Validation of a virtual source model of medical linac for Monte Carlo dose calculation using multi-threaded Geant4

Abstract

The use of phase space in medical linear accelerator Monte Carlo (MC) simulations significantly improves the execution time and leads to results comparable to those obtained from full calculations. The classical representation of phase space stores directly the information of millions of particles, producing bulky files. This paper presents a virtual source model (VSM) based on a reconstruction algorithm, taking as input a compressed file of roughly 800 kb derived from phase space data freely available in the International Atomic Energy Agency (IAEA) database. This VSM includes two main components; primary and scattered particle sources, with a specific reconstruction method developed for each. Energy spectra and other relevant variables were extracted from IAEA phase space and stored in the input description data file for both sources. The VSM was validated for three photon beams: Elekta Precise 6 MV/10 MV and a Varian TrueBeam 6 MV. Extensive calculations in water and comparisons between dose distributions of the VSM and IAEA phase space were performed to estimate the VSM precision. The Geant4 MC toolkit in multi-threaded mode (Geant4-[mt]) was used for fast dose calculations and optimized memory use. Four field configurations were chosen for dose calculation validation to test field size and symmetry effects, [Formula: see text] [Formula: see text], [Formula: see text] [Formula: see text], and [Formula: see text] [Formula: see text] for squared fields, and [Formula: see text] [Formula: see text] for an asymmetric rectangular field. Good agreement in terms of [Formula: see text] formalism, for 3%/3 mm and 2%/3 mm criteria, for each evaluated radiation field and photon beam was obtained within a computation time of 60 h on a single WorkStation for a 3 mm voxel matrix. Analyzing the VSM's precision in high dose gradient regions, using the distance to agreement concept (DTA), showed also satisfactory results. In all investigated cases, the mean DTA was less than 1 mm in build-up and penumbra regions. In regards to calculation efficiency, the event processing speed is six times faster using Geant4-[mt] compared to sequential Geant4, when running the same simulation code for both. The developed VSM for 6 MV/10 MV beams widely used, is a general concept easy to adapt in order to reconstruct comparable beam qualities for various linac configurations, facilitating its integration for MC treatment planning purposes.

A phase space model of a Versa HD linear accelerator for application to Monte Carlo dose calculation in a real‐time adaptive workflow

“…They also demonstrate good agreement between dose calculations resulting from the virtual source model and from the standard phase space, when using GEANT4 as the Monte Carlo engine. Their work 17 is for rectangular fields, and a subsequent report 18 describes the incorporation of a multileaf collimator into the virtual source model. Compared to these studies, the method in the current paper has the advantage of being related to a clinically commissioned commercial treatment planning system.…”

Developing a New Monte Carlo Algorithm as an Alternative Tool to Simulate Virtual Source Model on an Elekta Versa Hd Linac

“…Considered among the most accurate methods of calculating radiation dose in the radiotherapy, Monte Carlo (MC) simulation is currently a common method for benchmarking the other dose calculation methods in homogeneous and inhomogeneous cases. Some MC codes, including EGSnrc, MCSP, MCNP, Penelope, Geant4 and etc., are widely used in medical physics each coming with a number of pros and cons related to the simulation time as well as output of physical factors obtained at the end of the simulation (18,34). Every time experimental measurements are not possible, simulation studies are critically taken advantage of so that the physical factors are understood.…”

Development of a virtual source model for Monte Carlo‐based independent dose calculation for varian linac

“…A common solution to ease the I/O burden is to develop so-called virtual source models (VSMs), [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] which approximate the behavior of particles at the phase-space surface. VSMs have two distinct advantages: they take up significantly less disk space and allow for infinite sampling.…”

“…Numerous studies have been performed to generate VSM for a wide variety of radiotherapy treatment options; each was derived using either experimental measurements 7,9 or previously-generated PSF files. 5,6,8,[10][11][12][13][14][15][16][17][18][19][20][21] In this study, a generalized method to derive VSM from International Atomic Energy Agency (IAEA)-compliant PSF is introduced. Several VSMs were generated from PSF for the Varian (Varian Medical Systems, Palo Alto, CA) TrueBeam medical linear accelerator (Linac) for varying electron beam energies and treatment beam flattening configurations.…”

“…Particle parameters from the PSF must be modeled precisely for the VSM to yield quality simulations. Numerous studies have been performed to generate VSM for a wide variety of radiotherapy treatment options; each was derived using either experimental measurements 7,9 or previously‐generated PSF files 5,6,8,10–21 …”