RESUMO
INTRODUCTION: Current dosimetry protocols for clinical protons using air-filled ionization chambers assume that the perturbation correction factor is equal to unity for all ionization chambers and proton energies. Since previous Monte Carlo based studies suggest that perturbation correction factors might be significantly different from unity this study aims to determine perturbation correction factors for six plane-parallel and four cylindrical ionization chambers in proton beams at clinical energies. MATERIALS AND METHODS: The dose deposited in the air cavity of the ionization chambers was calculated with the help of the Monte Carlo code TOPAS/Geant4 while specific constructive details of the chambers were removed step by step. By comparing these dose values the individual perturbation correction factors pcel, pstem, psleeve, pwall, pcavâ pdis as well as the total perturbation correction factor pQ were derived for typical clinical proton energies between 80 and 250MeV. RESULTS: The total perturbation correction factor pQ was smaller than unity for almost every ionization chamber and proton energy and in some cases significantly different from unity (deviation larger than 1%). The maximum deviation from unity was 2.0% for cylindrical and 1.5% for plane-parallel ionization chambers. Especially the factor pwall was found to differ significantly from unity. It was shown that this is due to the fact that secondary particles, especially alpha particles and fragments, are scattered from the chamber wall into the air cavity resulting in an overresponse of the chamber. CONCLUSION: Perturbation correction factors for ionization chambers in proton beams were calculated using Monte Carlo simulations. In contrast to the assumption of current dosimetry protocols the total perturbation correction factor pQ can be significantly different from unity. Hence, beam quality correction factors [Formula: see text] that are calculated with the help of perturbation correction factors that are assumed to be unity come with a corresponding additional uncertainty.
Assuntos
Prótons , Radiometria , Método de Monte CarloRESUMO
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. fâ 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, fâ 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.
