Impact of new ICRU90 key data on stopping-power ratios and beam quality correction factors for carbon ion beams

Abstract

The recent update of dosimetric key data by the `International Commission on Radiation Units and Measurements' impacts the computation of beam quality correction factors $k_Q$ via several changes, e.g. for the mean excitation energies, $I$, which enters the stopping power computation for water and air, the computation procedure itself, the average energy expended in the production of an ion pair in air, $W/e$, as well as chamber perturbation factors for Cobalt-60. An accurate assessment of water-to-air stopping-power ratio, $s_{\rm w,air}$, in reference conditions with new recommendation is necessary to update the dosimetry protocols for carbon ion beams. The new ICRU90 key data were considered for computation of $s_{\rm w,air}$ for carbon ion beams using Monte Carlo transport simulations for a number of reference conditions, namely monoenergetic carbon ion beams with range in water from 3 to 30\,cm and Spread-Out Bragg peaks (SOBPs) of different widths and depths in water. New recommendations for $s_{\rm w,air}$ are presented, namely 1.1247 for the reference condition of 1\,g\,cm$^{-2}$ depth for monoenergetic carbon ion beams and 1.1274 at the center of physically-optimized SOBPs. The recommendation of a constant value (1.126) represents the stopping-power ratio within a 0.3\,\% variation of $s_{\rm w,air}$ for the different reference conditions. The impact of these new $s_{\rm w,air}$ values and the updated key data on the $k_Q$ for carbon ion beams was evaluated in a second step. The changes agree very well with experimental data for the case of cylindrical ionization chambers, but larger discrepancies are observed for plate-parallel ionization chambers.