Impact of new ICRU Report 90 recommendations on calculated correction factors for reference dosimetry

Abstract

In 2016 the ICRU published a new report dealing with key data for ionizing radiation dosimetry (ICRU Report 90). New recommendations have been made for the mean excitation energies I for air, graphite and liquid water as well as for the graphite density to use when evaluating the density effect. In addition, the ICRU Report 90 discusses renormalized photoelectric cross sections, but refuses to give a recommendation on the use of renormalization factors. However, the Consultative Committee for Ionizing Radiation (CCRI) recommends to use renormalized photoeffect cross sections. Goal of the present work is to evaluate the impact of these new recommendations on clinical reference dosimetry for high energy photon and electron beams. The beam quality correction factor kQ was calculated by Monte Carlo simulations for compact and parallel plate ionization chambers. In case of photons seven phase space files from clinical accelerators and twelve spectra taken from literature from 4 MV to 24 MV and additionally a 60Co source were applied. As electron source thirteen electron spectra available in literature were used in the range of 4 MeV to 21 MeV. The new ICRU recommendations have a small impact on Monte Carlo calculated kQ values for the chosen ionization chambers in the range of 0.1% to 0.35% only-the difference increases for higher photon energies. The impact of the ICRU Report 90 recommendations on Monte Carlo calculated stopping power ratios sw,a, perturbation factors p and beam quality correction factors kQ was investigated and confirmed a decrese of sw,a by a fraction of a percent for photon and electron beams. This study indicates that the impact of the new ICRU recommendation is within 0.35%. The determined deviations should be taken into account, when widely published Monte Carlo calculated values are examined.