There are a variety of tools used to deliver stereotactic radiosurgery. The technical differences between each and their relative strengths and weaknesses are beyond the scope of this article. The chance that stereotactic radiosurgery will completely obliterate a vascular lesion is a function of the targets volume and the amount of energy that may be safely delivered without causing adverse side-effects. As a rule of thumb therefore it is most effective when treating malformations up to a maximum volume of approximately thirty cubic centimeteres. Larger lesions may be treated by dividing the doses of energy delivered over several treatments- fractionation.
Gamma Knife and CyberKnife
The Gamma Knife was developed by Professor Lars Leksell in Sweden in 1967. It utilises 201 cobalt-60 sources arrayed in a semicircular fashion around the target which together allow gamma radiation to be focused on the lesion being treated. This means almost all the energy is disappated in the malformation or tumour and spares the nervous tissue around it from the effects of the radiation. The lesion is targeted precisely by fixing a metal frame to the patients head. This is usually secured under local anaestheisa. The patient undergoes CT and/or MR scans while wearing this frame and AVM treatments will also usually undergo catheter angiography. These images are then used to precisely plan where the radiation will be delivered. Complications are uncommon but include swelling of the brain around the area treated and occasionally seizures precipitated by the treatment.
The CyberKnife delivers radiation in a different way to the Gamma Knife. It is a Linear Accelerator (LINAC) based platform meaning that the type of radiation emitted is high energy x-ray or photons. In the CyberKnife the LINAC radiation source is married to a robotic mount. Realtime fluoroscopy (continuous x-rays) allows computer adjustment of the target while small movements occur. Consequently the rigidly fixed frame described above is not required something which makes this platform particularly useful for targetting lesions in the spinal cord and column. Instead a closely fitting mask is made for each patient ahead of treatment. Risks are similar to other varieties of sterotactic radiosurgery.
Other stereotactic radiosurgical platforms are available. These two are the ones in use for vascular malformations within our practice at present.