Characterization of an Indirect-Detection Amorphous Silicon Detector for Dosimetric Measurement of Intensity Modulated Photon Fields
Bailey, Daniel Wayne
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Indirect-detection amorphous silicon electronic imagers show much promise for measurement of radiation dose, particularly for pre-treatment verification of patient-specific intensity modulated radiotherapy plans. These instruments, commonly known as Electronic Portal Imaging Devices (EPIDs), have high data density, large detecting area, convenient electronic read-out, excellent positional reproducibility, and are quickly becoming standard equipment on today's medical megavoltage linear accelerators. However, because these devices were originally intended to be digital radiograph imagers and not dosimeters, the modeling, calibration, and prediction of their response to dose carries a number of challenges. For instance, EPID dose images exhibit off-axis dose errors of up to 18% with increasing distance from the central axis of the imager (as compared to dose predictions calculated by a commercially available treatment planning system). Furthermore, these off-axis errors are asymmetric, with higher errors in the in-plane direction than in the cross-plane direction. In this work, methods are proposed to account for EPID off-axis effects by precisely calculating off-axis output factors from experimental measurements to increase the accuracy of EPID absolute dose measurement. Using these methods, dose readings acquired over the entire surface of the detector agree to within 2% accuracy as compared to respective EPID dose predictions. Similarly, the percentage of measured dose points that agree with respective calculated dose points (using 3%, 3 mm criteria) improves by as much as 60% for off-axis intensity modulated photon fields. Furthermore, a number of clinical applications of EPID dosimetry are investigated, including pixel response constancy, the effect of data density on a common metric for quantitatively comparing measured vs. calculated dose, and the implementation of an electronic portal dosimetry program for radiotherapy quality assurance.