Design considerations for direct and indirect active matrix flat-panel portal imagers

During a radiation therapy treatment, many factors may influence the proper delivery of a calculated dose distribution. Some of these factors include misalignment of the treatment beam with respect to the patient, external or internal patient motion, or inaccurate positioning of beam-modifying devices.1 In order to quantify these geometric inaccuracies during treatment, a portal image is typically extracted from the megavoltage treatment beam with a radiation-sensitive detector. In this manner it is possible to verify the position of the radiation field relative to the bony anatomy. Portal imaging has also been studied with varying success for use in exit dosimetry, where the images are used to verify the dose distributions delivered to the patient.2,3 Traditionally, film has been used as the portal imaging detector. Over the past decade, several electronic portal imaging devices (EPIDs) have been developed for the Radiol Oncol 2001; 35(1): 63-71.