Portal film setup on the 14 MeV linear accelerator at the Antoni van Leeuwenhoek.
During portal film imaging, an x-ray image of the patient is taken with the megavolt radiation beam used to irradiate the tumour. On the resulting film it can be verified whether the radiation field is correctly placed. Since about 1980, megavolt portal film imaging has been routinely introduced as a control tool by some complex irradiations. The procedure is quite time consuming and cannot be introduced for all irradiations.
Radiotherapy is usually carried out by delivering a radiation dose in a number of irradiation sessions, the so-called fractionation. The patient comes every day during days or weeks to be irradiated in the same manner. The daily repositioning of the patient on the treatment table and setting up of the radiation beams on the target volume is one of the weakest links in the chain of activities .
Measuring and comparing portal images allows the detection of systematic and single deviations in the setup of the radiation beams. On this basis, it can be decided to adjust the patient setup during the irradiation series in order to correct a setup error or a shift trend. In practice, the adjustments of the setup take place afterwards, prior to the next irradiation from the series, since the development and analysis of the film are too time consuming to keep the patient waiting. The results are also useful to make the working methods more efficient by improving procedures and devices.
The sharpness and contrast of megavolt images are poor due to the physical properties of both the hard megavolt radiation and the linear accelerator. The hard radiation results in a low contrast and the radiation scattered by the accelerator and by the patient make the image even worse. The large size of the radiation focal spot in the accelerator, about 2 mm x 3 mm, explains the blurred imaging of details on the film.
Harm Meertens setting up image digitalisation.
As of about 1980, the medical physicist Harm Meertens develops a better method of analysing portal images. The megavolt imageis recorded on a light box with a video camera and stored digitally. Image enhancing software is used to improve the quality of the electronic image. The image processing is first derived from the practice of the CT scanner, in use since 1979 in the Antoni van Leeuwenhoek. After that, dedicated image-enhancing software has been developed In the NKI by, among others, Professor Marcel van Herk.
Megavolt image of a head and neck irradiation after digitalisation. Metal markers, placed on the skin or inside the patient, can be seen in the image and allow the localisation of important landmarks.The small circle is the X-ray shadow of a field-center marker fixed on the linear accelerator.
The corresponding simulator image after the digital image-enhancing procedure. The megavolt image is compared to a reference x-ray image previously acquired at the simulator as well as to the megavolt images of the next irradiations.
Analysis of the position of the edges of the radiation field during subsequent irradiations of a treatment series. The patient anatomy (bone structures) from a series of megavolt images is accurately superimposed (matched) and the position of the corresponding sets of field edges is plotted.