Simulation of real-time EPID images during IMRT using Monte-Carlo

Abstract This study is part of a project concerned with real-time EPID-based verification of the incremental dose delivered during IMRT radiation treatments. Three automated Monte-Carlo methods are devised to calculate the differential dose delivered to the EPID during treatment. All methods break down the normalized total monitor units into a number of equally spaced segments. A method models the dynamic simulation as a series of static fields, each field corresponding to an IMRT segment or a sub-segment. Another method models each segment as a separate dynamic IMRT file. A third method, which modifies the DYNVMLC module of the BEAMnrc code, uses the full-MLC file. The MLC positions for the simulated photons are sequentially selected within DYNVMLC to correspond to individual segments of the delivery. A bash script calls the BEAM shared-library to calculate and store the EPID dose for each segment. Validation is performed by comparing the average dose contributed by all segments with the dose predicted by a canonical dynamic IMRT simulation that uses the same MLC file. The best results are achieved by the methods based on dynamic simulations (where leaf positions within a segment are interpolated for simulated photons) whose normalized root mean square error is at the most 0.2% over the focal area. EPID images can be predicted for individual segments (or smaller intervals) of an IMRT delivery using Monte-Carlo methods. The MLC file can be externally spliced or a simple modification of the DYNVMLC code can achieve accurate results.