Characterization of a high-elbow, fluoroscopic electronic portal imaging device for portal dosimetry

The application of a newly developed fluoroscopic (CCD-camera based) electr onic portal imaging device (EPID) in portal dosimetry is investigated. A de scription of the EPID response to dose is presented in terms of stability, linearity and optical cross-talk inside the mechanical structure. The EPID has a relatively large distance (41 cm on-axis) between the fluorescent scr een and the mirror (high-elbow), which results in cross-talk with propertie s quite different from that of the low-elbow fluoroscopic EPIDs that have b een studied in the literature. In contrast with low-elbow systems, the maxi mum cross-talk is observed for points of the fluorescent screen that have t he largest distance to the mirror, which is explained from the geometry of the system. An algorithm to convert the images of the EPID into portal dose images (PDIs) is presented. The correction applied for cross-talk is a pos ition-dependent additive operation on the EPID image pixel values, with a m agnitude that depends on a calculated effective held width. Deconvolution w ith a point spread function, as applied for low-elbow systems, is not requi red. For a 25 MV beam, EPID PDIs and ionization chamber measurements in the EPID detector plane were obtained behind an anthropomorphic phantom and a homogeneous absorber for various field shapes. The difference in absolute d ose between the EPID and ionization chamber measurements, averaged over the four test fields presented in this paper, was 0.1 +/- 0.5% (1 SD) over the entire irradiation held, with no deviation larger than 2%.