Determination and validation of the initial beam parameters of Elekta Agility collimator head by Monte Carlo simulations

The availability of geometrical, physical, and initial beam parameters for Monte Carlo (MC) simulations of the Elekta Agility collimator head has become very difficult due to the proprietary nature of this data. This study presents strategies to independently determine the geometrical and physical properties of the components and initial beam parameters of the Agility collimator head for full beam simulations and postulates a benchmarking process using the EGSnrc MC toolkit. Target material of W (90%) and Re (10%) of 0.09 cm thickness, flattening filter of 1.77 cm thick stainless steel placed on 0.5 cm Al disc, and primary and secondary collimators of Tungsten alloy have been found to best fit the Agility head. The initial beam energy of 6.0 MeV with a radial distribution given as full-width half maxima (FWHM) of 0.301 cm (crossline) x 0.201 cm (inline) for 6 MV beam with a mean angular spread of 1.34 degrees has been found best fitting the model. Variations of 0.29% and 0.59% have been noted in the measured and calculated values of TPR20,10 and D-10 respectively. More than 90% dose points for all simulations passed the 2D gamma criteria of 3% DD, 3 mm DTA. This MC model of the Agility head can be used for dose calculation and validation of advanced treatment techniques.