The 'equivalent wedge' implementation of the Varian Enhanced Dynamic Wedge (EDW) into a treatment planning system

The purpose of this work was to establish procedures for the implementation of the Varian Enhanced Dynamic Wedge into a treatment planning system (TPS), based as much as possible on simple theoretical considerations and already available data. A method is presented for the calculation (rather than measurement) of off-axis relative wedge transmission curves that are required by the TPS for relative dose calculations. We also present a method for absolute dose (monitor unit) calculations, based on the calculation of an effective wedge factor on the prescription point. A simple formula has been derived for the calculation of the effective wedge factor for the most general case, i.e. an arbitrary effective wedge angle, field size and prescription point. Relative dose calculations have been verified by measurements performed on a Varian Clinac 2300C/D linear accelerator, for 6 MV and 20 MV photon energies. Monitor unit calculations have also been verified experimentally for several cases such as symmetric and asymmetric fields with prescription on the collimator axis or on the geometrical centre of the asymmetric field. The presented technique provides results within 2% for both relative and absolute dose calculations for clinically relevant cases.