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

In intensity modulated radiation treatments (IMRT),

For EPID dosimetry, the calibration should ensure that all pixels have a similar response to a given irradiation. A calibration method (MC), using an analytical fit of a Monte Carlo simulated flood field EPID image to correct for the flood field image pixel intensity shape, was proposed. It was compared with the standard flood field calibration (FF), with the use of a water slab placed in the beam to flatten the flood field (WS) and with a multiple field calibration where the EPID was irradiated with a fixed 10x10 field for 16 different positions (MF). The EPID was used in its normal configuration (clinical setup) and with an additional 3 mm copper slab (modified setup). Beam asymmetry measured with a diode array was taken into account in MC and WS methods. For both setups, the MC method provided pixel sensitivity values within 3% of those obtained with the MF and WS methods (mean difference<1%, standard deviation<2%). The difference of pixel sensitivity between MC and FF methods was up to 12.2% (clinical setup) and 11.8% (modified setup). MC calibration provided images of open fields (5x5 to 20x20 cm2) and IMRT fields to within 3% of that obtained with WS and MF calibrations while differences with images calibrated with the FF method for fields larger than 10x10 cm2 were up to 8%. MC, WS and MF methods all provided a major improvement on the FF method. Advantages and drawbacks of each method were reviewed.