Conceptualization of electronic dose to water and water-equivalent gas for ionometric dosimetry in external beam radiotherapy

Abstract

Electronic dose to water, which is the radiant energy imparted to a unit mass of water by electronic collisions, is conceptualized to exclude corrections and uncertainties associated with energy transfer to atomic nuclei and to enhance medical relevance in radiotherapy dosimetry. Based on the international code of practice, the procedures to determine electronic doses were formulated for high-energy photon, electron, proton, and ion beams. Nitrogen-based water-equivalent gas (WEG) mixtures were designed for use in gas-sealed ionization chambers for proton and ion beams. A thought experiment of reference dosimetry for electronic dose demonstrated its compatibility with absorbed dose for photon and electron beams. For proton and ion beams, the transition from absorbed dose to electronic dose will change the measured dose values by -1.4% and -2.2%, and decrease the uncertainty from 1.6% to 1.5% and from2.6% to 2.1%, respectively. With WEG ionization chambers, the uncertainty will further decrease to 1.2% for proton beams and 1.5% for ion beams. The formulated framework will be applicable to radiotherapy dosimetry with traceable dosimeter calibration under the international measurement system for radiation metrology.