Feasibility of using Geant4 Monte Carlo simulation for IMRT dose calculations for the Novalis Tx with a HD-120 multi-leaf collimator

Abstract

The aim of this study was to develop an independent dose verification system by using a Monte Carlo (MC) calculation method for intensity modulated radiation therapy (IMRT) conducted by using a Varian Novalis Tx (Varian Medical Systems, Palo Alto, CA, USA) equipped with a highdefinition multi-leaf collimator (HD-120 MLC). The Geant4 framework was used to implement a dose calculation system that accurately predicted the delivered dose. For this purpose, the Novalis Tx Linac head was modeled according to the specifications acquired from the manufacturer. Subsequently, MC simulations were performed by varying the mean energy, energy spread, and electron spot radius to determine optimum values of irradiation with 6-MV X-ray beams by using the Novalis Tx system. Computed percentage depth dose curves (PDDs) and lateral profiles were compared to the measurements obtained by using an ionization chamber (CC13). To validate the IMRT simulation by using the MC model we developed, we calculated a simple IMRT field and compared the result with the EBT3 film measurements in a water-equivalent solid phantom. Clinical cases, such as prostate cancer treatment plans, were then selected, and MC simulations were performed. The accuracy of the simulation was assessed against the EBT3 film measurements by using a gamma-index criterion. The optimal MC model parameters to specify the beam characteristics were a 6.8-MeV mean energy, a 0.5-MeV energy spread, and a 3-mm electron radius. The accuracy of these parameters was determined by comparison of MC simulations with measurements. The PDDs and the lateral profiles of the MC simulation deviated from the measurements by 1% and 2%, respectively, on average. The computed simple MLC fields agreed with the EBT3 measurements with a 95% passing rate with 3%/3-mm gamma-index criterion. Additionally, in applying our model to clinical IMRT plans, we found that the MC calculations and the EBT3 measurements agreed well with a passing rate of greater than 95% on average with a 3%/3-mm gamma-index criterion. In summary, the Novalis Tx Linac head equipped with a HD-120 MLC was successfully modeled by using a Geant4 platform, and the accuracy of the Geant4 platform was successfully validated by comparisons with measurements. The MC model we have developed can be a useful tool for pretreatment quality assurance of IMRT plans and for commissioning of radiotherapy treatment planning.

Development of a time-resolved mirrorless scintillation detector

“…In that study, the dose loss at a depth of 5.0 cm (source-axis distance, SAD: 100.0 cm) in a water phantom was approximately 2.25% for a 6 MV photon beam. Moreover, we calculated the effect of backscatter using a Monte Carlo simulation toolkit [32]. The energy of the medical linear accelerator (linac, Novalis Tx; Varian Medical Systems, CA, USA) was 6 MV, the field size was 10.0 × 10.0 cm 2 , the standard source-to-surface distance (SSD) was 100.0 cm, and the water…”

A convolution-superposition fluence model for the Siemens HD120 multi leaf collimator with application to a 3D VMAT dose engine

“…Purchasable software dose engines are by enlarge purpose-built for their application and not customizable or able to be integrated into software developed in-house, therefore are not an option. Currently, open-source Monte Carlo (MC) software [2,3] is the only fully customizable dose engine option for the HD120 since it can be incorporated into software developed in-house and accurately calculates the transmission, interleaf leakage, and tongue-and-groove effects for this MLC [4][5][6][7][8]. Unfortunately, unlike their expensive commercial MC counterparts such as SciMoCa (IBA Dosimetry, Schwarzenbruck, Germany), SureCalc (MIM Software, Cleveland, USA), and RayStation (Ray-Search, Stockholm, Sweden) which can calculate dose in minutes, open-source MC modulated dose calculation times can be prohibitive.…”

“…Unfortunately, unlike their expensive commercial MC counterparts such as SciMoCa (IBA Dosimetry, Schwarzenbruck, Germany), SureCalc (MIM Software, Cleveland, USA), and RayStation (Ray-Search, Stockholm, Sweden) which can calculate dose in minutes, open-source MC modulated dose calculation times can be prohibitive. Even with variance reduction techniques and with the use of multiple processors, open-source MC computation takes several hours [7,8], making calculations impractical especially in the clinical setting. Analytic calculation methods such as convolution-superposition and collapsed-cone (CC) techniques could be options for a quick-calculating dose engine, however an accurate fluence model of the HD120 is required as a starting point to implement these techniques.…”