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A Monte Carlo model of a novel electronic portal imaging device (EPID) has been developed using Geant4 and its performance for imaging and dosimetry applications in radiotherapy has been characterised. The EPID geometry is based on a physical prototype under ongoing investigation and comprises an array of plastic scintillating fibres in place of the metal plate/phosphor screen in standard EPIDs. Geometrical and optical transport parameters were varied to investigate their impact on imaging and dosimetry performance. Detection efficiency was most sensitive to variations in fibre length, achieving a peak value of 36% at 50 mm using 400 keV x-rays for the lengths considered. Increases in efficiency for longer fibres were partially offset by reductions in sensitivity. Removing the extra-mural absorber surrounding individual fibres severely decreased the modulation transfer function (MTF), highlighting its importance in maximising spatial resolution. Field size response and relative dose profile simulations demonstrated a water-equivalent dose response and thus the prototype's suitability for dosimetry applications. Element-to-element mismatch between scintillating fibres and underlying photodiode pixels resulted in a reduced MTF for high spatial frequencies and quasi-periodic variations in dose profile response. This effect is eliminated when fibres are precisely matched to underlying pixels. Simulations strongly suggest that with further optimisation, this prototype EPID may be capable of simultaneous imaging and dosimetry in radiotherapy.

Purpose: Current amorphous silicon electronic portal imaging devices (a-Si EPIDs) that are frequently used in radiotherapy applications employ a metal platephosphor screen configuration to optimize x-ray detection efficiency. The phosphor acts to convert x rays into an optical signal that is detected by an underlying photodiode array. The dosimetric response of EPIDs has been well characterized, in part through the development of computational models. Such models, however, have generally made simplifying assumptions with regards to the transport of optical photons within these detectors. The goal of this work was to develop and experimentally validate a new Monte Carlo (MC) model of an a-Si EPID that simulates both x-ray and optical photon transport in a self-contained manner. Using this model the authors establish a definitive characterization of the effects of optical transport on the dosimetric response of a-Si EPIDs employing gadolinium oxysulfide phosphor screens. Methods: The Geant4 MC toolkit was used to develop a model of an a-Si EPID that employs standard electromagnetic and optical physics classes. The sensitivity of EPID response to uncertainties in optical transport parameters was evaluated by investigating their effects on the EPID point spread function (PSF). An optical blur kernel was also calculated to isolate the component of the PSF resulting purely from optical transport. A 6 MV photon source model was developed and integrated into the MC model to investigate EPID dosimetric response. Field size output factors and relative dose profiles were calculated for a set of open fields by separately scoring energy deposited in the phosphor and optical absorption events in the photodiode. These were then compared to quantify effects resulting from optical photon transport. The EPID model was validated against experimental measurements taken using a research EPID. Results: Optical photon scatter within the phosphor screen noticeably broadened the PSF. Variat

Purpose: Standard amorphous silicon electronic portal imaging devices (a-Si EPIDs) are x-ray imagers used frequently in radiotherapy that indirectly detect incident x-rays using a metal plate and phosphor screen. These detectors may also be used as two-dimensional dosimeters; however, they have a well-characterized nonwater-equivalent dosimetric response. Plastic scintillating (PS) fibers, on the other hand, have been shown to respond in a water-equivalent manner to x-rays in the energy range typically encountered during radiotherapy. In this study, the authors report on the first experimental measurements taken with a novel prototype PS a-Si EPID developed for the purpose of performing simultaneous imaging and dosimetry in radiotherapy. This prototype employs an array of PS fibers in place of the standard metal plate and phosphor screen. The imaging performance and dosimetric response of the prototype EPID were evaluated experimentally and compared to that of the standard EPID. Methods: Clinical 6 MV photon beams were used to first measure the detector sensitivity, linearity of dose response, and pixel noise characteristics of the prototype and standard EPIDs. Second, the dosimetric response of each EPID was evaluated relative to a reference water-equivalent dosimeter by measuring the off-axis and field size response in a nontransit configuration, along with the off-axis, field size, and transmission response in a transit configuration using solid water blocks. Finally, the imaging performance of the prototype and standard EPIDs was evaluated quantitatively by using an image quality phantom to measure the contrast to noise ratio (CNR) and spatial resolution of images acquired with each detector, and qualitatively by using an anthropomorphic phantom to acquire images representative of human anatomy. Results: The prototype EPID's sensitivity was 0.37 times that of the standard EPID. Both EPIDs exhibited responses that were linear with delivered dose over a range of 1

Radiation therapy treatment verification is currently limited to image-guided radiotherapy to verify patient and target location. There is no widely available method for direct verification of the delivered dose. Imaging has been widely implemented with amorphous silicon (a-Si) flat panel imagers. a- Si imagers can also be used to verify dose delivered to the patient, but current detector designs are problematic for dosimetry. Our group is investigating new detector designs optimized for simultaneous imaging and dosimetry. Detector specifications for megavoltage radiographic imaging and radiation dosimetry are in some respects contradictory to each other, presenting a significant technical challenge for detector design. The first generation of our prototype detectors consist of plastic scintillator fiber arrays interfaced directly onto a-Si imagers. Experimental and modeling studies are being conducted to optimize this design and determine the feasibility. Results to-date demonstrate excellent dosimetry and promising imaging performance, with significant potential for improvement. Ongoing detector developments are focused on improving detective quantum efficiency for imaging performance. We have demonstrated the feasibility of the plastic scintillator based detector and continue to optimize the design to develop a detector for comprehensive radiation therapy treatment verification.

Current electronic portal imaging devices (EPIDs) are generally used for megavoltage imaging in radiotherapy and employ a thin Cu plate/phosphor screen to convert x-ray energies into optical photons . In order to achieve a high spatial resolution, thin screens are used which subsequently results in low x-ray absorption and thus a low detective quantum efficiency (DQE) for megavoltage x-rays. Additionally, the high atomic number Cu/phosphor screen materials is not ideal for dosimetric applications. To improve the imaging and dosimetry dual-functionality of EPIDs, water equivalent plastic scintillators have been proposed. Plastic scintillator fibers may however be susceptible to radiation damage caused primarily by ionizations from low energy secondary electrons. An accumulation or clustering of these ionization events, within regions corresponding to the volume of the plastic polymer chains, may lead to chain breaks. This could result in changes to the optical photon absorption properties and optical yield of the fiber, affecting the overall imaging performance of the detector. Here we used Monte Carlo radiation transport simulations for a preliminary investigation into the distribution of ionizations within a single plastic fiber. We find a large number of ionization events can accumulate along the fiber length, which over repeated exposures could lead to damage. To determine the effect of damage on the imaging performance, two fiber arrays were modeled with and without areas of damage. The damaged fiber array was found to produce approximately half the number of counts as the undamaged array.