AbstractID: 6578 Title: Investigation into the front plate thickness of thick portal detectors Metal front plates have been traditionally used for portal film detectors, both to increase the quantum efficiency and reduce lower-energy patient scatter. Typically a thickness equivalent to about 1-1.5 mm copper (or equivalent mass thickness of a denser metal) is used. Modern megavoltage detectors, however, are typically thicker than film. This study investigates whether such thick detectors should still use the same front plate thickness which has been optimized for film. The effect of the front plate on the detective quantum efficiency (DQE), contrast, and differential signal-to-noise ratio (DSNR) is explored. The DQE is calculated using a cascade analysis formalism, which is evaluated with Monte Carlo techniques. The contrast and DSNR are affected by the scatter rejection properties of the front plate, which is calculated using Monte Carlo. Graphs are presented which indicate the dependence of each parameter on the front plate for various detector thicknesses. We focus on metal/phosphor and metal/a-Se detectors since these are under investigation for use in active matrix flat-panel imagers. The results for a 6 MV beam indicate that although 1-1.5 mm of copper is adequate for sensitive layers less than about 300 µm, a smaller front plate of about 0.4 mm Cu may be optimal for larger thicknesses of the sensitive layer.