Stopping-Power Ratios and Beam Quality Factors for Ion Carbon Beams – Impact of New ICRU90 Key Data

Reference dosimetry for carbon ion beams relies on calibrated ionization chamber measurements and dose-to-water-based protocols. Since no primary standard for carbon beams exists, the most prominent code of practice – IAEA's TRS-398 – defines the relation between dose to water and charge measured by an ionization chamber by means of beam quality correction factors k_Q. The recent update of dosimetric key data by the 'International Commission on Radiation Units and Measurements' impacts the computation of k_Q factors via changes of several key data, such as the 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, and the chamber perturbation factors. Andreo et al. estimated the effect of the new key data on the beam quality correction factors to be -0.5 %, and on the stopping power ratios for carbon ions water to air s_w,air -0.4 % - mainly due to the changes in values. An accurate assessment of sw,air in reference conditions with new recommendation is necessary to update the dosimetry protocols for carbon ion beams. This contribution presents an evaluation of the impact of the updated key quantities on the stopping power ratio s_w,air and the k_Q factors for carbon ion beams in different reference conditions. In addition, recommendation of new values of s_w,air are presented.