Poster - Thurs Eve-10: The accuracy of MU calculations for enhanced dynamic wedge with the Varian's anisotropic analytical algorithm.

The Anisotropic Analytical Algorithm (AAA) of the Varian Eclipse treatment planning system calculates absolute dose for a variety of beam settings including those with an enhanced dynamic wedge (EDW). This algorithm has gone through a number of version updates since it was first released in July 2005. Previous versions of AAA have come with instructions on how to manually modify the several parameters that affect the EDW output factors. The current version of AAA (8.1.17) has had the EDW module modified so that problems associated with the second-source parameter calculation are resolved. The purpose of this paper is to report on the observed discrepancies between measured and calculated EDW output factors for the current version (8.1.17) and the most recent version (8.0.05) of the AAA algorithm. EDW output factors were measured in a water-equivalent phantom on a Varian 2100iX LINAC running 6MV and 15MV photon beams. A similar geometry was modelled in the treatment planning system to calculate the number of monitor units required to deliver a specified dose at a depth with or without an EDW. Measurements and calculations were performed for a wide range of EDW wedge angles, field sizes and depths. The best EDW output factor results possible with version 8.0.05 yielded discrepancies up to 7% despite considerable parameter adjustment. The new version (8.1.17) gave results within 1% of measured without any parameter adjustment. Care should be taken when using any Eclipse AAA algorithm earlier than version 8.1.17 to calculate MUs for Varian enhanced dynamic wedge.