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

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 [Formula: see text] 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, [Formula: see text] factors, [Formula: see text] factors, and [Formula: see text] ratios (which are the basis of Monte Carlo calculated beam quality correction factors [Formula: see text]) 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 [Formula: see text] and [Formula: see text] factors agreed within 0.5% or better for the individual codes for both beam qualities. The resulting [Formula: see text] 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 [Formula: see text] and [Formula: see text] factors between the codes are 0.5% at maximum. The differences in the [Formula: see text] ratios are 0.7% at maximum.