Conceptualization of ionization dose to water for dosimetry in external beam radiotherapy

Purpose: This paper aims to conceptualize the ionization dose to water, which is the dose absorbed to water and expended on ionization, to represent more relevant medical doses, and formulate its dosimetry procedures for high-energy photon, electron, proton, and ion beams. It also aims to design ionization chamber gases to reduce dosimetric uncertainty for proton and ion beams. Methods: Based on the international code of practice, the dosimetry procedure excluding the energy-per-ionization correction and that further excluding the stopping-power-ratio correction were formulated. For the latter, water-equivalent gases (WEG) were designed to be used in gas-sealed ionization chambers for proton and ion beams. The proposed dosimetry procedures, where the ionization dose is calibrated to a reference absorbed dose in the standard dosimeter calibration with 60Co gamma rays, were simulated for test beams. Results: The calibrated ionization dose virtually measured with a common ionization chamber was demonstrated to be compatible with the absorbed dose for high-energy photon and electron beams, and with slightly reduced uncertainty from 1.4% to 1.3% for a proton beam and from 2.4% to 1.9% for an ion beam. With a WEG ionization chamber, the uncertainty was further reduced to 0.7% for the proton beam and 1.0% for the ion beam. Conclusions: The new dose concept, calibrated ionization dose to water, may be used compatibly in current radiotherapy practice. Its minimized beam-quality correction will be advantageous for complex clinical beams of various types and in non-reference conditions not covered by the standard dosimetry procedures.