Abstract
Geant4 is a multi-purpose Monte Carlo simulation tool for modeling particle transport in matter. It provides a wide range of settings, which the user may optimize for their specific application. This study investigates GATE/Geant4 parameter settings for proton pencil beam scanning therapy. GATE8.1/Geant4.10.3.p03 (matching the versions used in GATE-RTion1.0) simulations were performed with a set of prebuilt Geant4 physics lists (QGSP_BIC, QGSP_BIC_EMY, QGSP_BIC_EMZ, QGSP_BIC_HP_EMZ), using 0.1mm-10mm as production cuts on secondary particles (electrons, photons, positrons) and varying the maximum step size of protons (0.1mm, 1mm, none). The results of the simulations were compared to measurement data taken during clinical patient specific quality assurance at The Christie NHS Foundation Trust pencil beam scanning proton therapy facility. Additionally, the influence of simulation settings was quantified in a realistic patient anatomy based on computer tomography (CT) scans. When comparing the different physics lists, only the results (ranges in water) obtained with QGSP_BIC (G4EMStandardPhysics_Option0) depend on the maximum step size. There is clinically negligible difference in the target region when using High Precision neutron models (HP) for dose calculations. The EMZ electromagnetic constructor provides a closer agreement (within 0.35mm) to measured beam sizes in air, but yields up to 20% longer execution times compared to the EMY electromagnetic constructor (maximum beam size difference 0.79mm). The impact of this on patient-specific quality assurance simulations is clinically negligible, with a 97% average 2%/2mm gamma pass rate for both physics lists. However, when considering the CT-based patient model, dose deviations up to 2.4% are observed. Production cuts do not substantially influence dosimetric results in solid water, but lead to dose differences of up to 4.1% in the patient CT. Small (compared to voxel size) production cuts increase execution times by factors of 5 (solid water) and 2 (patient CT). Taking both efficiency and dose accuracy into account and considering voxel sizes with 2mm linear size, the authors recommend the following Geant4 settings to simulate patient specific quality assurance measurements: No step limiter on proton tracks; production cuts of 1mm for electrons, photons and positrons (in the phantom and range-shifter) and 10mm (world); best agreement to measurement data was found for QGSP_BIC_EMZ reference physics list at the cost of 20% increased execution times compared to QGSP_BIC_EMY. For simulations considering the patient CT model, the following settings are recommended: No step limiter on proton tracks; production cuts of 1mm for electrons, photons and positrons (phantom/range-shifter) and 10mm (world) if the goal is to achieve sufficient dosimetric accuracy to ensure that a plan is clinically safe; or 0.1mm (phantom/range-shifter) and 1mm (world) if higher dosimetric accuracy is needed (increasing execution times by a factor of 2); most accurate results expected for QGSP_BIC_EMZ reference physics list, at the cost of 10-20% increased execution times compared to QGSP_BIC_EMY.
Highlights
Geant[4] is a C++ based, object-oriented, multi-purpose Monte Carlo (MC) tool to simulate particle transport for energies ranging from eV to TeV scale,[1,2] with applications ranging from the modeling of high energy particle colliders to space and shielding simulations.[3]
A set of in-house developed scripts (AUTOMC22) written in GNU Octave[23] (v.4.4.1), which automatically create GATE macro files, launch simulations and analyse simulation results, have been developed for clinical proton therapy dose calculations at The Christie NHS Foundation Trust. This system performs all its MC calculations using GATE v.8.1/ Geant[4] v.10.3.3.2 These are the same versions included in GATE-RTion v.1.0,12 and in keeping with the goals of GATE-RTion, this work aims to provide recommendations in terms of simulation parameters to adopt for proton pencil beam scanning
Simulations performed with the EMY and EMZ did not depend on the step limiter, whereas for simulations performed with Option[0] energies varied by up to 0.5 MeV, indicating that for this physics option the range in water depends on the step limiter
Summary
Geant[4] is a C++ based, object-oriented, multi-purpose Monte Carlo (MC) tool to simulate particle transport for energies ranging from eV to TeV scale,[1,2] with applications ranging from the modeling of high energy particle colliders to space and shielding simulations.[3]. [4–6] for proton radiotherapy simulations using Geant[4] or Geant[4] wrappers). This study is focused on GATE/Geant[4] simulations for patient dosimetry in proton pencil beam scanning. Geant[4] based Architecture for Medicine Oriented Simulation (GAMOS),[7] TOol for PArticle Simulation (TOPAS)[8] and Geant[4] Application for Tomographic Emission (GATE)[9,10,11] provide user-friendly interfaces to Geant[4]. This research takes place within a recent initiative called GATE-RTion,[12] which is aimed at providing a validated long-term application, based on a GATE and Geant[4] version, along with associated software tools for clinical dosimetric implementation to facilitate collaborations between hadron therapy institutes. Current collaborators include the Centre Antoine Lacassagne (Nice, France), the Christie NHS Foundation Trust (Manchester, UK) and MedAustron (Wiener Neustadt, Austria)
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