Investigating the impact of the effective point of measurement for plane-parallel ionization chambers in clinical proton beams

<i>Objective.</i>To investigate the impact of the positioning of plane-parallel ionization chambers in proton beams on the calculation of the chamber-specific factor<i>f_Q</i>and, hence, the beam quality correction factorkQ,Q0.<i>Approach.</i>Monte Carlo simulations were performed to calculate the chamber-specific factor<i>f_Q</i>in monoenergetic proton beams for six different plane-parallel ionization chambers while positioning the chambers with a) their reference point and b) their effective point of measurement accounting for the water equivalent thickness of the entrance window.<i>Main results.</i>For all ionization chamber models investigated in this study, the difference in<i>f_Q</i>between both positioning approaches was larger for steeper dose gradients and bigger differences between the geometrical thickness and water-equivalent thickness of the entrance window. The largest effect was 1.2% for the IBA PPC-05 ionization chamber at an energy of 60 MeV.<i>Significance.</i>The positioning of plane-parallel ionization chambers in proton beams has a systematic impact on the<i>f_Q</i>factor. This is especially of relevance for thekQ,Q0factors presented in the recently updated TRS-398 code of practice (CoP) from IAEA. The background is that a positioning with the effective point of measurement is prescribed in TRS-398 CoP, however, all Monte Carlo derived data that have been employed for the update are based on a positioning of the ionization chambers with their reference point. Hence, the updatedkQ,Q0factors for plane-parallel ionization chambers in proton beams are subject to systematic errors that can be as large as 0.5%.