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TOPAS/Geant4 configuration for ionization chamber calculations in proton beams
Jörg Wulff1, Kilian-Simon Baumann2,3, Nico Verbeek1
1Westdeutsches Protonentherapiezentrum Essen, Essen, Germany.
Physics in Medicine and Biology
|May 9, 2018
Summary
Monte Carlo (MC) simulations using Geant4/TOPAS provide reliable results for ionization chamber (IC) dosimetry in proton beams. Adjusting production cuts and considering hadronic models ensures accuracy, aligning with established dosimetry protocols.
Area of Science:
- Medical Physics
- Radiation Dosimetry
- Computational Physics
Background:
- Monte Carlo (MC) calculations are essential for ionization chamber (IC) dosimetry in radiation fields.
- Geant4, utilized by TOPAS, is a general-purpose code suitable for investigating ICs in proton beams.
- Understanding parameter settings and condensed history (CH) algorithm limitations is crucial for reliable MC results.
Purpose of the Study:
- To evaluate the reliability of Geant4/TOPAS for ionization chamber (IC) dosimetry in proton beams.
- To assess the impact of parameter settings and production cuts on MC calculations for ICs.
- To validate MC results against experimental data and established dosimetry protocols.
Main Methods:
- Implemented a Fano cavity test in Geant4 for protons to validate the condensed history (CH) algorithm.
- Tested various user-selectable parameters of the CH implementation in the EMStandardOpt4 physics-list.
- Calculated the overall factor fQ for detailed IC geometries (NACP-02, NE2571) using TOPAS.
Main Results:
- The Fano test confirmed the suitability of the EMStandardOpt4 physics-list with the WentzelIV model for IC calculations.
- Specific step-size limitations were required for isotropic proton sources to achieve high accuracy in the Fano test.
- Reduced production cuts in TOPAS yielded constant fQ values for bare air-filled cavities and agreement with published data.
- MC-calculated fQ values for detailed chamber models agreed with published data within statistical uncertainties (<0.3%).
- Hadronic scattering model selection impacted results by ~0.3% at higher energies.
Conclusions:
- Geant4/TOPAS MC code is reliable for ionization chamber (IC) calculations in proton dosimetry.
- Optimizing production cuts is essential for achieving accurate results comparable to detailed IC models and published data.
- MC calculations align with IAEA-TRS398 protocols, with an additional ~0.3% uncertainty from hadronic interactions.
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