Flagged uniform particle splitting for variance reduction in proton and carbon ion track-structure simulations

Abstract

Flagged uniform particle splitting was implemented with two methods to improve the computational efficiency of Monte Carlo track structure simulations with TOPAS-nBio by enhancing the production of secondary electrons in ionization events. In Method 1 the Geant4 kernel was modified. In Method 2 Geant4 was not modified. In both methods a unique flag number assigned to each new split electron was inherited by its progeny, permitting reclassification of the split events as if produced by independent histories. Computational efficiency and accuracy were evaluated for simulations of 0.5–20 MeV protons and 1–20 MeV/u carbon ions for three endpoints: (1) mean of the ionization cluster size distribution, (2) mean number of DNA single-strand breaks (SSBs) and double-strand breaks (DSBs) classified with DBSCAN, and (3) mean number of SSBs and DSBs classified with a geometry-based algorithm. For endpoint (1), simulation efficiency was 3 times lower when splitting electrons generated by direct ionization events of primary particles than when splitting electrons generated by the first ionization events of secondary electrons. The latter technique was selected for further investigation. The following results are for Method 2, with relative efficiencies about 4.5 times lower for Method 1. For endpoint (1), relative efficiency at 128 split electrons approached maximum, increasing with energy from 47.2±0.2 to 66.9±0.2 for protons, decreasing with energy from 51.3±0.4 to 41.7±0.2 for carbon. For endpoint (2), relative efficiency increased with energy, from 20.7±0.1 to 50.2±0.3 for protons, 15.6±0.1 to 20.2±0.1 for carbon. For endpoint (3) relative efficiency increased with energy, from 31.0±0.2 to 58.2±0.4 for protons, 23.9±0.1 to 26.2±0.2 for carbon. Simulation results with and without splitting agreed within 1% (2 standard deviations) for endpoints (1) and (2), within 2% (1 standard deviation) for endpoint (3). In conclusion, standard particle splitting variance reduction techniques can be successfully implemented in Monte Carlo track structure codes.

Monte Carlo simulation of chemistry following radiolysis with TOPAS-nBio

“…This can be an obstacle for users that require different configurations, including the onerous requirement of rebuilding the application for each configuration. TOPAS-nBio provides the means to easily configure track structure simulations, access to scoring information at different stages along the simulation, variance reduction techniques (Ramos-Méndez et al 2017) and complex geometries at micrometer and nanometer scales (McNamara et al 2017), making TOPAS with the TOPAS-nBio extension a powerful yet easy-to-use MC platform for track structure simulations. In this work, TOPAS-nBio has been extended to allow the user to easily configure parameters for water radiolysis simulation in a clear way that mitigates user error and allows the user to perform regression tests when updating to the latest version.…”

“…To help circumvent the need for specialized expertise in programming and Geant4, the TOPAS tool (Perl et al 2012) wraps and extends Geant4, allowing a much wider group access to the modeling of complex radiotherapy applications without the need for advanced programming skills. The TOPAS-nBio project aims to extend TOPAS to facilitate the configuration of track structure simulations by providing an interface to the Geant4-DNA physics and chemistry processes as well as the use of complex radiobiological geometries (McNamara et al 2017), variance reduction techniques (Ramos-Méndez et al 2017) and scorers for radiobiological quantities of interest. These features are compatible with advanced TOPAS capabilities such as 4D simulation (Shin et al 2012) and advanced Geant4 capabilities such as multithreading (Dong et al 2012), both critical to full water radiolysis simulation.…”

Geometrical structures for radiation biology research as implemented in the TOPAS-nBio toolkit

“…TOPASnBio provides users with a catalogue of specialised biological geometries and a flexible interface to extend the reactions involved in non-homogeneous chemistry simulations (Ramos-Mêndez et al 2018). TOPAS-nBio also offers specialised scorers for biological damage (e.g., double strand breaks in DNA) and includes a set of variance reduction techniques specific to biological modelling (Ramos-Mêndez et al 2017).…”

Radiobiological impact of gadolinium neutron capture from proton therapy and alternative neutron sources using TOPAS‐nBio

“…() can be established to determine the parameter, where and are the linear quadratic parameters for the reference radiation for the human salivary gland (HSG) cell‐line, is the dose‐mean lineal energy, is the mean chord length and m is the mass domain of the site. A default 10% cell survival relative to 200 kVp x‐rays for HSG cells were utilized for the RBE calculations within this method 43 . The parameter is quantified within the TOPAS MC code from the input of the other parameters.…”