Verification of Varian Enhanced Dynamic Wedge implementation in Masterplan Treatment Planning System

Abstract

This paper investigates the accuracy of the two available calculation algorithms of the Oncentra-OTP three dimensional Treatment Planning System, the pencil beam method and collapsed-cone convolution, in modeling the Varian Enhanced Dynamic Wedge. Measurements were carried out for a dual high energy (6-15 MV) Varian DHX-S linear accelerator using ionization chambers for beam axis measurements (wedge factors and depth doses), film dosimetry for off-axis dose profiles analysis and a diode matrix detector for bidimensional absolute dose distributions. Using both calculation algorithms, different configuration of symmetric and asymmetric fields varying the wedge’s angle in the permitted range were tested. Accuracy of the Treatment Planning System was evaluated in terms of percentage differences between measured and calculated values for wedge factors, depth doses and profiles. As far as the absolute dose distribution was concerned, the gamma index method was used with 2% and 2 mm as acceptance criteria for dose difference and distance to agreement respectively. Wedge factors and percentage depth doses were within 2% deviation between calculated and measured values. The comparison of measured and calculated dose profiles shows that the Van Dyck’s acceptance criteria are generally met; a disagreement can be noted for large wedge angles and field size limited to the low dose-low gradient region only. The 2D absolute dose distribution analysis confirms the good accuracy of the two calculation algorithms in modeling the enhanced dynamic wedge.