Determination of field size dependent wedge factors from a few selected measurements

Abstract

Some modern treatment planning systems (TPSs) provide for input of wedge factor (WF) tables covering the entire range of square and elongated fields available on the accelerator. Depending on the field size increment chosen and the number of available wedge orientations, one may have to take more than one hundred measurements per wedge and photon energy to commission the TPS. To expedite TPS commissioning while maintaining high accuracy, we demonstrate a simple method that requires only a few measurements per wedge, from which the remaining wedge factors can be found through linear interpolation based on field area. For the externally mounted wedges of two common linear accelerators, we have shown that WFs are proportional to field area and are nearly independent of field elongation and wedge orientation. WFs computed from 5 to 7 measurements comprised of square fields and a single, large rectangular field agreed with direct measurements throughout the entire range of achievable field dimensions within 0.6% at 6 MV, and within 1% at 15 MV. Making the same set of measurements and using the equivalent square method to find wedge factors at other field sizes leads to errors up to 2%. Measuring the wedge factor for a 10x10 cm2 field and applying the same value to all field sizes can lead to errors of up to 10% at both 6 MV and 15 MV.