Introduction: Monte Carlo method is often applied in radiation therapy as utilized in all the branches of science. An important requirement for successful radiotherapy is carefully examine the dose distribution specifications and decrease the difference between these features with experience to an acceptable level. In this study, the characteristics of 6, 15 and 20 MeV incident x-rays are provided for 10×10 cm2 fields. This information is necessary to achieve an accurate treatment planning.
Materials and Methods: The MCNP code is employed to simulate 6 MeV photon beam from a major manufacturer of a Varian linac accelerator. The used phantom in this research was water. The output data charts have illustrated, by two software origin and table curve. Results: In order to validate the performance and accuracy of the simulated head, the parameters associated with 5×5 cm2 field were compared to empirical values. In this study, 6, 15, 20 MeV (6, 15, 20 photon modes) energy spectra, percentage depth dose, beam profile are calculated. The comparisons between calculated and experimental results show good agreement (with ±4%). Conclusion: The MCNP code has a suitable precision in calculation of dose distribution for these modes of Varian. There is a good agreement between empirical and simulation result for 6 MV photon beam for both percentage depth dose and beam profiles. For the next two modes, comparing results with references indicates that the results are acceptable as well as other applications in radiotherapy.