The problem of accurately setting up patients has been understood for some time, and it is now common practice to take portal films during a course of external beam radiotherapy as part of a complete quality assurance programme.

A method for registration of digitized portal and simulator films was developed based on the identification of anatomical fiducial points on a simulator image and its corresponding portal image to provide a method of patient position verification.

Experiments conducted on pelvic phantom images to check the consistency and accuracy of the registration process indicate that single intra-user accuracy was 0.5° for rotations and 1mm for shifts. Multiple inter-user variation was also examined and found to be 1° for rotations and 1.2mm for shifts.

The technique appears to be suitable as a portal image verification system for radiotherapy treatment.