Radiation of the acoustic neuroma with the Cyber Knife (radiosurgery)

The CyberKnife System is a revolutionary further development of radiosurgery by using the linear accelerator. The further development involves a radiation device on the one hand and a image locating system on the other.

Other than the fractionated radiation therapy, the artificially generated photon radiation in the linear accelerator are given in one single radiation dosage. In special cases the dosage normally applied in one single shot are divided into 2 – 5 smaller doses.

The radiation apparatus – a linear accelerator – is compact, but at the same time light and on the robot arm there is a place that has six axles and therefore can move freely in six directions. This way the radiation can be carried out from approx. 1,200 positions in the room. The accuracy of the robot's positioning is within 0.2 mm. Therefore, all body regions can be optimally reached with the beams, which makes it possible to treat very irregularly formed tumours in critical places.

A completely new image guidance system it to do without the head frame or fixed mask, which are essential for fractionated radiation therapy or the GammaKnife. The only essential items are an individual lying-down support and a type of network, to hold the head as still as possible without any difficulty. The guidance system comprises two x-ray systems and an image processing computer. The axles of the two x-ray systems are placed vertically next to each other and separate in the centre of the radiation area. There is a three dimensional, stereotactic image of the target area. In the image processing computer this image is constantly compared to the computer tomography, which was made previously as the patient model. The significant bone structures are compared this way. If their positions do not match, correction values are reported to the robot. The robot is repositioned accordingly, the arms are shifted linear or/and they rotate, so that the position changes of the patient are immediately adjusted, ensuring an optimal radiation geometry (number, intensity and direction of the rays). The Munich Centre specifies in a video that the radiation dose can be reduced by 80% in a 3 mm distance from the tumour. This also shows that a razor-sharp dividing line between the tumour and the surrounding area is not drawn.

The treatment procedure usually comprises the following steps for radiation: preliminary discussion, creation of a virtual patient model through imaging systems (CT, MRT, amongst others), treatment planning (the patient does not need to be present for this), the actual radiation and agreed follow-up examinations. The patient lies relaxed on the treatment table. He can communicate with the doctor. Everything is monitored by video.

In regular distances, the digital image guidance system checks the current position of the head and compares it with the date in the patient model. Deviations up to one centimetre are automatically offset by the robot. Some 150 rays are then transmitted from different positions to the acoustic neuroma. The actual radiation lasts only a few seconds.