This is a guest post by Amanda who writes on behalf of the San Diego Gamma Knife Center, a shared resource for the San Diego neurosurgical and radiation oncology community.Gamma Knife surgery has become one of the most popular and extensively tested types of what is known as “stereotactic radiosurgery”, or SRS. SRS (as depicted in the accompanying illustration) is a non-invasive medical procedure that uses highly focused radiation beams as a treatment for brain tumors and other kinds of intracranial maladies. Gamma Knife surgery has been used for treating trigeminal neuralgia, and the treatment of both benign and malignant brain tumors. But the Gamma Knife is not the only SRS system available. In recent years, the popularity and acceptance of radiosurgery procedures has led to the development of newer SRS systems. The most well known among these newer systems is the Cyber Knife. Both the Gamma Knife and the Cyber Knife can be highly effective in treating many of the same maladies, but they operate on patients in fundamentally different ways. Here is a brief comparison of the two SRS systems.
How They Work
During Gamma Knife surgery, patients undergo a CT scan and sometimes also an MRI scan prior to the actual procedure. These scans are used by a team of specialists to plan out the radiosurgery in minute detail. Once the planning is finished, the patient is brought over to the Gamma Knife table and his/her head is fixed onto a rigid head frame that automatically positions the target area of the brain in the spot where the radiation beams will intersect. The Gamma Knife system then delivers approximately 201 highly focused cobalt-60 source radiation beams into a single target area of the brain. The beams are delivered all at once, multiple times and in short bursts. The procedure can last as little as 30 minutes and usually a single session is all that’s needed to complete the treatment.Unlike the Gamma Knife system, which is designed exclusively for use in non-invasive brain surgery, the Cyber Knife system is designed to conduct SRS on lesions anywhere in the body, including organs that move with the patient’s respiration. It can do so because, unlike the Gamma Knife, it does not deliver the radiation beams to the target area simultaneously. Rather, it uses a single, high-energy photon beam that’s attached to a robot arm. The arm moves to different positions during the surgery, aiming at the target area from different angles. A head frame is not necessary either. Instead, patients are secured to the treatment table with a plastic mask that helps the Cyber Knife update the position of the skull in real time, allowing the robot arm to compensate for patient movement. More than one treatment session is sometimes required.
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The quoted Gamma Knifeaccuracy of 0.5 mm is the ability of the Gamma Knife to accurately hit a phantom target not taking anything else into consideration. The 1 mm accuracy of the CyberKnife is the over all total accuracy of the entire radiosurgery procedure. We call it “end to end” accuracy. And 1 mm is the upper limit, most often it is half or less than this number. To get the end to end number you scan a head phantom, import the scans to a planning computer, calculate a treatment plan, place the phantom on the treatment table, image the phantom and properly localize the target, then deliver the treatment with films located inside the target. Then measure how accurately you deliver the treatment. The 0.5 mm Gamma Knife figure only takes into consideration the last step, how accurately a target can be placed at the Gamma Knife isocenter. A true end to end test on a Gamma Knife will actually result in numbers fairly close to CyberKnife. The 0.5 mm does not take into consideration the possible MRI image distortion, the MRI pixel size (0.5 mm), the accuracy of localizing the head frame fiducial markers.
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