An investigation of the accuracy of Monte Carlo portal dosimetry for verification of IMRT with extended fields

Abstract

This work investigated the accuracy of Monte Carlo (MC) simulations of amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) for the dosimetric verification of intensity-modulated radiotherapy (IMRT). In particular, the suitability of the method for verification of head and neck IMRT with extended field segments (≈20 cm superior–inferior), covering almost the entire detector area, was studied. A solution involving schematic modelling of backscatter materials has been established to account for non-uniform backscatter to the imager from supporting structures. 96% of points within the IMRT fields evaluated passed a ‘gamma’ evaluation criterion of 2%, 2 mm at isocentre at a dose rate of 100 MU min−1 with this solution included. Only 79% of points passed this gamma criterion without the correction for backscatter included. This work has also demonstrated the ability of the technique to detect systematic delivery errors in step and shoot IMRT. The technique identified a systematic overshoot on the first segment and an undershoot on the final segment. Results were verified by ion chamber measurements and agreed well with those reported in the literature, averaging approximately 0.1 and 0.3 MU for 100 and 300 MU min−1 deliveries, respectively. MC portal verification has the potential to become a key tool in the verification of IMRT and can also facilitate selection of optimal delivery parameters, thus improving treatment accuracy. This approach can be applied to the verification of other new treatment techniques and should also enable development of methodologies to detect and correct for delivery errors, both before and during treatment.