Conceptualization of ionization dose to water for dosimetry in external beam radiotherapy
Nobuyuki Kanematsu
TL;DR
The paper argues that absorbed dose to water may not fully reflect radiobiological effects in external beam radiotherapy and introduces ionization dose to water $D_i$ as a more biologically relevant quantity, calibratable within the IAEA ICP framework. It derives $D_i$ from the standard dose $D$ via $D_i = D\,\frac{\overline{E_i}_{\mathrm{air}}}{W_{\mathrm{air}}_Q}$ and provides a practical calibration form $D_i_Q0 = M\,N_D_{Q_0}\,(s_{\mathrm{w/air}}\,p_{\mathrm{ch}})_{Q/Q_0}$; to reduce gas-related corrections for protons/ions, it proposes nitrogen-based water-equivalent gas (WEG) mixtures with $I_{weg}=I_w$, yielding $s_{\mathrm{w/weg}}\approx (Z/A_r)_{\mathrm{w/weg}}$ and $D_i_Q0 = M\,N_D_{Q_0}\,p_{\mathrm{ch},Q/Q_0}$. A thought-reference dosimetry test with air and WEG chambers demonstrates that $D_i_Q0$ can agree with $D$ for photons/electrons and remains close for protons and ions, with uncertainties reduced to about $0.7\%$–$1.0\%$ using ideal WEG. If validated broadly, this approach could be adopted in clinical practice to improve dosimetric accuracy, particularly for complex clinical beams and non-reference conditions, by focusing on electronic excitations and reducing beam-quality corrections and their uncertainties. The study also highlights the implicit neglect of endothermic nuclear energy in absorbed dose and argues that it is intrinsically irrelevant to ionization dose.
Abstract
Absorbed dose or temperature rise may not be essential for radiotherapy due to its limited biological relevance. This study conceptualizes ionization dose to water, which is the absorbed dose to water expended on ionization, to represent biologically more relevant doses. Based on the international code of practice for ionization chamber dosimetry incorporating cobalt calibration, the procedures to measure ionization dose were formulated for high-energy photon, electron, proton, and ion beams. Nitrogen-based water-equivalent gas (WEG) mixtures were designed for proton and ion beams. The proposed dosimetry procedures were tested in a thought experiment. With a common ionization chamber, calibrated ionization doses were generally compatible with corresponding absorbed doses, and identical for photon and electron beams. The dosimetric uncertainty was reduced from \SI{1.4}{\%} to \SI{1.3}{\%} for a proton beam and from \SI{2.4}{\%} to \SI{1.9}{\%} for an ion beam. With an ideal WEG ionization chamber, the uncertainty was further reduced to \SI{0.7}{\%} for the proton beam and \SI{1.0}{\%} for the ion beam. The new dose concept, calibrated ionization dose to water, may be used directly in current radiotherapy practice with reduced uncertainty. 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 protocols. The endothermic nuclear energy being wasted in radiotherapy has been overlooked in absorbed dose and will be intrinsically irrelevant to ionization dose.