PET monitoring of cancer therapy with 3He and 12C beams:: a study with the GEANT4 toolkit

We study the spatial distributions of beta(+)-activity produced by therapeutic beams of He-3 and C-12 ions in various tissue-like materials. The calculations were performed within a Monte Carlo model for heavy-ion therapy (MCHIT) based on the GEANT4 toolkit. The contributions from positron-emitting nuclei with T-1/2 > 10 s, namely C-10,C-11, N-13, O-14,O-15, F-17,F-18 and P-30, were calculated and compared with experimental data obtained during and after irradiation, where available. Positron-emitting nuclei are created by a C-12 beam in fragmentation reactions of projectile and target nuclei. This leads to a beta(+)-activity profile characterized by a noticeable peak located close to the Bragg peak in the corresponding depth-dose distribution. This can be used for dose monitoring in carbon-ion therapy of cancer. In contrast, as most of the positron-emitting nuclei are produced by a 3He beam in target fragmentation reactions, the calculated total beta(+)-activity during or soon after the irradiation period is evenly distributed within the projectile range. However, we predict also the presence of N-13, O-14, F-17,F-18 created in charge-transfer reactions by low-energy He-3 ions close to the end of their range in several tissue-like media. The time evolution of beta(+)-activity profiles was investigated for both kinds of beams. We found that due to the production of F-18 nuclides the beta(+)-activity profile measured 2 or 3 h after irradiation with He-3 ions will have a distinct peak correlated with the maximum of depth-dose distribution. We also found certain advantages of low-energy He-3 beams over low-energy proton beams for reliable PET monitoring during particle therapy of shallow-located tumours. In this case the distal edge of beta(+)-activity distribution from F-17 nuclei clearly marks the range of He-3 in tissues.