Electronic portal imaging devices (EPIDs) based on indirect detection, active matrix flat panel imagers (AMFPIs) have become the technology of choice for geometric verification of patient localization and dose delivery in external beam radiotherapy. However, current AMFPI EPIDs, which are based on powdered-phosphor screens, make use of only approximately 2% of the incident radiation, thus severely limiting their imaging performance as quantified by the detective quantum efficiency (DQE) (approximately 1%, compared to approximately 75% for kilovoltage AMFPIs). With the rapidly increasing adoption of image-guided techniques in virtually every aspect of radiotherapy, there exist strong incentives to develop high-DQE megavoltage x-ray imagers, capable of providing soft-tissue contrast at very low doses in megavoltage tomographic and, potentially, projection imaging. In this work we present a systematic theoretical and preliminary empirical evaluation of a promising, high-quantum-efficiency, megavoltage x-ray detector design based on a two-dimensional matrix of thick, optically isolated, crystalline scintillator elements. The detector is coupled with an indirect detection-based active matrix array, with the center-to-center spacing of the crystalline elements chosen to match the pitch of the underlying array pixels. Such a design enables the utilization of a significantly larger fraction of the incident radiation (up to 80% for a 6 MV beam), through increases in the thickness of the crystalline elements, without loss of spatial resolution due to the spread of optical photons. Radiation damage studies were performed on test samples of two candidate scintillator materials, CsI(Tl) and BGO, under conditions relevant to radiotherapy imaging. A detailed Monte Carlo-based study was performed in order to examine the signal, spatial spreading, and noise properties of the absorbed energy for several segmented detector configurations. Parameters studied included scintillator material, septal wall material, detector thickness, and the thickness of the septal walls. The results of the Monte Carlo simulations were used to estimate the upper limits of the modulation transfer function, noise power spectrum and the DQE for a select number of configurations. An exploratory, small-area prototype segmented detector was fabricated by infusing crystalline CsI(Tl) in a 2 mm thick tungsten matrix, and the signal response was measured under radiotherapy imaging conditions. Results from the radiation damage studies showed that both CsI(Tl) and BGO exhibited less than approximately 15% reduction in light output after 2500 cGy equivalent dose. The prototype CsI(Tl) segmented detector exhibited high uniformity, but a lower-than-expected magnitude of signal response. Finally, results from Monte Carlo studies strongly indicate that high scintillator-fill-factor configurations, incorporating high-density scintillator and septal wall materials, could achieve up to 50 times higher DQE compared to current AMFPI EPIDs.

Image guided PT provided excellent biochemical control rates for patients with localized prostate cancer. The actuarial rates of high-grade toxicity were low after PT. From pretreatment to 5 years of follow-up, a significant decline was found only in mean EPIC sexual summary scores. Prospective clinical studies are needed to determine the comparative effectiveness of PT and other radiation treatment strategies.

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Active matrix, flat-panel x-ray imagers based on a-Si:H thin film transistors offer many advantages and are widely utilized in medical imaging applications. Unfortunately, the detective quantum efficiency (DQE) of conventional flat-panel imagers incorporating scintillators or a-Se photoconductors is significantly limited by their relatively modest signal to noise ratio, particularly in applications involving low x-ray exposures or high spatial resolution. For this reason, polycrystalline HgI 2 is of considerable interest by virtue of its low effective work function, high atomic number, and the possibility of large-area deposition. In this study, a detailed investigation of the properties of prototype, flat-panel arrays coated with two forms of this high-gain photoconductor are reported. Encouragingly, high x-ray sensitivity, low dark current, and spatial resolution close to the theoretical limits were observed from a number of prototypes. In addition, input-quantum-limited DQE performance was measured from one of the prototypes at relatively low exposures. However, high levels of charge trapping, lag, and polarization, as well as pixel-to-pixel variations in x-ray sensitivity are of concern. While the results of the current study are promising, further development will be required to realize prototypes exhibiting the characteristics necessary to allow practical implementation of this approach.