Proton therapy is a high-quality radiation therapy which uses a proton beam to irradiate cancer tissue. The advantage of this type of treatment is a highly conformal dose deposition due to the presence of the Bragg peak. It is often required to irradiate the tumor volume with a precision better than 1 - 2 mm, which means that proton therapy needs not only precise treatment planning but also monitoring and proton range verification during the treatment. One way to monitor the proton range is Prompt Gamma Imaging (PGI) which means to detect gamma rays produced by
the excitation of the target nuclei by incident protons. In this work, the results of the Geant4 simulation (version 10.6.3.) of interactions of protons with a carbon target are presented. This includes the study of 4.4 MeV and 9.6 MeV line properties, as multiple differences were observed between simulation and experiment, one of which is a double peak for the C12(p, p’𝛾 4.44)C12 spectral line. The shortcomings of the current physical models in Geant4 in describing the shape and intensity of the 4.4 MeV and 9.6 MeV gamma lines will be discussed.