Conceptualization of ionization dose to water to quantify radiotherapy doses

Abstract: \emph{Objective.} This study conceptualizes the ionization dose to water as the dose absorbed to water and expended exclusively on ionization, which may be more relevant to biological responses than absorbed dose, and formulates its dosimetric procedures for high-energy photon, electron, proton, and ion beams. It also aims to propose ionization chambers with reduced ionization-dose uncertainty for proton and ion beams. \emph{Approach.} Based on the international code of practice, the dosimetric procedure without W-value correction was formulated for all these beam types, and that also without stopping-power-ratio correction was formulated for proton and ion beams. For the latter, water-equivalent gas (WEG) was considered for gas-sealed ionization chambers. For dose representation, the ionization dose is calibrated to the reference absorbed dose in the standard dosimeter calibration with $^{60}$Co \gamma\ rays. The proposed dosimetry procedures were simulated for virtual test beams. \emph{Main results.} The calibrated ionization dose measured with a common ionization chamber was compatible with the absorbed dose for all beams, and with a slightly reduced uncertainty from \SI{1.4}{\%} to \SI{1.3}{\%} for proton beams and from \SI{2.4}{\%} to \SI{1.9}{\%} for ion beams. With a WEG ionization chamber, the uncertainty was further reduced to \SI{0.7}{\%} for proton beams and \SI{1.0}{\%} for ion beams. \emph{Significance.} The new dose concept, calibrated ionization dose, can be easily used in radiotherapy practice with conventional dose prescriptions. The minimized beam-quality correction for ionization doses will be advantageous for non-reference conditions and complex clinical beams of various types, including those not covered by the standard dosimetry procedures.