Monte Carlo calculations of output factors for clinically shaped electorn fields

Abstract

We report on the use of EGS4/BEAM Monte Carlo technique to predict the output factors for clinically relevant, irregularly shaped inserts as they intercept the electron beams provided by a linear accelerator. The output factor for a particular combination; energy, cone, insert, and source to surface distance is defined in accordance with AAPM TG-25 as the product of cone correction factor and insert correction factor evaluated at the depth of maximum dose. Since cone correction factors are easily obtained we focus our investigation on the insert correction factors (ICF). An analysis of the inserts used in routine clinical practice resulted in the identification of a set of seven (7) ?idealized? shapes characterized by specific parameters. The ICF?s for these shapes were calculated using a Monte Carlo method (EGS4/BEAM) and measured for a subset of them using an ion chamber and well established measurement methods. Analytical models were developed to predict the MC calculated ICF values for various electron energy, cone size, shape and SSD. The goodness of fit between predicted and MC calculated ICF?s values were tested using the Kolmogorov-Smirnoff statistical test. Results show that MC calculated ICF?s match the measured values within 2.0% for most of the shapes considered, except for few highly elongated fields where deviations up to 4.0% were recorded. Predicted values based on analytical modeling agree with measured ICF?s values within 2-3% for all configurations. It is concluded that the predicted values of ICF?s based on modeling of MC calculated values could be introduced in clinical use.