Irradiation of a patient on the linear accelerator onto which an EPID megavolt image cassette is mounted. The cassette holder is made in the NKI and was used during the 1990's until the robot arm became available.
In 1986 the first clinical megavolt images are made with the Electronic Portal Imaging Device (EPID). From 1988 prototypes of the EPID are routinely used during radiotherapy. In 1990 the first publication appears with a report on the experiences with quality control based on the images produced by the EPID . In 1992 physicist Jurrien Bijholt receives his doctorate with a thesis on "Evaluation of treatment set up in radiotherapy using an electronic portal imaging device" .
Irradiation of a patient on the linear accelerator. Illustration Bert van Vliet
The patient on the treatment table is treated with a beam of high energy photon radiation. The dimensions of the treatment fields are set using four independently operated diaphragm blocks within the collimator. The radiation is directed to the tumour by positioning of the table and rotation of the gantry. The flat cassette with the EPID is in a holder opposite the collimator, behind the patient. The EPID detects the radiation beam that is attenated by passage through the patient and produces an image of the anatomy which is irradiated. The arrows in the sketch indicate which settings for the irradiation are made according to a prescribed value.
During this phase, the prototype EPID is still mounted on the accelerator by hand. Many settings are made by hand after that. The accurate settings demand a great deal of concentration and effort by the radiation therapists who carry out the radiation treatment. The increasing complexity of the treatments and the heavy work load, as a result of waiting lists and limited capacity, mean that the introduction of portal imaging is delayed. In order to use the EPID efficiently, new computer-operated machines are considered by which part of the settings can be made automatically. From 1988 to 1992 in the Netherlands Cancer Institute (NKI), two new computer controlled accelerators by Brown Boveri (4 MV) and Philips (SLi, 6-20 MeV) are acquired.
In 1990, Harm Meertens and colleagues publish a report on the first clinical experience with the EPID in the NKI. Two EPID prototypes are evaluated in clinical practice. Both have a field of view of 32 cm x 32 cm; a LoRes (Low Resolution) EPID with a pixel size of 2.54 mm in a matrix of 128x128 pixels, and a HiRes (High Resolution) EPID with a pixel size of 1.27 mm in a matrix of 256x256 pixels. In LoRes, the time needed is 2 to 3 seconds for both scanning and imaging; in HiRes, it is 2 to 3 seconds for scanning and 3 to 6 seconds for imaging.
Various EPID imaging techniques are available:
The conclusions from the evaluation of Meertens and colleagues are positive. The images from the electronic megavolt camera are of the same quality as those from conventional megavolt films. The position of the irradiation field can be checked. When a megavolt image is made every 2 to 6 seconds the dynamic behaviour of both the patient and the treatment machine can be analysed.