Evaluation of the distribution of prompt gammas emission during proton therapy using the GATE Monte Carlo toolkit

One of the methods for monitoring the range of proton beams is the detection of instantaneous gammas resulting from proton-nucleus interactions inside the patient's body. In this study, the interaction of proton beams with energies of 90, 120, and 150 MeV with a PMMA phantom was simulated using the GATE Monte Carlo tool. The correlation between the proton dose distribution and the spatial distribution of instantaneous gammas in different energy windows, which were selected according to the resolvable peaks of the instantaneous gamma spectrum, was investigated. The results showed that the choice of energy window for detecting instantaneous gammas has a great impact on the accuracy of estimating the proton range using the instantaneous gamma distribution. Then, ideal detector cylinders with radii of 29, 49, 69, and 89 mm were simulated. The anisotropic distribution of the instantaneous gamma rays detected by the 89 mm radius detector cylinder showed that there is a preferred position concerning the proton beam position for detecting instantaneous gamma rays, which can be used to maximize the geometric efficiency of instantaneous gamma detection. The results showed that the preferred position for detecting instantaneous gamma rays depends on the proton beam energy and that with increasing proton energy this preferred position is found at a location further back than the proton beam position. In addition, the preferred position showed significant changes in the energy window. Next, using the equation of the line passing through the preferred positions obtained by the detector cylinders, we estimated the position with the maximum instantaneous gamma efficiency produced inside the phantom and compared it with the original position.