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

We study the spatial distributions of ${\beta }^{+}$-activity produced by therapeutic beams of ${}^3$He and ${}^{12}$C 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 ${}^{10,11}$C, ${}^{13}$N, ${}^{14,15}$O, ${}^{17,18}$F and ${}^{30}$P positron-emitting nuclei were calculated and compared with experimental data obtained during and after irradiation. Positron emitting nuclei are created by ${}^{12}$C beam in fragmentation reactions of projectile and target nuclei. This leads to a ${\beta }^{+}$-activity profile characterised by a noticeable peak located close to the Bragg peak in the corresponding depth-dose distribution. On the contrary, as the most of positron-emitting nuclei are produced by ${}^3$He 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 ${}^{13}$N, ${}^{14}$O, ${}^{17,18}$F created in charge-transfer reactions by low-energy ${}^3$He 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. Due to the production of ${}^{18}$F nuclide the ${\beta }^{+}$-activity profile measured 2 or 3 hours after irradiation with ${}^3$He ions will have a distinct peak correlated with the maximum of depth-dose distribution. We found certain advantages of low-energy ${}^3$He 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 ${}^{17}$F nuclei clearly marks the range of ${}^3$He in tissues.