As the survival rate for primary cancers improves, physicians are seeing a corresponding increase in metastatic brain tumors. At least 10-15 percent of all patients detected with a primary cancer will develop a secondary cancer in the brain, with metastatic brain tumors affecting about 200,000 people a year.
More and more brain tumor patients are joining the nearly half a million people worldwide who have been treated with Gamma Knife surgery. Studies show that Gamma Knife radiosurgery results in local control exceed an average of 85 percent for the management of tumors in any brain location.1
By its very nature, radiosurgery has inherent radiation risks, including the risk associated with radiation dose to normal tissue outside the defined target. It is understood that for low doses, the carcinogenic risk is proportionate to the dose2, and that the effects may not be seen for as many as 10 or more years following treatment.
Because of the significant risks associated with millimeter errors in targeting for the brain, Gamma Knife surgery combines physical immobilization of the patient's head with ultra-accurate targeting and delivery. Patients are secured with a stereotactic head frame, which not only assists in targeting, it ensures total immobilization of the head during imaging and treatment.3
With accuracy proven during more than 50 years of use, the stereotactic frame has become the technique of choice for precision neurosurgery.
The CyberKnife, which claims to be "as good as" Gamma Knife, uses a thermo plastic mask meant to immobilize the patient's head. This immobilization device was tried and discarded for use with Leksell Gamma Knife, because, as stated in a clinical study, "Head movement was restrained but not eliminated."4
A clinical study showed that with the mask, target movement was not only possible, it was likely. Of the 250 cases examined, 146 cases (58%) had shifts of targets >2 mm during treatment. One patient's CyberKnife treatment was cancelled after only seven nodes of treatment due to movement, and to avoid uncomfortably long treatment sessions, patient positioning was monitored every second minute, not every 10 seconds as recommended. Intra-fraction errors of >2mm occurred between repositioning.
There are three types of accuracy commonly measured for radiosurgery. The first is mechanical accuracy, which is the sum of all mechanical tolerances. Leksell Gamma Knife is guaranteed to <0.3mm5 (based on 170 measurements over five years) compared to CyberKnife's 0.5mm pointing precision.
The second is radiological accuracy, which includes the mechanical accuracy plus beam delivery accuracy. For Gamma Knife this is guaranteed to <0.50mm, with an average achievable accuracy of 0.15mm, based on 332 measurements over a period of two years on 189 installed systems. The average radial error of the CyberKnife system is 2.1mm.
The third and most important measurement of accuracy is the total clinical accuracy. This is an end-to-end measurement combining mechanical and radiological accuracy plus imaging. Gamma Knife's average achievable clinical accuracy is 0.48mm5 compared to CyberKnife's average total of 2.1mm as measured in the study.
Not only must physicians worry about the effects of the treatment dose, but also leakage - stray radiation emanating from the linac or cobalt source. As cancer patients live longer, the need to be attuned to this body dose becomes increasingly important. Also, pediatric patients and those with benign indications should not be subjected to unnecessary radiation. Leksell Gamma Knife was engineered to minimize doses to normal tissue surrounding the target as well as to the whole body.
A clinical study showed that CyberKnife delivers a dose outside the target that is two to seven times higher than the older B-model Gamma Knife for the same target dose6. The main source is leakage from the CyberKnife's "lightweight" linac on the robotic arm. In addition, the entrance and exit dose in the CyberKnife is significant, and a larger number of beams must be used per isocenter.
In contrast, Leksell Gamma Knife achieves a higher degree of shielding, thereby lowering the entrance and exit dose. The new Leksell Gamma Knife Perfexion™ delivers a body dose that is 20 to 100 times lower than the CyberKnife.