Aim The purpose of this work was to estimate the dependency between the produced positron emitters and the proton dose distribution as well as the dependency between points of annihilation and the proton dose distribution. Background One important feature of proton therapy is that, through the non-elastic nuclear interaction of protons with the target nuclei such as 12C, 14N and 16O, it produces a small number of positron-emitting radioisotopes along the beam-path. These radioisotopes allow imaging the Bragg peak position which is related to the proton dose distribution by using positron emission tomography. Methods In this study, the GEANT4 toolkit was applied to simulate a soft and bone tissue phantom in proton therapy to evaluate the positron emitter productions and the actual annihilation points of β+. Simulation was done by delivering pencil and spread-out Bragg peak (SOBP) proton beams. Results The findings showed that (15O, 11C, 13N) and (11C, 15O, 38K, 30P, 39Ca, 13N) are the most suitable positron emitters in the soft and bone tissue respectively. By increasing the proton energy, the distance between the peak of annihilation profile and Bragg peak is almost constant, but the distance between the Bragg peak position and positron annihilation point peak in bone tissue is smaller than that in the soft tissue. The peak of β+ activity distribution becomes sharper at higher proton energies. Conclusions There is a good relationship between the positions of positron annihilation profile and positron emitters radioactive decay. Also, GEANT4 is a powerful and suitable tool for simulation of nuclear interactions and positron emitters in tissues.

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