Investigations of parameters affecting the photon energy spectra of an 18 MV Varian 2100C/D linear accelerator

Radiotherapy using linear accelerators is known as an effective modality for cancer treatment. The photons energy of treatment beams significantly affect the dose distribution. Therefore, it is important to accurately evaluate the photon energy spectra. In this study, MCNPX Monte Carlo code (version 2.6.0) was used to simulate an 18 MV photon beam of a Varian 2100C/D linear accelerator. By matching computed and measured percent depth and profile doses, the optimum values of mean energy and full width at half maximum (FWHM) of the radial distribution of beam were found to be 18.5 MeV and 0.14 cm, respectively. The simulation was also used to investigate the impact of parameters, such as depth, source-to-surface distances (SSD), field size, flattening filter material and geometry of treatment head components on the photon spectra. The results showed that the photon spectra were decreased as an exponential function by increasing depth in phantom and SSD. Results also indicated that the photon spectra depend on the Z of the flattening filter materials. Photon spectra for low-Z materials, such as Al, were significantly increased (up to 31.6%) in comparison with using the original material due to the decrease in the photon absorption cross-section. Each component of the linac head has a different effect on the photon spectrum due to its material and special shape. Based on the obtained results, primary collimator and MLC have, respectively, maximum and minimum effect on the mean energy of photons. Moreover, photon spectra were changed considerably with field size. Change in the photon spectra up to 28.3% was obtained when using 40 × 40 cm2 field size compared to the 5 × 5 cm2 because of the increased scatter from the collimator and the phantom.