Abstract
In fractionated radiation therapy with high energy photon beams, patient set-up is verified by analysing the position of the radiation field relative to the patient anatomy. This analysis is performed in an X-ray film, which has been exposed at the beam exit side of the patient during irradiation. Electronic portal image detectors, such as fluorescent screens, scanning diode arrays, or matrix ionization chambers produce an image within a few seconds, and enable an instantaneous verification of the patient set-up for every treatment session. An approach to detect radiation field edges in portal images using global threshold segmentation and local gradient information is presented.