Comparison of penh, fluka, and Geant4/topas for absorbed dose calculations in air cavities representing ionization chambers in high‐energy photon and proton beams

Abstract

Purpose The purpose of this work is to analyze whether the Monte Carlo codes penh, fluka, and geant4/topas are suitable to calculate absorbed doses and fQ/fQ0 ratios in therapeutic high‐energy photon and proton beams. Methods We used penh, fluka, geant4/topas, and egsnrc to calculate the absorbed dose to water in a reference water cavity and the absorbed dose to air in two air cavities representative of a plane‐parallel and a cylindrical ionization chamber in a 1.25 MeV photon beam and a 150 MeV proton beam — egsnrc was only used for the photon beam calculations. The physics and transport settings in each code were adjusted to simulate the particle transport as detailed as reasonably possible. From these absorbed doses, fQ0 factors, fQ factors, and fQ/fQ0 ratios (which are the basis of Monte Carlo calculated beam quality correction factors kQ,Q0) were calculated and compared between the codes. Additionally, we calculated the spectra of primary particles and secondary electrons in the reference water cavity, as well as the integrated depth–dose curve of 150 MeV protons in water. Results The absorbed doses agreed within 1.4% or better between the individual codes for both the photon and proton simulations. The fQ0 and fQ factors agreed within 0.5% or better for the individual codes for both beam qualities. The resulting fQ/fQ0 ratios for 150 MeV protons agreed within 0.7% or better. For the 1.25 MeV photon beam, the spectra of photons and secondary electrons agreed almost perfectly. For the 150 MeV proton simulation, we observed differences in the spectra of secondary protons whereas the spectra of primary protons and low‐energy delta electrons also agreed almost perfectly. The first 2 mm of the entrance channel of the 150 MeV proton Bragg curve agreed almost perfectly while for greater depths, the differences in the integrated dose were up to 1.5%. Conclusion penh, fluka, and geant4/topas are capable of calculating beam quality correction factors in proton beams. The differences in the fQ0 and fQ factors between the codes are 0.5% at maximum. The differences in the fQ/fQ0 ratios are 0.7% at maximum.

Monte Carlo simulated beam quality and perturbation correction factors for ionization chambers in monoenergetic proton beams

“…Therefore, the Type B uncertainty associated with the / ratios presented here may differ from the estimation in Wulff et al 9 and Baumann et al 8 due to differences in the cross‐section data and particle transport of the codes. Baumann et al 40 investigated the impact of combining a factor determined in TOPAS/Geant4 and a calculated in EGSnrc and found that the resulting difference from using the same code for both factors is (0.3 ± 0.2)%.…”

Beam Monitor Calibration for Radiobiological Experiments With Scanned High Energy Heavy Ion Beams at FAIR

“…The type B uncertainty of Monte Carlo calculated (D w /D air )60 Co ratios (due to uncertainties in the radiation physics and transport models, geometry, etc.) were estimated to be 0.5% based on an intercode comparison by Baumann et al [33] and a benchmark experiment by Renner et al [40]. The corresponding type B uncertainty of the (D w /D air ) heavy ion ratios from FLUKA simulations is more difficult to estimate but certainly larger than the uncertainty for 60 Co photons.…”

“…Today advanced computational methods, in particular powerful Monte Carlo codes, are available to study the response of ionization chambers in different radiation fields [29][30][31][32]. The calculation of k Q factors by means of Monte Carlo simulation can be also described by Equation (7) [33,34]:…”

“…The transport settings were chosen to be the same as reported by Baumann et al [33] because they were optimized specifically for ionization chamber calculations with heavy charged particles. All simulations were performed with full electromagnetic transport (photon and electron transport down to 1 keV) and with the physics models set to the highest precision level (e.g., full Rayleigh and Coulomb scatter corrections, heavy fragment evaporation and coalescence).…”

“…The material definitions of air, graphite and water were defined according to the ICRU 90 recommendations [25] (details reported by Baumann et al [33]). For the other materials (e.g., PMMA, aluminum), the standard FLUKA definitions were used.…”