Intensity modulated radiotherapy (IMRT) has in recent years become the standard of care for radiotherapy treatments. The high precision of IMRT delivery makes patient set-up accuracy more important than for past procedures. The complexity of delivery techniques and large dose gradients around the target volume make the verification process of patient treatment crucial to the success of treatment. Verification procedures involve imaging the patient prior to treatment to compare the patient setup

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... with the planned set-up (or reference position) to ensure the maximum coverage of the target volume. The non-uniform intensities of the intensity-modulated beams create a "mask" that hides the patient's anatomy in electronic portal images (EPI) acquired during the delivery of these beams. This imaging difficulty during IMRT is the main concern of this study investigation. With IMRT treatment it is currently possible to image the patient pre-and posttreatment but not during the treatment field delivery due to the modulations in the fields that mask the patient's anatomy. Previous studies have indicated methods of calibration to remove these intensity modulations. The studies showed that the calibration of images is feasible for measured IMRT portal images, but there were no reference images to compare these results with except the 'day-one' measured reference image that was assumed to be correct. They recommended the comparison of those images with MC reference image to assess the systematic set-up errors of the patient set-up during the treatment course. This study focuses on generating Monte Carlo (MC) IMRT portal images to be used as reference images (after calibration) in the verification process of patient set-up errors during IMRT treatment. Additionally, a proposed imaging procedure to be developed to estimate the set-up errors during IMRT treatment using the calibrated portal images measured and simulated (reference image). The initial stage of this study investigated the ability of MC code to simulate a simple IMRT field using square fields (5×5, 10×10, 15×15, 20×20, and 25×25 cm 2 ). The study then developed a simulation of a clinical IMRT field (prostate) and demonstrated the feasibility of simulating these simple and clinical IMRT field using a step-and-shoot delivery technique. A model lung phantom was designed v