Automatic comparison of bony structures and field borders in the reference image of the simulator with the megavolt EPID image during the irradiation. This is an important step towards "image-guided" radiotherapy.
During the 1990's, the electronic portal imaging device (EPID) is introduced on a large scale in the Netherlands Cancer Institute (NKI). A simple EPID holder is designed and each of the six linear accelerators is fitted with a fixture that holds the EPID, so that a megavolt image can be made under all rotation angles of the gantry of the accelerator. Initially, the EPID had to be fixed to the accelerator each time before it could be used. At the end of the 1990's an in-house built robot arm was used with the EPID on most of the accelerators. This was folded flat against the gantry when not in use. See the item "Robot arm makes quality control with EPID images easier".
The development of electronic portal imaging into an advanced technique in the NKI is documented in, among others, the doctoral theses of Jurrien Bijholt in 1992  and Kenneth Gilhuijs in 1995 . EPID images are enhanced by advanced digital processing. The group of Van Herk is extended with programmers and other project co-workers (among these are Jurrien Bijholt and Kenneth Gilhuijs and many others) so that the necessary software for image processing can be developed in house.
The group grows steadily and in around 2012 has about 25 positions. Imaging software development is a spearhead of research in the radiotherapy department of the NKI.
Real time detection and correction using EPID portal imaging is used more and more to verify the alignment of the patient in the treatment position. The development of automatic analysis of the set-up error during the irradiation is an important step. For this, the edges of the irradiation field and the bony structures in both the reference image and the megavolt image are automatically recognized and compared.