Enhanced dynamic wedge factors at off-axis points in asymmetric fields.

Abstract

Several recent reports have described methods for calculating enhanced dynamic wedge factors (EDWFs). Many of these reports use the monitor-unit (MU) fraction method to predict EDWFs as a function of field size. Although simple in approach, MU fraction methods do not produce accurate EDWFs in large or asymmetric fields. A recently described technique, based on the MU fraction method works well for large and asymmetric fields, but only when the calculation point is in the center of the field. Other existing methods based on beam-segment superposition do not have this limitation. These beam summation methods, however, are difficult to implement in routine clinical MU calculation schemes. In this paper, we present a simple calculation method that estimates EDWFs at off-axis calculation points in both symmetric and asymmetric fields. Our method, which also is based on the MU fraction method, similarly uses empirically determined field-size corrections but also applies wedged-field profiles to estimate EDWFs that are independent of calculation-point location and field symmetry. EDWF measurements for a variety of field sizes and calculation-point locations for both 6- and 18-MV x-ray beams were performed to validate our calculations and those of our ADAC Pinnacle3 Treatment Planning System. The disagreement between the calculated and measured EDWFs over the useful clinical range of field sizes and calculation-point locations was less than 2%. The worst disagreement was 3% and occurred at a point 8.5 cm from the center of an asymmetric 25 (wedged direction)x20 cm2 60 degrees-wedged field. Detailed comparisons of measurements with calculations and wedge factors obtained from the ADAC Pinnacle3 Treatment Planning System will be presented. In addition, the strengths and weaknesses of this calculation method will be discussed.