Monte Carlo calculation of beam quality correction factors in proton beams using TOPAS/GEANT4

Abstract

To provide Monte Carlo calculated beam quality correction factors (k _Q ) for monoenergetic proton beams using [Formula: see text], a toolkit based on the Monte Carlo code [Formula: see text]. Monte Carlo simulations of six plane-parallel and four cylindrical ionization chambers were carried out. The latest ICRU 90 recommendations on key data for ionizing-radiation dosimetry were used to calculate the electronic stopping powers and to select the mean energy necessary to create an ion pair in air ([Formula: see text]). [Formula: see text] factors were calculated for a ⁶⁰Co spectrum at a depth of 5 g cm^-2. f _Q factors and [Formula: see text] ratios as well as k _Q factors were calculated at the entrance region of monoenergetic proton beams with energies between 60 MeV and 250 MeV. Additionally, perturbation correction factors for the Exradin A1SL ionization chamber at an energy of 250 MeV were calculated. [Formula: see text] factors agreed within 0.7% or better, f _Q factors within 1.7% or better and [Formula: see text] ratios within 2.2% or better with Monte Carlo calculated values provided in the literature. Furthermore, k _Q factors calculated in this work were found to agree within 1% or better with experimentally determined k _Q factors provided in the literature, with only two exceptions with deviations of 1.4% and 2.4%. The total perturbation correction factor for the Exradin A1SL chamber was 0.969(7) and hence significantly different than unity in contrast to the assumption from the IAEA TRS-398 code of practice (CoP). [Formula: see text] can be used to calculate k _Q factors in clinical proton beams. k _Q factors for six plane-parallel and four cylindrical ionization chambers were calculated and provided for the upcoming update of the IAEA TRS-398 CoP.