A semi-empirical method for calculating the effect of air gap space on output factors on electron beam dose distributions

Background: A simple approach to calculate the effect of air gap on output factors on electron beam dose distribution is presented. Methods: The method accounts for variations of pencil beam parameters using a model developed by Bruinvis et al. [4,5]. The evaluation of this method is based on measurements of the output factors at various distances between the final collimating device and the phantom surface. Results: Comparison of calculations and measurements of output factors for various cone sizes and 0, 2 and 4 cm air gaps show agreement to within approximately 1.5% for electron energies of 6-13.5 MeV and field sizes of 5.3-10 cm in diameter. Conclusion: The accuracy of this semi-empirical method can be considered clinically acceptable and reduces the amount of experimental work needed. (C) 1998 Elsevier Science Ireland Ltd.