An MCNP-based model of a linear accelerator x-ray beam

The Monte Carlo N-Particle radiation transport computer code (MCNP) has bee n employed on a personal computer to develop a simple model simulating the major components within the beam path of a linear accelerator radiation hea d, namely the electron target, primary conical collimator, beam flattening filter, wedge filter and the secondary collimators. The model was initially used to calculate the energy spectra and angular distributions of the x-ra y beam for the Philips SL 75/5 linear accelerator, in a plane immediately b eneath the flattening filter. These data were subsequently used as a 'sourc e' of x-rays at the target position, to assess the emergent beam from the s econdary collimators. The depth dose distributions and dose profiles at con stant depth for Various field sizes have been calculated for a nominal oper ating potential of 4 MV and found to be within acceptable limits. It is con cluded that the technique may be used to calculate the energy spectra of an y linear accelerator upon specification of the component dimensions, materi als and nominal accelerating potential. It is anticipated that this work wi ll serve as the basis of a quality control tool for Linear accelerators and treatment planning systems.