beams: a study with the GEANT4 toolkit

We study the spatial distributions of β-activity produced by therapeutic beams of He and 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 positronemitting nuclei with T1/2 > 10 s, namely 10,11C, N, 14,15O, 17,18F and P, were calculated and compared with experimental data obtained during and after irradiation, where available. Positron emitting nuclei are created by C beam in fragmentation reactions of projectile and target nuclei. This leads to a β-activity profile characterised 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. On the contrary, as the most of positron-emitting nuclei are produced by He 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 N, O, 17,18F created in charge-transfer reactions by low-energy He 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 F nuclide the β-activity profile measured 2 or 3 hours after irradiation with He ions will have a distinct peak correlated with the maximum of depth-dose distribution. We also found certain advantages of low-energy 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 β-activity distribution from F nuclei clearly marks the range of He in tissues. Submitted to: Phys. Med. Biol. PACS numbers: 87.53.Pb, 87.53.Wz, 87.53.Vb E-mail: [email protected]