Monte Carlo-calculated beam-quality and perturbation correction factors for helium-, carbon-, and neon-ion beams

A new technique, multi-ion radiotherapy, which uses He, O, and Ne ions in addition to C ions, has been proposed, though there is still limited information on the dosimetric quantities, such as the beam quality and perturbation correction factors, necessary to determine the absorbed dose-to-water for various ion species. In this study, beam-quality correction factors, k_Q, for He, C, and Ne ions in five types of ionization chambers were derived through Monte Carlo simulations. Additionally, perturbation effects attributed to the ionization chambers were analyzed in detail, and differences in these effects across chamber types and ions were clarified.

The k_Q values for 150 MeV/u-He, 290 MeV/u-C, and 400 MeV/u-Ne ions were evaluated using the Geant4 Monte Carlo code. The f_Q (the product of the water-to-air stopping power ratio and the perturbation correction factor) was obtained using the absorbed doses to water and to air inside the active volume of the ionization chamber. The k_Q values were then calculated from the f_Q and the literature-extracted factors, $f_{Q_0}$ and W_air. To evaluate individual perturbation effects, each chamber component (such as the central electrode, wall, and stem in the modeled chambers) was replaced by water in turn, and the dose was recalculated for each modified geometry. From the ratio of these doses, the contribution of each component to the perturbation effect was estimated.

The derived k_Q values were consistent with the code of practice for dosimetry, the revised TRS-398, and varied slightly with the ion species, from He to Ne ions. Additionally, the k_Q values increased by up to 1% compared with the current protocol values, particularly in plane-parallel chambers. Analysis of contributions to the perturbation factors revealed that the perturbation effect due to the cavity, P_cav, is significantly greater for light ions than for protons and had the most substantial impact among the evaluated individual perturbation factors.

The revision of k_Q values for light ions may result in the absorbed dose-to-water varying by up to 1% compared with doses calculated using the currently recommended k_Q values. At the current stage, it has been confirmed that common k_Q values can be applied to light ions from He to Ne in both Farmer and Markus type ionization chambers.