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/sub oc/) by multiplying the central axis wedge factor (WF/sub 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/sub oc/. To determine the accuracy of TPS calculations, WF/sub 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/sub oc/s for all wedges were in good agreement with measured values (<2% for 15/spl deg/, 30/spl deg/, and 45/spl deg/ wedges, and approximately 3% for 60/spl deg/ wedge). For 6/100R and Co-60 unit, the difference between calculated and measured WF/sub oc/ varied from 4-10% depending on the wedge angle and off-axis distance. To improve the accuracy of WF/sub 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 /spl ap/1%) calculation of WF/sub oc/s.