International Journal of Advances in Applied Sciences (IJAAS) Vol. 14, No. 1, March 2025, pp. 227~234 ISSN: 2252-8814, DOI: 10.11591/ijaas.v14.i1.pp227-234  227 Journal homepage: http://ijaas.iaescore.com Evaluation of 6 MV photon beam characteristics on Varian Clinac iX: a Monte Carlo study Josua Timotius Manik 1 , Anisza Okselia 2 , Junios 3 , Angelica Margaretha 1 , Freddy Haryanto 4 1 Department of Physics, Faculty of Science Computer and Mathematics, Matana University, Tangerang, Indonesia 2 Department of Radiotherapy, Dr. Hasan Sadikin Central General Hospital, Bandung, Indonesia 3 Department of Medical Informatics, Faculty of Science Social and Education, Nusantara Prima University, Bukittinggi, Indonesia 4 Departement of Physics, Faculty of Mathematics and Natural Sciences, Bandung Institute of Technology, Bandung, Indonesia Article Info ABSTRACT Article history: Received May 31, 2024 Revised Dec 24, 2024 Accepted Jan 26, 2025 This work aims to study the characteristics of photon beams through phase space file (PSF) analysis using Monte Carlo (MC) simulation. 6 MV photon beams from the Varian Clinac iX were simulated using PRIMO software. The beam parameters were validated by evaluating the percentage depth dose and dose profile. A full PSF was scored at the downstream end of the linear accelerator (LINAC) upper and lower parts and analyzed to determine the beam fluence profile, energy fluence profile, angular distribution, and spectral distribution. The results show that within PSF 1, the photon beam has an average scattering angle of 10.74° and a mean energy of 1.18 MeV. In PSF 2, the average scattering angle decreases to 2.63° while the mean energy increases to 1.50 MeV. The field size variation at 20×20, 30×30, and 40×40 cm 2 affects both the angular and spectral distribution of the photon beam. The photon beam in PSF 2 exhibits an average scattering angle of 4.56, 6.31, and 6.66°, with corresponding mean energy values of 1.40, 1.32, and 1.30 MeV, respectively. These findings show that as the field size increases, the photon beam scatters at a larger angle while the energy decreases. Keywords: Angular distribution Beam characteristic Monte Carlo Phase space file Spectral distribution This is an open access article under the CC BY-SA license. Corresponding Author: Josua Timotius Manik Department of Physics, Faculty of Science Computer and Mathematics, Matana University St. CBD Barat Kav, Tangerang, Banten-15810, Indonesia Email: josua.timotius@matanauniversity.ac.id 1. INTRODUCTION Radiotherapy stands as the main modality for treating cancer, utilizing ionizing radiation such as electrons, photons, and protons to deposit the maximum dose to the target cancer cells while minimizing dose to the surrounding organ at risk (OAR). Dose calculation requires information about beam characteristics, including beam profile and its spectra, which can be obtained through Monte Carlo (MC) simulation [1]–[3]. In medical physics, the MC method is applied to simulate particle interaction stochastically in radiation transport. MC simulation for photon beam transport begins with generating the primary photon with a predefined initial state such as a position, energy, and direction of motion. The primary particle then follows its trajectory, interacting with the materials it traverses, such as air and the components of the linear accelerator (LINAC) head, through processes including the photoelectric effect, Compton scattering, and pair production. There interactions produce secondary particles, which are simulated in the same way as the primaries. The MC dose calculation technique offers the advantage of completely decoupling dose calculation within the phantom or computed tomography (CT) from the treatment head model.