Evaluation of an electronic portal imaging device for transit dosimetry

The possibility of using a commercially available electronic portal imaging device for transit dosimetry was investigated. The detection unit of the d evice comprises a metal plate:fluorescenes screen;and a camera. Basic param eters of this system were investigated: stability, detector uniformity. dos e-response curve, field-size dependence and phantom-thickness dependence. I t was found that in terms of relative dosimetry, portal images corrected fo r detector nea-uniformity are in good agreement (within 5%) with transit do se distributions measured by film dosimetry. For dose determination, it was found that the use of the device is hampered by an important field-size de pendence and phantom-thickness dependence. Both correction factors should b e applied if the device is to be used for this purpose.