A Monte Carlo simulation was used to study imaging and dosimetric characteristics of a novel design of megavoltage (MV) X-ray\r\ndetectors for radiotherapy applications. The new design uses Cerenkov effect to convert X-ray energy absorbed in optical fibres\r\ninto light for MV X-ray imaging. The proposed detector consists of a matrix of optical fibres aligned with the incident X rays\r\nand coupled to an active matrix flat-panel imager (AMFPI) for image readout. Properties, such as modulation transfer function,\r\ndetection quantum efficiency (DQE), and energy response of the detector, were investigated. It has been shown that the proposed\r\ndetector can have a zero-frequency DQE more than an order of magnitude higher than that of current electronic portal imaging\r\ndevice (EPID) systems and yet a spatial resolution comparable to that of video-based EPIDs. The proposed detector is also less\r\nsensitive to scattered X rays from patients than current EPIDs.
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