Stereotactic radiosurgery is a technique to precisely deliver therapeutic radiation to a target within the nervous system while minimising the dose of radiation to adjacent tissue. This reduces the side effects of therapy and allows for repeated treatments in selected cases as well as safer treatment of multiple targets simultaneously e.g. brain metastases.
How Radiosurgery Works
Unlike radiotherapy which often seeks to arrest cell division by damaging the reproductive ability of the cell at the DNA level, radiosurgery seeks to eventually physically destroy the cells in-situ. Multiple beams of radiation which individually pose little if any risk of injury are delivered from multiple directions to a target which receives the sum of their energies.
"Stereotactic" refers to the use of a three-coordinate system to describe the position of the target lesion in a patient and correlate it to a virtual target generated from patient imaging. Originally this relied on the application of a frame directly to the patient's head but it is also possible to use a mask or robotic positioning systems to achieve a similar accuracy. In most cases radiosurgery can be delivered under a local anaesthetic and treatment will often not require hospital admission.
There are different tools that may be used to deliver stereotactic radiosurgery and which may deliver the radiation energy in different forms.
Gamma Knife
The Gamma Knife is perhaps the best established radiosurgery platform worldwide. It delivers high-intensity gamma radiation to the target originating from 201 Cobalt-60 sources arrayed around the patient's head. It was developed by Swedish neurosurgeon Lars Leksell together with Ladislau Steiner, and Börje Larsson.
Traditionally the tremendous accuracy of Gamma Knife was used by utilising a stereotactic frame, affixed to the patient's head under local anaesthesia. This same device has also been used to guide neurosurgical procedures including biopsies and the placement of electrodes deep within the brain to treat movement disorders. Modern evolutions of the Gamma Knife system can also be used without the need for a frame although in some situations its use is recommended to aid accuracy. For brain AVM treatments a frame still usually sited before angiography as the most accurate means to describe the nidus in three dimensional space.
LINAC Systems- Cyberknife, Novalis
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 stereotactic radiosurgery although there may be small differences in the dose delivered to target and tissue.
These LINAC machines that may be used to treat tumours of the body and spine in a similar way- aiming to destroy offending diseased tissue as opposed to arresting it's cell's division. This is referred to as Stereotactic Ablative Radiotherapy (SABR).
Radiosurgery and Arteriovenous Malformations
In our practice frame-based Gamma Knife is currently usually preferred for the treatment of brain AVM given its proven track record and precision with this pathology. Not every AVM is suitable for radiosurgical treatment.
The goal of treatment should be to obliterate the malformation completely as leaving smaller residual does not reliably reduce the risk of bleeding. Obliteration is more likely when the volume of the AVM nidus is relatively small. Using higher doses of radiation can increase the effect but also increases the likelihood of injury to surrounding tissue, something that is also effected by how quickly the radiation is delivered to tissues and the vulnerability of different tissue types to radiation. Therefore the ideal AVM treatment will be to a relatively small volume nidus in a single session.
Fractionated Treatment
Radiosurgery has been used over multiple treatment sessions in attempts to treat larger AVMs (dose- or volume- fractionated treatment). Obliteration is less often achieved and side-effects of treatment are more common.
Pre-treatment of an AVM with embolisation may reduce the nidus volume sufficiently to increase the efficacy of radiosurgery but it can also introduce difficulties in targeting the area needing treatment or impose local effects on how the radiation interacts with tissues. For that reason or practice is generally to consider radiosurgery before significant embolisation of the nidus. Individual points of weakness such as aneurysms may secured endovascularly leaving the nidus of the AVM to be treated with radiosurgery alone.
Embolisation and Radiosurgery
Pre-treatment of an AVM with embolisation may reduce the nidus volume sufficiently to increase the efficacy of radiosurgery but it can also introduce difficulties in targeting the area needing treatment or impose local effects on how the radiation interacts with tissues. For that reason or practice is generally to consider radiosurgery before significant embolisation of the nidus. Individual points of weakness such as aneurysms may secured endovascularly leaving the nidus of the AVM to be treated with radiosurgery alone.
Pre-surgical Radiosurgery to AVM
Radiosurgical treatment with the intention of reducing the volume of an nidus to facilitate microsurgical removal later has been advocated. The difficulty with that strategy is that any volume reduction is somewhat unpredictable. Another potential drawback is that it will take several years to realise the full effect of the radiosurgical treatment meaning a patient would live with a partially treated AVM for a long time. Any risk of bleeding would persist until the nidus is completely obliterated.
When surgery is carried out because radiosurgery has failed to obliterate an AVM, the preceding radiosurgical treatment does not usually produce additional technical difficulties in removing the nidus.
