Type-B uncertainties related to ionization chamber modelling in Monte Carlo based reference dosimetry for clinical proton and carbon ion beams

Background and Purpose: Monte Carlo (MC) simulations are increasingly used to derive beam quality correction factors (kQ,Q0) for reference dosimetry in clinical proton and carbon ion beams. Type-B uncertainties represent their dominant source of uncertainty, among which the impact of ionization chamber (IC) geometry and material properties on absorbed dose calculations has not been fully quantified yet.Materials and Methods: Seven ICs have been modelled in geant4, and independent perturbations of the geometrical dimensions (±10%), material mass densities (±1.3%) and I-values (±10%) were performed. For each IC, the absorbed dose in the sensitive volume was computed under clinical proton and carbon ion irradiation. The deviations from unperturbed simulations were used to estimate type-B uncertainties, while type-A uncertainties were quantified from simulation statistics.Results: All perturbed configurations showed deviations within three standard deviations of the reference. Maximum deviations of 0.4% were observed across the three types of perturbation, independently of beam energy or chamber type. Each source of uncertainty contributed to roughly 0.2% individually, with a combined effect of 0.4% on the absorbed dose.Conclusions: The type-B uncertainties associated with IC modelling, including geometry, mass density, and I-values, have a negligible effect on MC-derived absorbed doses and kQ,Q0 factors for clinical proton and carbon ion beams compared to the total uncertainty of kQ,Q0 (1.4% for protons and 2.4% for carbon ions). This study establishes an upper limit of 0.4% for this contribution.