Output factors of asymmetric and dynamic wedge fields: differential output factor

The dose calculation algorithms of irregular fields often use the principle of integration over the field area to describe the contribution of each irradiated area or volume element to the dose at the point of interest. We have developed a technique, differential output factor (DOF) for calculating the output factor at any point of an asymmetric field. Since the dynamic wedge (DW) field can be considered as a sequence of asymmetric subfields, this method can be extended for DW fields as well. In the DOF method the quantity DOF(x,y) = (1(4)) (∂2OF(2x,2y)(∂x∂y) is integrated over the corresponding field area that can be asymmetric and non-static. OF(2x,2y) is the relative output factor of the symmetric field 2x*2y. For static asymmetric fields the method simplifies to the average of four quarter fields. The method utilizes the beam data of symmetric fields and DW segmentation tables including the relative weights for the sequence of asymmetric fields to produce the DW field. Therefore, the need for additional measurements is minimal. For the output factors of 6 and 15 MV DW fields the accuracy of better than 0.8 % has been achieved