Monte Carlo simulation of dynamic wedged fields

Abstract

This thesis studied the dosimetric characteristics of the dynamic wedge (DW) for a linear accelerator using a Monte Carlo technique based on the EGS4/BEAM system. The geometry of a linear accelerator was simulated. For the DW technique, the segmented treatment table (STT) contains information on the collimator position versus cumulative weighting of monitor units. The simulation of DW was accomplished by weighting the incidence electron fluence with the STT. Calculations were performed for DW and physical wedge (PW) with wedge angles ranging from 15° to 60° for both 6 and 18 MV photon beams and for a whole range of field sizes. The calculated percentage depth doses and beam profiles agreed with the corresponding measurements within 2%. Our calculations show that the effects of a DW on the spectrum and angular distributions (as well as electron contamination) are much less significant than with a PW. A 45° PW in the 6 MV photon beam results in a 30% increase in mean photon energy due to beam hardening. It also results in a 5% dose reduction in the build‐up region due to filtration of contaminated electrons. Neither this mean photon energy increase nor dose reduction is found for a DW. Field size dependence of a DW was also investigated thoroughly. The mean photon energy on the beam axis was reduced by 12.3% as the field size increases from 4×4 to 20×20 for both DW and open fields. The dose in the build‐up region is increased up to 10% due to the increase of the contaminated electrons for a large field size. The results of this work should improve radiotherapy treatment planning with dynamic wedges.