Monte Carlo simulation of dynamic wedged fields

Dynamic wedges generated by moving one set of independent jaw are equipped with modern Linac and are used in routine phone-beam radiotherapy. This work studies the dosimetric characteristics of dynamic wedges (DW) using a Monte Carlo technique developed presently based on EGS4 BEAM system. The exact geometry of a linac was simulated. The calculation of DW was accomplished by weighting the incidence electron fluence with the STT. The calculation was validated by the measurement. It was found that the calculated depth doses and beam profiles agreed within 2% with the measurements. Calculations were performed for DW with wedge angle ranging from 15° to 60° for both 6 and 18 MV photon beams in whole range of field sizes. To compare dosimetric differences between physical wedges (PW) and DW, calculations were also carried out for PW under the same conditions as DW. Our calculation reveals that the effects of a dynamic wedge on beam spectral and angular distributions, as well as electron contamination are much less significant as compared with a physical wedge. For the 6-MV photon beam, a 45°PW can result in a 30% increase in mean photon energy due to the effect of beam hardening. It also can introduce a 5% dose reduction in the built up region due to the physical-wedge filtration of contaminated electrons. Neither this mean-energy increase nor such dose reduction is found for a dynamic wedge. Field size dependence of DW dosimetry was also investigated thoroughly. The calculated DW factors agree with the measurements within +/-2%. Both the calculated and measured DW factors are significantly dependent on the field size. The mean photon energy was reduced by 12.3% as the field size increases from 4 x 4 to 20 x 20 cm2 for both DW and open fields. The dose in the build up region is increased up to 10% (from 4 x 4 to 20 x 20 cm2) due to the increase of the contaminated electrons of the large field size. Our study demonstrates that the Monte Carlo method is a useful tool to study the dosimetry of a dynamic wedge. The data presented in this work provide important information for treatment planning involving dynamic wedge.