Technical Possibilities of IGRT

Image guided radiotherapy (IGRT) is the process of different kind of imaging in the treatment room, before between or after radiation therapy. The goal of the IGRT process is to improve the accuracy of the radiation field placement, and to reduce the exposure of healthy tissue during radiation treatments. This is very useful since tumours can move between treatments due to differences in organ filling or movements while breathing. On the base of these results the patient set-up can be corrected and the dose delivery can be modified. IGRT needs a special workflow and quality control protocol using predetermined frequency and methods of special processes.

Technical possibilities of IGRT

IGRT can be applied in several different ways. In the following table the different technical solutions are summarized.

Figure 1.: Linear accelerator with kV cone beam CT. The X-ray source and detector are rotated 90° degrees relative to the MV source.

Figure 2.: Linear accelerator with MV cone beam CT. The red lines show the incident beam used for imaging by the EPID.

Figure 3.: Linear accelerator and rail mounted spiral CT installed in the same treatment room.

 
Inaccuracies of the clinical application of IGRT can be separated as intra-fraction and inter-fraction set-up inaccuracies.

 
Intra-fraction tumour localisation can performed with stereoscopic kV X-ray imaging, if the tumour is visible on the radiograph (lung cancer). In the most cases the tumour is invisible, so the target volume in some way should make “visible”. Therefore is used implanted tumour marker in clinical practice, which is usually made from gold and gives a very good X-ray shadow. The actual position of the tumour and its movement during treatment can be determined with detection of these markers. Intra-fraction tumour displacement can investigated by small antennas, implanted in the tumour, which can be excited by radio frequency waves. The spatial coordinates of antennas can be determined by the continuous detection of reflected signals.