Electronic portal imaging detectors (EPID) have initially been developed for imaging purposes but they also present a great potential for dosimetry. This is of special interest for intensity modulated radiation treatment (IMRT), where the complexity of the delivery makes quality assurance necessary. By comparing a predicted EPID image of an IMRT field with a measured image, it is possible to verify that the beam is properly delivered by the linear accelerator and that the dose is delivered to the correct location in the patient. This study focused on predicting the EPID image of IMRT fields in air with Monte Carlo methods. As IMRT treatments consist of a series of segments of various sizes which are not always delivered on the central axis, large spectral variations may be observed between the segments. The effect of these spectral variations on the EPID response was studied. A detailed description of the EPID was
Significant differences were found between soft tissue alignment and the predicted FM position. FMs are useful in reducing inter-observer variability compared with soft tissue alignment. Consideration needs to be given to margin design when using soft tissue matching due to increased inter-observer variability. This study highlights some of the complexities of soft tissue guidance for prostate radiation therapy.
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