Investigation of the effect of flattening filters on photon spectra in Medical Linac with MCNP 6.2

Medical linear accelerators (LINAC) are one of the main means of dose delivery in external beam radiotherapy. Monte Carlo simulation is widely accepted as a reliable method in designing and optimization of LINACS as well as treatment planning. In this research we used MCNP 6.2 general use radiation transport code to design a 12 MV LINAC with detailed parts including target to produce bremsstrahlung, flattening filter (FF), ion chamber, primary collimator, and secondary collimators with adjustable X and Y jaws. With the help of the MCNP 6.2 program used in the design of the linear accelerator, a system of optimizing the linear accelerator components for different energy ranges in the modeling was realized.We investigated the effects of different FF materials in resulting photon spectra, percentage depth dose and electron contamination in a cubic water phantom with side length of 40 cm, at a 100 cm source to surface distance and 10×10 cm2 field size. Flattening filter materials of Al, SS, Sn, Ta and Hg were used as well as a flattening filter free (FFF) system. Keywords: Photon, Monte Carlo, Linear Accelerator and Water Phantom.