Dynamic wedge versus physical wedge: a Monte Carlo study

The purposes of this study are (1) to analyze the characteristics of dynamic wedge (DW) and (2) to compare DW with physical wedge (PW) in terms of their differences in spectrum, energy fluence, angular distribution, electron contamination, and dose distributions in inhomogeneous phantom. The EGS4/BEAM Monte Carlo codes were used to simulate the exact geometry of a 6 MV beam generated by a Varian 2100C linac and the DOSXYZ code was employed to calculate 3-D dose distribution in phantom. The DW was simulated according to the segmented treatment tables supplied with the Varian linac. The phase-space data files, which contain all particle information, were obtained at various scoring planes for DW, PW and open fields. These phase-space files are then used for beam characterizations and also used as the input data for the dose calculation in homogeneous and heterogeneous phantoms. The percentage depth dose curves and beam profiles for PW, DW, and open fields were measured and used to verify the Monte Carlo simulations. The simulated results were found to agree with the measurements within 1%. The Monte Carlo calculation shows that, for PW, beam-hardening effect is significant and the beam quality changes across the field in wedged direction, while no such phenomena were observed for DW. The spectrum for contaminated electrons for DW was different from that for PW and the open fried. This results in up to 5% difference on the dose in the build-up region. The different spectra between DW and PW results in noticeable changes on dose distribution near a heterogeneity. This is demonstrated by a calculation with an air slab in place for both DW and PW.