Background: It is well known that the main components of clinical linac such as the primary collimator

flattening filter and secondary collimator affect the X-ray energy spectra. How does it appear is the foundation of dose calculation

and it is helpful for building up a beam model. Purpose: This paper aims to make a detailed study on the effects of these main components on the X-ray energy spectra of a clinical linac beam. Methods: Monte Carlo program EGSnrc/BEAMnrc is employed to build up the beam model of Varian 600C linac

as well as the other three different beam models: without primary collimator

without flattening filter

without secondary collimator at the same time. After respective simulations

we use Beamdp program to analyze the phase space files

so as to obtain energy spectra for different models. Results: Flattening filter has heavy influence on X-ray beam’s energy spectra

the mean energy increases by 48.9% compared with its absence. But the primary collimator and secondary collimator have little influence on energy spectra

the change of mean energy is within 1% compared their absence. In the center field size of 10 cm×10 cm

flattening filter could let the energy fluence decrease by 52.4%

but the distribution becomes more flat and the unflatness changes from 15.09% to 1.82%; primary collimator could increase energy fluence by 1.58%; but secondary collimator could decrease the energy fluence by 2.00%. Conclusion: Flattening filter has mainly influence on the energy spectra of X-ray beam

it could let the mean energy increase and the energy fluence distribution become more flat; the scatter of primary collimator for X-ray beam must be taken into account whilst the scatter of secondary collimator for X-ray beam can be neglected. The results are meaningful to build a X-ray beam model for clinical linac. Abstract