In this study simulation was carried out using the Monte CarloMCNPX code.Results: By using mono energetic proton pencil beams, energy depositions in tissues, espe-cially inside the brain, as well as estimating the neutron and photon production as a resultof proton interactions in the body, together with their energy spectra, were calculated orobtained. The amount of energy escaped from the head by secondary neutrons and photonswas determined.Conclusions: It was found that for high energy proton beams the amount of escaped energyby neutrons is almost 10 times larger than that by photons. We estimated that at 110 MeVbeam energy, the overall proton energy “leaked” from the head by secondary photons andneutrons to be around 1%.

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