Experimental determination of beam quality correction factors in scanned proton beams using water calorimetry

<i>Objective.</i>To measure beam quality correction factors (kQ) in single-layer scanned proton beams using water calorimetry for three ionization chamber types commonly used in clinical proton dosimetry.<i>Approach.</i>Measurements were performed at two proton therapy centers using clinical proton beams with nominal energies of 150, 220 and 226 MeV, at a reference depth of 4 cm. ThekQ-factors were obtained by comparing absorbed dose-to-water determinations from a water calorimeter with ionization chamber readings under identical conditions. Three ionization chamber models were investigated: the IBA FC65-G (cylindrical), PPC05 and PPC40 (plane-parallel). Two independent calorimeter setups were used across three measurement campaigns.<i>Main results.</i>The measuredkQ-factors showed strong agreement with current TRS-398 Rev. 1 (2024) recommendations and literature data. For the FC65-G chamber, excellent alignment was observed with previous water calorimetry-based measurements. For the PPC40 chamber, both chambers yielded consistent results within 0.5% of TRS-398 Rev. 1. For the PPC05 chambers, a maximum deviation of 1.4% was observed relative to TRS-398 Rev. 1, and inter-chamber variability was within 0.5%. The use of two calorimeter setups yielded consistent results within 0.2%, supporting their validity.<i>Significance.</i>This study presents new experimentalkQ-factors for ionization chambers in single-layer scanned proton beams, contributing to the currently scarce experimental database, providing further validation of the TRS-398 Rev. 1 (2024) recommendations, and supplying benchmark data for future Monte Carlo-basedkQcalculations.