Water calorimetry-based beam quality correction factors for carbon and helium ion beams

Objective.To determine beam quality correction factors,, in single-layer scanned carbon and helium ion beams using water calorimetry, thereby reducing uncertainties in dosimetry for light ion therapy.Approach.Water calorimetry measurements were performed under harmonized conditions at two synchrotron-based ion beam therapy centers (MedAustron, Austria and HIT, Germany). Measurements were conducted in the plateau region of single-layer scanned beams. Carbon ion beams covered nominal energies from 213.4-402.8 MeV/u, while helium ion beams were measured at 196.3 and 198.8 MeV/u. Multiple cylindrical (IBA FC65-G, PTW 30013) and plane-parallel (IBA PPC05, PPC40; PTW 34001, 34045) ionization chamber types were investigated.Results.-factors were determined with standard uncertainties of approximately 1%, depending on chamber type and measurement condition. For all investigated chambers, inter-center agreement was observed within the combined standard uncertainties. Chamber-to-chamber variability was below 0.5% for chamber types for which two samples were available, except for the IBA PPC05, for which differences up to 1.3% were observed. For carbon ion beams, the measured-factors are consistent with previously published water calorimetry-based results. However, systematic deviations up to 3% persist between experimentally determined and Monte Carlo-derivedvalues. The measured-factors for helium ions show trends that are broadly similar to those observed in carbon ion beams across all chamber types.Significance.This work expands the experimental-database for light ions by providing new carbon ion data and the first experimental-factors for helium ion beams. These results provide essential experimental data for future refinement of dosimetry recommendations, and highlight persistent discrepancies between the experimental and Monte Carlo-based-factors that require further investigation.