AbstractID: 6827 Title: MU Calculation for the Enhanced Dynamic Wedge Fields at Offaxis Points
The Enhanced Dynamic Wedge MU calculation is different from that of the physical
wedge in two ways: (1) the wedge factor is a function of field size and the final moving
jaw position; (2) each dynamic wedge field (symmetric or asymmetric) is treated as an
individual wedge field, and the shape of the wedge depends on the two initial jaw
positions in the wedge direction. The wedge factor at the center of the field is determined
by multiplying the value of the Normalized Golden Segmented Treatment Table
(NGSTT) at the center of the field by the NGSTT value at the final moving jaw position.
We found that the largest EDW field profile at different depths can be used as a relative
EDW off-axis factor. Therefore, the MU calculation for a 600 EDW field at an off-axis
point can be performed using the conventional wedge MU calculation method, where the
wedge factor is determined using the NGSTT method described above, and the wedge
off-axis factor is determined by the ratio of the output at the off-axis point to that at the
center of the field. Since an arbitrary wedge angle can be obtained by combining open
and 600 EDW beams using the dose-ratio relationship of Petti et al., the MU calculation
at an off-axis point for a given EDW angle can be determined using this equation.
Measured and calculated MU comparisons at off-axis points are discussed. The
maximum difference between measured and calculated data is within 2%.