Dynamic wedge versus physical wedge: A Monte Carlo study

The purpose of this study is to analyze the characteristics of dynamic wedg es (DW) and to compare DW to physical wedges (PW) in terms of their differe nces in affecting beam spectra, energy fluence, angular distribution, conta minated electrons, and dose distributions. The EGS4/BEAM Monte Carlo codes were used to simulate the exact geometry of a 6 MV beam and to calculate 3- D dose distributions in phantom. The DW was simulated in accordance with th e segmented treatment tables (STT). The percentage depth dose curves and be am profiles for PW DW, and open fields were measured and used to verify the Monte Carlo simulations. The Monte Carlo results were found to agree withi n 2% with the measurements performed using film and ionizing chambers in a water phantom. The present EGS4 calculation reveals that the effects of a D W on beam spectral and angular distributions, as well as electron contamina tion, are much less significant than those for a PW. For the 6 MV photon be am, a 45 degrees PW can result in a 30% increase in mean photon energy due to the effect of beam hardening. It can also introduce a 5% dose reduction in the build-up region due to the reduction of contaminated electrons by th e PW. Neither this mean-energy increase nor such dose reduction is found fo r a DW. Compared to a DW, a PW alters the photon-beam spectrum significantl y. The dosimetric differences between a DW and a PW are significant and cle arly affect the clinical use of these beams. The data presented may be usef ul for DW commissioning. (C) 2001 American Association of Physicists in Med icine.