Monte Carlo Evaluation of the Impact of Flattening Filter Removal and Collimator on the Energy Properties of a 6 MV Photon Beam Using GATE

I. Lagrini (LSIB Laboratory, Faculty of Sciences and Technologies Mohammedia, Hassan II University of Casablanca, BP 146, Mohammedia 28806, Morocco)
M. Bencheikh (LSIB Laboratory, Faculty of Sciences and Technologies Mohammedia, Hassan II University of Casablanca, BP 146, Mohammedia 28806, Morocco)
I. Maouhoubi (Solid Physics Laboratory, Group of Nanomaterial and Renewable Energies, Department of Physics, Faculty of Sciences Dhar El Mahraz, Sidi Mohamed Ben Abdellah University, BP. 1796, Fez, Morocco)
A. Khallouqi (Laboratory of Health Sciences and Technologies, Higher Institute of Health Sciences, Hassan 1st University, BP 539, Settat, Morocco)
H. Sekkat (Sciences and Engineering of Biomedicals, Biophysics and Health Laboratory, Higher Institute of Health Sciences, Hassan First University, Settat 26000, Morocco)
O. Berradi (Laboratory of Applied Physics, Computer Science and Statistics, Faculty of Sciences Dhar El Mahraz, Sidi Mohamed Ben Abdellah University, BP. 1796, Fez, Morocco)
M. Bougtib (Department of Radiation Oncology, University Hassan II, Hospital of Fez, Fez, Morocco)

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Study aims to evaluate the impact of removing the Flattening Filter (FF) and the secondary collimator on the energetic properties of a 6 MV photon beam. The Monte Carlo (MC) simulation platform GATE/Geant4 (v9.3) was used to evaluate particle fluence, energy fluence distribution, and energy deposited at the phantom surface with a 10 × 10 cm² field size and a Source-to-Surface Distance (SSD) of 100 cm, as well as the production of secondary particles. Validation using Varian Phase-Space (PS) data showed excellent agreement, with gamma pass rates of 99% for Percentage Depth Dose (PDD) and 98% for lateral profiles at depths of 5 and 10 cm, based on a 2% - 2 mm gamma index criterion. Beam quality parameters (TPR20/10) and Relative Dose Differences (RDD) confirmed a margin of error of less than 2%, validating the accuracy of the geometric simulation model. Removing the FF significantly increased the energy fluence of the particles responsible for energy deposition on the surface of the water phantom, increasing relatively by 136% for photons and 55% for electrons in the energy ranges of 0.49 - 0.525 MeV and 0.28 - 0.315 MeV, respectively. Moreover, the secondary collimator contributes significantly to the production of secondary particles that affect the dose distribution and beam energy properties.