Report on G4-Med, a Geant4 benchmarking system for medical physics applications developed by the Geant4 Medical Simulation Benchmarking Group.

BACKGROUND: Geant4 is a Monte Carlo code extensively used in medical physics for a wide range of applications, such as dosimetry, micro- and nanodosimetry, imaging, radiation protection, and nuclear medicine. Geant4 is continuously evolving, so it is crucial to have a system that benchmarks this Monte Carlo code for medical physics against reference data and to perform regression testing. AIMS: To respond to these needs, we developed G4-Med, a benchmarking and regression testing system of Geant4 for medical physics. MATERIALS AND METHODS: G4-Med currently includes 18 tests. They range from the benchmarking of fundamental physics quantities to the testing of Monte Carlo simulation setups typical of medical physics applications. Both electromagnetic and hadronic physics processes and models within the prebuilt Geant4 physics lists are tested. The tests included in G4-Med are executed on the CERN computing infrastructure via the use of the geant-val web application, developed at CERN for Geant4 testing. The physical observables can be compared to reference data for benchmarking and to results of previous Geant4 versions for regression testing purposes. RESULTS: This paper describes the tests included in G4-Med and shows the results derived from the benchmarking of Geant4 10.5 against reference data. DISCUSSION: Our results indicate that the Geant4 electromagnetic physics constructor G4EmStandardPhysics_option4 gives a good agreement with the reference data for all the tests. The QGSP_BIC_HP physics list provided an overall adequate description of the physics involved in hadron therapy, including proton and carbon ion therapy. New tests should be included in the next stage of the project to extend the benchmarking to other physical quantities and application scenarios of interest for medical physics. CONCLUSION: The results presented and discussed in this paper will aid users in tailoring physics lists to their particular application.

Influence of track structure and condensed history physics models of Geant4 to nanoscale electron transport in liquid water.

The Geant4 toolkit offers a range of electromagnetic (EM) models for simulating the transport of charged particles down to sub-keV energies. They can be divided to condensed-history (CH) models (like the Livermore and Penelope models) and the track-structure (TS) models included in the Geant4-DNA low-energy extension of Geant4. Although TS models are considered the state-of-the-art for nanoscale electron transport, they are difficult to develop, computationally intensive, and commonly tailored to a single medium (e.g., water) which prohibits their use in a wide range of applications. Thus, the use of CH models down to sub-keV energies is particularly intriguing in the context of general-purpose Monte Carlo codes. The aim of the present work is to compare the performance of the CH models of Geant4 against the recently implemented TS models of Geant4-DNA for nanoscale electron transport. Calculations are presented for two fundamental quantities, the dose-point-kernel and the microdosimetric lineal energy. The influence of user-defined simulation parameters (tracking and production cuts, and maximum step size) on the above calculations is also examined. It is shown that Livermore offers the best performance among the CH models of Geant4 for nanoscale electron transport. However, even under optimally-chosen simulation parameters, the differences between the CH and TS models examined may be sizeable for low energy electrons (<1 keV) and/or nanometer size targets (<100 nm).

ASSESSING THE CONTRIBUTION OF CROSS-SECTIONS TO THE UNCERTAINTY OF MONTE CARLO CALCULATIONS IN MICRO- AND NANODOSIMETRY.

Within EURADOS Working Group 6 'Computational Dosimetry', the micro and nanodosimetry task group 6.2 has recently conducted a Monte Carlo (MC) exercise open to participants around the world. The aim of this exercise is to quantify the contribution to the uncertainty of micro and nanodosimetric simulation results arising from the use of different electron-impact cross-sections, and hence physical models, employed by different MC codes (GEANT4-DNA, PENELOPE, MCNP6, FLUKA, NASIC and PHITS). Comparison of the participants' simulation results for both micro and nanodosimetric quantities using different MC codes was the first step of the exercise. The deviation between results is due to different cross-sections but also different tracking methods and particle transport cut-off energies. The second step of the exercise will involve using identical cross-section datasets to account only for the other variations in the first step, thus enabling the determination of the uncertainty contribution due to different cross-sections. This paper presents a comparison of the MC simulation results obtained in the first part of the exercise. For the microdosimetric simulations, particularly in the configuration where the electron source is contained within the micrometric target, the choice of MC code has a small influence on the results. For the nanodosimetric results, on the other hand, the mean ionisation cluster size distribution (ICSD) was sensitive to the physical models used in the MC codes. The ICSD was therefore chosen to study the influence of different cross-section data on the uncertainty of simulation results.

Geant4-DNA example applications for track structure simulations in liquid water: A report from the Geant4-DNA Project.

