Monte Carlo-derived ionization chamber correction factors in therapeutic carbon ion beams

Abstract

The accuracy of electromagnetic transport in the GEANT4 Monte Carlo (MC) code was investigated for carbon ion beams and ionization chamber (IC)-specific beam quality correction factors were calculated. This work implemented a Fano cavity test for carbon ion beams in the 100–450 MeV/u energy range to assess the accuracy of the default electromagnetic physics parameters. The Urban and the Wentzel-VI multiple Coulomb scattering models were evaluated and the impact of maxStep, dRover, and final range parameters on the accuracy of the transport algorithm was investigated. The optimal production thresholds for an accurate calculation of values, which is the product of the water-to-air stopping power ratio and the IC-specific perturbation correction factor, were also studied. The correction factors were calculated for a cylindrical and a parallel-plate IC using carbon ions in the 150–450 MeV/u energy range. Modifying the default electromagnetic physics parameters resulted in a maximum deviation from theory of 0.3%. Therefore, the default EM parameters were used for the remainder of this work. The factors were found to converge for both ICs with decreasing production threshold distance below 5 μm. The values obtained in this work agreed with the TRS-398 stopping power ratios and other previously reported results within uncertainty. This study highlights an accurate MC-based technique to calculate the combined stopping power ratio and the perturbation correction factor for any IC in carbon ion beams.

On the perturbation effect and LET dependence of beam quality correction factors in carbon ion beams

“…The mean excitation energies for water and air were set to 78 eV and 85.7 eV, respectively, adhering to the recommendation provided in ICRU 90 report 22 . Throughout this work, the production thresholds were set to 1 mm globally and 1 μm for a 2 × 2 × 2 cm 3 region around the scoring volume to fulfill the Spencer‐Attix condition of delta‐ray equilibrium 8,18,19 . In a previous study, using a production threshold larger than 5 μm was found to severely overestimate the beam quality correction factor 19 .…”

“…Tables 1 and 2 show the transport parameters used in this investigation for 60 Co and carbon ion beam simulations, respectively. The G4EMStandardOpt4 was previously found to pass the Fano cavity test for carbon ion beams within 0.3% and is considered to be highly accurate for both light and heavy charged particle transport 19,28,29 . The default stopping power tables for carbon ions in TOPAS use ICRU 73 values, which were overridden with the ICRU 90 data only for water and air, whereas, all other materials utilized the ICRU 73 data 21 .…”

“…Throughout this work, the production thresholds were set to 1 mm globally and 1 μm for a 2 × 2 × 2 cm 3 region around the scoring volume to fulfill the Spencer‐Attix condition of delta‐ray equilibrium 8,18,19 . In a previous study, using a production threshold larger than 5 μm was found to severely overestimate the beam quality correction factor 19 . A 30 × 30 × 50 cm 3 , with 50 cm being in the direction of the beam propagation, water phantom was simulated with a uniform 10 × 10 cm 2 parallel beam incident on the surface.…”

“…10,11,[16][17][18] Previously, MC-derived f Q correction factors were reported in pristine carbon ion beams in the 150-450 MeV/u energy range for a cylindrical and a parallel-plate IC. 19 A nonnegligible perturbation effect was observed; however, the magnitude of the perturbation correction due to the specific IC subcomponents was not included. Furthermore, the stopping power data presented in the International Commission on Radiation Units and Measurements (ICRU) report 73 were used, whereas the latest stopping power data have been reported in the ICRU report 90.…”

“…Monte Carlo (MC) methods can be employed to calculate the k Q correction factor 10,11,16–18 . Previously, MC‐derived f Q correction factors were reported in pristine carbon ion beams in the 150–450 MeV/u energy range for a cylindrical and a parallel‐plate IC 19 . A non‐negligible perturbation effect was observed; however, the magnitude of the perturbation correction due to the specific IC subcomponents was not included.…”