Abstract
Objective. The boundary crossing algorithm available in Geant4 10.07-p01 general purpose Monte Carlo code has been investigated for a 12 and 200 MeV electron source by the application of a Fano cavity test. Approach. Fano conditions were enforced through all simulations whilst varying individual charged particle transport parameters which control particle step size, ionisation and single scattering. Main Results. At 12 MeV, Geant4 was found to return excellent dose consistency within 0.1% even with the default parameter configurations. The 200 MeV case, however, showed significant consistency issues when default physics parameters were employed with deviations from unity of more than 6%. The effect of the inclusion of nuclear interactions was also investigated for the 200 MeV beam and was found to return good consistency for a number of parameter configurations. Significance. The Fano test is a necessary investigation to ensure the consistency of charged particle transport available in Geant4 before detailed detector simulations can be conducted.
Highlights
The derivation of absorbed dose-to-water from the amount of charge measured in an air-filled ionisation chamber requires, among other steps, a conversion from absorbed dose to the cavity to absorbed dose-to-water
The Fano test is a necessary investigation to ensure the consistency of charged particle transport available in Geant4 before detailed detector simulations can be conducted
Novel dosimetry techniques such as very high energy electrons (VHEEs) are growing in popularity (DesRosiers et al 2000), the same detailed reference dosimetry protocols used for clinical techniques do not currently exist
Summary
The derivation of absorbed dose-to-water from the amount of charge measured in an air-filled ionisation chamber requires, among other steps, a conversion from absorbed dose to the cavity to absorbed dose-to-water. For a calibrated ionisation chamber, the change of this conversion between the calibration beam and the user beam needs to be known and this change is normally embedded in the data used in codes of practice for reference dosimetry These strict dosimetry protocols are available for all current clinical radiotherapy treatment modalities and are described in detail in the IAEA TRS-398 Code of practice for dosimetry (IAEA 2000). A recent study by McManus et al (2020) using ultra-short pulsed 200 MeV electrons highlighted significant issues which arise in plane-parallel ion chambers due to the lack of traceable dosimetry One of these elements is the absence of a chamber specific calibration coefficient, ND,w,Q, which converts the charge measurement of the chamber into dose-to-water for this beam modality (McManus et al 2020). When deriving other ion chamber correction factors such as absolute ion recombination, from direct comparison between an ion chamber measurement and a calorimeter measurement as per the above study, incorrect determination of the calibration coefficient can lead to underestimations of the ion recombination occurring in the chamber or result in unphysical evaluations of ion collection efficiencies greater than 100% (McManus et al 2020)
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