Monte Carlo simulation of a 6 MV Varian Trilogy clinical linear accelerator using Geant4 for small-field dosimetry analysis

In recent decades, radiotherapy has advanced significantly due to the emergence of advanced clinical linear accelerators, enabling safer and faster treatments with reduced side effects. In this study, we present a Monte Carlo simulation of a 6 MV Varian Trilogy clinical linear accelerator using the Geant4 toolkit to analyze dosimetric parameters, with a focus on small photon fields. The simulation models phase-space files generated at key beam-modification stages. Validation was performed by comparing simulated depth dose curves (PDD) and dose profiles with experimental measurements for the 6 MV photon beam, employing the gamma index (3%/3 mm criteria) to quantify the degree of agreement in the PDDs. The results agreed within a 2% difference, with over 95% of points passing the gamma analysis for a 10 x 10 cm² field. The developed framework allows for the calculation of correction factors for small-field dosimetry, addressing challenges such as lateral electronic disequilibrium in high-energy 6 MV beams, and comparing them with established values in the literature. These capabilities are particularly important for measurements of small fields used in advanced radiotherapy techniques, including Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT), where high precision in dose deposition is essential. Overall, this work establishes a robust and flexible Geant4-based platform for simulating clinical linear accelerators, contributing to the development of more accurate dosimetric protocols for high-energy photon therapy in small fields. Preliminary observations suggest notable output factor discrepancies in small photon fields (<3x3 cm2), indicating the potential need for detector-specific correction factors. Ongoing analysis aims to confirm these trends.