seyed Mojtaba Karbalaee - M.Sc of medical physics
Mitra alsadat Karbalaee - M.Sc of medical physics
Mohammad reza Kheirolah bayatiany - M.Sc of medical physics
Fatemeh Seif - M.Sc of medical physics

Introduction: A linear accelerator (LINAC) is the device most commonly used for external beam radiation therapy for patients whit cancer. The medical radiation physicist and the dosimetrist, determine how to deliver the prescribed dose, calculate the amount of time it will take the accelerator to deliver that dose and evaluate quality and quantity of the beam. It must to be known information about beam quality and it's energy spectrum. This information can be obtained from Percentage Depth Dose (PDD) curves. Direct measurement of the beam energy spectrum is very difficult. Monte-Carlo stimulation is the best method to analyzing nuclear process. This method uses microscopic physical phenomena to approach macroscopic and measurable values such as PDD. Method: This model was used to simulate x-ray beam that comes from 6Mv Varian 2100C/D linac head. After measuring the geometric parameters of all parts in linac head, model was written in EGS5 code in Fortran99 language. The model was tested for 109 history, 10x10 field size, SSD=100cm at 50x50x50 water phantom. After comparing the results of this model with experimental data and measuring the agreement, photon spectrum was measured. Results: the calculation was shown to agree with experimental data at the 0.7% to 2.1%. The most probable photon energy is less than 1Mev

Monte Carlo, EGS code, linear accelerator