Monte Carlo calculated beam quality correction factors for light ions used in particle therapy

Abstract Objective. This work aims to enhance the current data set of beam quality correction factors ( k Q ) for light ions used in particle therapy, and to assess the contribution of secondary fragments in the case of helium and carbon ion beams. Approach. A Fano cavity test was performed for proton, helium and carbon ions before k Q values were derived for 20 ionisation chambers using Geant4-v11.2. For helium and carbon ions, three fragmentation models were compared (binary intranuclear cascade, intranuclear cascade mode++ and quantum molecular dynamics model), and the influence of secondary particles was investigated through the introduction of new correction factors. Main results. For helium ions, an energy dependency of the k Q values was observed, in addition to significant deviations between the different fragmentation models, especially at the highest clinical energies. For carbon ions, while no significant deviations were observed between the different beam energies or fragmentation models considered, a slight energy-dependency in addition to deviations up to 3 % with experimental values were highlighted. The factorisation of the k Q factors showed that the perturbation from secondary ions was smaller than 0.9 % for helium ions and smaller than 0.3 % with carbon ions. Significance. This work highlights the need for experimental k Q data for helium ions, in particular to distinguish between various fragmentation models provided by Geant4. In addition, although in agreement with the IAEA TRS-398 Code of Practice, the discrepancy between Monte Carlo and experimental results subsists for carbon ions. The latter is unlikely to be solved without an intensification of experimental measurement campaigns focussing, in particular, on the derivation and uncertainty reduction of the W air values.