The first major radiosurgery device was introduced by Swedish neurosurgeon Dr. Lars Leksell with the invention of the Gamma Knife in 1968. Gamma Knife uses radioactive cobalt as the radiation source. The targeting strategy of Gamma Knife is based on the fact that the location of any target is fixed relative to points of reference on the skull. A rigid head frame fixed to the skull that employs three dimensional stereotactic coordinates can therefore be used for precise targeting of a brain tumor. The head frame limits the use of Gamma Knife to treatment of intracranial targets and therefore does not allow for spinal or body radiosurgery.
With advances in technology, in 1994 Stanford neurosurgeon Dr. John Adler introduced the CyberKnife which uses a LINAC (a machine that produces high energy x-rays) as the radiation source mounted on a robotic arm. Having been a student under Dr. Leksell, Dr. Adler sought to create a radiosurgery device that can be used in other areas of the body in a more comfortable manner without a rigid head frame. It was also believed that the nearly limitless degrees of freedom of the robotic arm allowed for more versatile dose coverage of irregularly shaped targets.
The major differences between Gamma Knife and CyberKnife involve the source of radiation and how radiation is delivered to the target. With respect to the radiation sources, both gamma rays (Gamma Knife) and high-energy X-rays (CyberKnife) produce photon energy that has similar effects (Compton-scatter) on target tissues. The targeting methods are perhaps the major differences between the two technologies. The Gamma Knife is frame based, which means it requires a rigid head frame that is fixed to the patient’s skull during treatment while the CyberKnife is frameless. The Gamma Knife commits multiple radiation beams to a target that is positioned at the center of the radiation field. Once the target is fixed in position, multiple small cylindrical collimators open to expose the target to radiation in a controlled manner. Any movement of the target during treatment would result in an inaccurate delivery of radiation. Hence the reason why Gamma Knife employs a rigid head frame for intracranial targeting.
CyberKnife, on the other hand, delivers single radiation beams in succession to a target that is localized in three dimensional space. During treatment, x-ray guidance is used to track and correct for target movement which allows the CyberKnife robotic arm to follow a target throughout treatment delivery. Furthermore, without the requirement for rigid fixation, CyberKnife allows for hypofractionation (i.e. delivery of radiation over 2-5 consecutive days) a technique that is used to protect healthy tissue, especially when targets are embedded within critical structures such as the spinal cord or brain stem.
A common question often arises as to which device is most accurate. The answer is a bit complicated since the two targeting strategies are very different. Both technologies deliver radiation to an accuracy of less than 1 millimeter. However most will argue that the frame-based system (Gamma Knife) is slightly more accurate compared to the frameless system (CyberKnife). However, although accuracy is important, the ultimate effect on the target tissue also depends upon how dose is distributed to the target. For example, Gamma Knife uses isocentric targeting whereas CyberKnife uses non-isocentric targeting. This basically refers to the way radiation dose is distributed to a target (Gamma knife uses overlapping spheres or isocenters whereas CyberKnife uses more conformal dose coverage). Lastly, CyberKnife often employs hypofractionation that further distinguishes the two technologies and their respective targeting strategies.
Since Gamma Knife and CyberKnife are different technologies, they require different treatment planning strategies. In experienced hands, the differences among these two radiosurgery technologies should not change the clinical outcome. A skilled radiosurgeon is expected to create a treatment plan that is both safe and effective. Determining dose allocation to the target and setting limits to critical structures as well as determining the appropriate dosing schedule are all factors that ultimately have the greatest effect on outcome. However, in select cases, the differences among these two technologies may be important. It is therefore necessary that your neurosurgeon recognize these situations in order to design the best treatment plan.