A Monte Carlo simulation was used to study imaging and dosimetric characteristics of a novel design of megavoltage (MV) X-ray detectors for radiotherapy applications. The new design uses Cerenkov effect to convert X-ray energy absorbed in optical fibres into light for MV X-ray imaging. The proposed detector consists of a matrix of optical fibres aligned with the incident X rays and coupled to an active matrix flat-panel imager (AMFPI) for image readout. Properties, such as modulation transfer function, detection quantum efficiency (DQE), and energy response of the detector, were investigated. It has been shown that the proposed detector can have a zero-frequency DQE more than an order of magnitude higher than that of current electronic portal imaging device (EPID) systems and yet a spatial resolution comparable to that of video-based EPIDs. The proposed detector is also less sensitive to scattered X rays from patients than current EPIDs. 1. Introduction Radiation therapy is widely used today to treat patients with tumors [1]. Megavoltage (MV) X-ray beams generated from a linear accelerator are commonly used to deliver the prescribed radiation dose to the tumor while minimizing the dose to the surrounding healthy tissues. The geometric accuracy of such treatment is crucial for its success. Currently, there are a number of ways to verify the positional accuracy of the treated target. One of them is to use MV cone beam CT (MV-CBCT) to locate the position of the target in the treatment room prior to the start of the treatment [2, 3]. MV-CBCT uses an electronic portal imaging device (EPID) [4] attached to the LINAC to acquire CT data by rotating the MV X-ray source (emitting a cone beam) and the EPID around the patient. One of the main challenges with this approach is that the imaging dose currently required to achieve sufficient soft tissue contrast to visualize and delineate a soft tissue target; for example, the prostate is prohibitively large for daily verification. This is due to the poor X-ray absorption, that is, low quantum efficiency (QE) of the EPID used. For most EPIDs developed so far, the QE is typically on the order of 2–4% at 6？MV as compared to the theoretical limit of 100% [5]. This is because the total combined thickness of the energy conversion layer and the metal buildup in most EPIDs is only ~2？mm. In contrast, the first half value layer (HVL) for 6？MV X-ray beams is ~13？mm of lead. Thus, a significant increase of QE is required in order to reduce the dose currently required to visualize and delineate the prostate using MV-CBCT. Efforts
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