A method that significantly improves accuracy of MU/time calculations for wedged asymmetric fields

Wedged asymmetric fields are commonly encountered in radiation therapy. Most treatment planning systems (TPSs) calculate the off-axis wedge factors (WF_oc) by multiplying the central axis wedge factor (WF_c) by the ratio of transmissions along the off-axis ray to that along the central-axis. Therefore the wedge shape determines the off-axis wedge factors. However, during TPS commissioning, it is often necessary to modify the physical wedge shape in order to fit the measured wedged dose distributions. This can result an inaccurate calculation of WF_oc. To determine the accuracy of TPS calculations, WF_oc measurements for various asymmetric beams produced by dual energy VARIAN 2100C and 6/100R linear accelerators and a Theratronics-1000 Co-60 unit were done. For the 2100C linear accelerator, the FOCUS calculated WF_ocs for all wedges were in good agreement with measured values (<2% for 15°, 30°, and 45° wedges, and approximately 3% for 60° wedge). For 6/100R and Co-60 unit, the difference between calculated and measured WF_oc varied from 4-10% depending on the wedge angle and off-axis distance. To improve the accuracy of WF_oc calculations, wedges were represented in the TPS in two versions: planning and calculational. The planning wedge has a shape that produces accurate calculations of dose distributions and the calculational wedge shape assures an accurate (within ≈1%) calculation of WF_ocs