Type-B uncertainties related to ionization chamber modelling in Monte Carlo based reference dosimetry for clinical...

Monte Carlo simulations are increasingly used to derive beam quality correction factors ( $k_{Q,Q_0}$ ) 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 geometry and material properties on absorbed dose calculations has not been fully quantified yet.

Seven ionization chamber models have been modelled in Image 1001 , and independent perturbations of the geometrical dimensions, material mass densities and $I$ -values were performed. For each chamber model, the absorbed dose in the sensitive volume was computed for clinical proton and carbon ion beams. The deviations from unperturbed simulations were used to estimate type-B uncertainties.

All perturbed configurations showed deviations within three standard uncertainties (type-A) of the reference result. 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 $\sim 0.4\text{\%}$ on the absorbed dose.

The type-B uncertainties associated with ionization chamber modelling do not dominate the overall uncertainty of ionization response functions $f_Q$ for clinical proton and carbon ion beams which are the basis of Monte Carlo calculated $k_{Q,Q_0}$ factors.