This Special Report presents a description of Geant4-DNA user applications dedicated to the simulation of track structures (TS) in liquid water and associated physical quantities (e.g., range, stopping power, mean free path…). These example applications are included in the Geant4 Monte Carlo toolkit and are available in open access. Each application is described and comparisons to recent international recommendations are shown (e.g., ICRU, MIRD), when available. The influence of physics models available in Geant4-DNA for the simulation of electron interactions in liquid water is discussed. Thanks to these applications, the authors show that the most recent sets of physics models available in Geant4-DNA (the so-called "option4" and "option 6" sets) enable more accurate simulation of stopping powers, dose point kernels, and W-values in liquid water, than the default set of models ("option 2") initially provided in Geant4-DNA. They also serve as reference applications for Geant4-DNA users interested in TS simulations.

Implementation of new physics models for low energy electrons in liquid water in Geant4-DNA.

A new alternative set of elastic and inelastic cross sections has been added to the very low energy extension of the Geant4 Monte Carlo simulation toolkit, Geant4-DNA, for the simulation of electron interactions in liquid water. These cross sections have been obtained from the CPA100 Monte Carlo track structure code, which has been a reference in the microdosimetry community for many years. They are compared to the default Geant4-DNA cross sections and show better agreement with published data. In order to verify the correct implementation of the CPA100 cross section models in Geant4-DNA, simulations of the number of interactions and ranges were performed using Geant4-DNA with this new set of models, and the results were compared with corresponding results from the original CPA100 code. Good agreement is observed between the implementations, with relative differences lower than 1% regardless of the incident electron energy. Useful quantities related to the deposited energy at the scale of the cell or the organ of interest for internal dosimetry, like dose point kernels, are also calculated using these new physics models. They are compared with results obtained using the well-known Penelope Monte Carlo code.

Track structure modeling in liquid water: A review of the Geant4-DNA very low energy extension of the Geant4 Monte Carlo simulation toolkit.

Understanding the fundamental mechanisms involved in the induction of biological damage by ionizing radiation remains a major challenge of today's radiobiology research. The Monte Carlo simulation of physical, physicochemical and chemical processes involved may provide a powerful tool for the simulation of early damage induction. The Geant4-DNA extension of the general purpose Monte Carlo Geant4 simulation toolkit aims to provide the scientific community with an open source access platform for the mechanistic simulation of such early damage. This paper presents the most recent review of the Geant4-DNA extension, as available to Geant4 users since June 2015 (release 10.2 Beta). In particular, the review includes the description of new physical models for the description of electron elastic and inelastic interactions in liquid water, as well as new examples dedicated to the simulation of physicochemical and chemical stages of water radiolysis. Several implementations of geometrical models of biological targets are presented as well, and the list of Geant4-DNA examples is described.

Dose point kernels in liquid water: an intra-comparison between GEANT4-DNA and a variety of Monte Carlo codes.

Modeling the radio-induced effects in biological medium still requires accurate physics models to describe the interactions induced by all the charged particles present in the irradiated medium in detail. These interactions include inelastic as well as elastic processes. To check the accuracy of the very low energy models recently implemented into the GEANT4 toolkit for modeling the electron slowing-down in liquid water, the simulation of electron dose point kernels remains the preferential test. In this context, we here report normalized radial dose profiles, for mono-energetic point sources, computed in liquid water by using the very low energy "GEANT4-DNA" physics processes available in the GEANT4 toolkit. In the present study, we report an extensive intra-comparison of profiles obtained by a large selection of existing and well-documented Monte-Carlo codes, namely, EGSnrc, PENELOPE, CPA100, FLUKA and MCNPX.

[Evaluation of a new table, Exact IGRT(Varian), on the treatment beam attenuation and the image quality]. / Evaluation d'une nouvelle table, Exact IGRT (Varian), sur l'atténuation du faisceau de traitement et la qualité des images.

PURPOSE: A new couch top designed for IGRT procedures, Exact IGRT (EIC), has been developped. It has the advantage of having no removable support bars and is thicker than a standard table (Exact(EC)) in order to create the appropriate rigidity. The aim of this study was to quantify the beam attenuation caused by the EIC and to evaluate the EIC contribution to on-board image quality improvement. MATERIALS AND METHODS: The treatment beam attenuation and the surface-dose of the patient were measured for simple radiation geometries at different gantry angles and for two clinical cases, head and neck and posterior pelvis. The 2D and 3D image quality was analysed. RESULTS: The beam attenuation at 215 degrees was 2.0% to 4.1% for 6 MV and 1.2% to 2.2% for 18 MV according to the EIC part. The dose-surface tripled between the measurement without table and with table into the beam (similar results between EIC and EC). The EIC influence was significative for the posterior pelvic tumor, acceptable for the head and neck case. The EIC showed a clear improvement of the low contrast resolution (visualisation of a 7mm and 1% contrast disk [15mm for the EC]). CONCLUSION: In order to take into account the beam attenuation, the EIC should be modeled in the treatment planning system. However, the good quality of 3D images obtained with the EIC allows us to easily visualize soft tissues.