Modelling PET radionuclide production in tissue and external targets using Geant4

The Proton Therapy Facility in TRIUMF provides 74 MeV protons extracted from a 500 MeV H- cyclotron for ocular melanoma treatments. During treatment, positron emitting radionuclides such as C-11, O-15 and N-13 are produced in patient tissue. Using PET scanners, the isotopic activity distribution can be measured for in-vivo range verification. A second cyclotron, the TR13, provides 13 MeV protons onto liquid targets for the production of PET radionuclides such as F-18, N-13 or Ga-68, for medical applications. The aim of this work was to validate Geant4 against FLUKA and experimental measurements for production of the above-mentioned isotopes using the two cyclotrons. The results show variable degrees of agreement. For proton therapy, the proton-range agreement was within 2 mm for C-11 activity, whereas N-13 disagreed. For liquid targets at the TR13 the average absolute deviation ratio between FLUKA and experiment was 1.9 +/- 2.7, whereas the average absolute deviation ratio between Geant4 and experiment was 0.6 +/- 0.4. This is due to the uncertainties present in experimentally determined reaction cross sections.