Monte Carlo simulation of NovalisTx linear accelerator using GATE/Geant4 code for dosimetry analysis

External radiotherapy is one of the most efficient tools for cancer treatments. This method uses intense radiation in the form of a prescribed dose emitted by a medical linear accelerator in order to sterilize the tumor while minimizing the dose received by the normal surrounding healthy tissues and nearby organs at risk. Monte Carlo simulation is generally used for studying the physical process of radiation-matter interaction. It is widely used in radiotherapy for dose calculation, which achieves realistic accuracy even in complex cases. In the present work, the 6 MV beam from the NovalisTx™ accelerator was modeled by the GATE/Geant4 v8.2 code in a homogeneous water phantom. The simulation has been performed by applying the phase space and the variance reduction techniques to speed up the calculation time without losing accuracy. The percentage depth dose and transverse profiles for different field sizes from 3x3 to 20x20 cm2 at 100 cm from the source were calculated. The simulation results were analyzed by the ROOT platform. We obtained good agreement between the measured and calculated dose distributions for all field sizes as the mean dose error was less than 1% and more than 98% of the points passed the 2%/2mm gamma index criterion.