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

Abstract

We study the spatial distributions of β+-activity produced by therapeutic beams of 3He and 12C 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 T1/2 > 10 s, namely 10,11C, 13N, 14,15O, 17,18F and 30P, were calculated and compared with experimental data obtained during and after irradiation, where available. Positron-emitting nuclei are created by a 12C beam in fragmentation reactions of projectile and target nuclei. This leads to a β+-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 β+-activity during or soon after the irradiation period is evenly distributed within the projectile range. However, we predict also the presence of 13N, 14O, 17,18F created in charge-transfer reactions by low-energy 3He ions close to the end of their range in several tissue-like media. The time evolution of β+-activity profiles was investigated for both kinds of beams. We found that due to the production of 18F nuclides the β+-activity profile measured 2 or 3 h after irradiation with 3He ions will have a distinct peak correlated with the maximum of depth–dose distribution. We also found certain advantages of low-energy 3He beams over low-energy proton beams for reliable PET monitoring during particle therapy of shallow-located tumours. In this case the distal edge of β+-activity distribution from 17F nuclei clearly marks the range of 3He in tissues.