Energy- and intensity-modulated electron beams forradiotherapy

Abstract

This work investigates the feasibility of optimizing energy- andintensity-modulated electron beams for radiation therapy. A multileafcollimator (MLC) specially designed for modulated electron radiotherapy (MERT)was investigated both experimentally and by Monte Carlo simulations. Aninverse-planning system based on Monte Carlo dose calculations was developedto optimize electron beam energy and intensity to achieve dose conformity fortarget volumes near the surface. The results showed that an MLC with 5 mm leafwidths could produce complex field shapes for MERT. Electron intra- andinter-leaf leakage had negligible effects on the dose distributions deliveredwith the MLC, even at shallow depths. Focused leaf ends reduced the electronscattering contributions to the dose compared with straight leaf ends. Asanticipated, moving the MLC position toward the patient surface reduced thepenumbra significantly. There were significant differences in the beamletdistributions calculated by an analytic 3-D pencil beam algorithm and theMonte Carlo method. The Monte Carlo calculated beamlet distributions wereessential to the accuracy of the MERT dose distribution in cases involvinglarge air gaps, oblique incidence and heterogeneous treatment targets (at thetissue-bone and bone-lung interfaces). To demonstrate the potential of MERTfor target dose coverage and normal tissue sparing for treatment ofsuperficial targets, treatment plans for a hypothetical treatment werecompared using photon beams and MERT.