Monte Carlo simulation of 6‐MV dynamic wave VMAT deliveries by Vero4DRT linear accelerator using EGSnrc moving sources

Abstract

The commissioning and benchmark of a Monte Carlo (MC) model of the 6-MV Brainlab-Mitsubishi Vero4DRT linear accelerator for the purpose of quality assurance of clinical dynamic wave arc (DWA) treatment plans is reported. Open-source MC applications based on EGSnrc particle transport codes are used to simulate the medical linear accelerator head components. Complex radiotherapy irradiations can be simulated in a single MC run using a shared library format combined with BEAMnrc "source20." Electron energy tuning is achieved by comparing measured vs simulated percentage depth doses (PDDs) for MLC-defined field sizes in a water phantom. Electron spot size tuning is achieved by comparing measured and simulated inplane and crossplane beam profiles. DWA treatment plans generated from RayStation (RaySearch) treatment planning system (TPS) are simulated on voxelized (2.5 mm 3) patient CT datasets. Planning target volume (PTV) and organs at risk (OAR) dose-volume histograms (DVHs) are compared to TPS-calculated doses for clinically deliverable dynamic volumetric modulated arc therapy (VMAT) trajectories. MC simulations with an electron beam energy of 5.9 MeV and spot size FWHM of 1.9 mm had the closest agreement with measurement. DWA beam deliveries simulated on patient CT datasets results in DVH agreement with TPS-calculated doses. PTV coverage agreed within 0.1% and OAR max doses (to 0.035 cc volume) agreed within 1 Gy. This MC model can be used as an independent dose calculation from the TPS and as a quality assurance tool for complex, dynamic radiotherapy treatment deliveries. Full patient CT treatment simulations are performed in a single Monte Carlo run in 23 min. Simulations are run in parallel using the Condor High-Throughput Computing software 1 on a cluster of eight servers. Each server has two physical processors (Intel Xeon CPU E5-2650 0 @2.00 GHz), with 8 cores per CPU and two threads per core for 256 calculation nodes.

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“…An MC model of the Vero4DRT has been built in-house and validated for dose distribution simulations without accounting for DTT beam panning and tilting motions. [ 14 15 ] The EGSnrc/BEAMnrc code was used to simulate the 6MV photon beams. [ 16 17 ] To obtain a dose calculation uncertainty of <2%, 1.5 × 10 9 electrons were incident on the Vero4DRT target, and the resulting bremsstrahlung photons and scattered electrons were transported through the linac head, including the primary collimator, flattening filter, ionization chambers, and stationary secondary collimators.…”

“…Particles were then directed through the MLCs and the patient’s CT image using a shared library format of BEAMnrc code combined with “source20” of DOSXYZnrc code. [ 14 15 ] The photon cutoff energy used was 0.01 MeV and the electron cutoff energy was 0.521 MeV. All simulations were conducted using the Condor High-Throughput Computing software[ 19 ] on a cluster with eight servers with the Red Hat Enterprise Linux operating system (v. 6.4).…”