Monte Carlo simulation of treatment machine Varian Clinac iX

Abstract

This report presents methods and results from the development of a Monte Carlo model of a Varian Clinac iX linear accelerator of nominal energy 6 MV . The simulations are made by the BEAMnrc/EGSnrc Monte Carlo code package [1]. The model is adjusted for open fields. Wedge fields can be simulated by step-and-shoot method. In a future work, the model will be further developed by including MLC component. This report describes in detail how model parameters are optimised and how the quality of the model is verified. The parameters adjusted in the model are the energy of the electrons (monoenergetic) incident (normally) on the target as well as the width of the spatial distribution of the electrons (assumed to be Gaussian). The accelerator head is simulated in one step and the dose distribution in water is calculated in a subsequent step. Simulated data are compared to measured data visually, quantatively by directly comparing the numbers and by statistically weighting the di erences in a chi2=NDF analysis. The optimum parameter set is found to be 5.7 MeV and 0.1 cm (FWHM). The agreement between measured and simulated data is found to be good. The measured and simulated data agreed to within 1% except in the case of depth dose for a 2x2 cm2 field and for profiles at dose maximum.