Prompt-gamma emission in GEANT4 revisited and confronted with experiment.

The field of online monitoring of the beam range is one of the most researched topics in proton therapy over the last decade. The development of detectors that can be used for beam range verification under clinical conditions is a challenging task. One promising possible solution are modalities that record prompt-gamma radiation produced by the interactions of the proton beam with the target tissue. A good understanding of the energy spectra of the prompt gammas and the yields in certain energy regions is crucial for a successful design of a prompt-gamma detector. Monte-Carlo simulations are an important tool in development and testing of detector concepts, thus the proper modelling of the prompt-gamma emission in those simulations are of vital importance. In this paper, we confront a number of GEANT4 simulations of prompt-gamma emission, performed with different versions of the package and different physics lists, with experimental data obtained from a phantom irradiation with proton beams of four different energies in the range 70-230 MeV. The comparison is made on different levels: features of the prompt-gamma energy spectrum, gamma emission depth profiles for discrete transitions and the width of the distal fall-off in those profiles. The best agreement between the measurements and the simulations is found for the GEANT4 version 10.4.2 and the reference physics list QGSP_BIC_HP. Modifications to prompt-gamma emission modelling in higher versions of the software increase the discrepancy between the simulation results and the experimental data.

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