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The CyberKnife is a frameless robotic radiosurgery system used for treating benign tumors, malignant tumors and other medical conditions. The system was invented by John R. Adler, a Stanford University professor of neurosurgery and radiation oncology, and Peter and Russell Schonberg of Schonberg Research Corporation. It is made by the Accuray company headquartered in Sunnyvale, California.
The CyberKnife system is a method of delivering radiotherapy, with the intention of targeting treatment more accurately than standard radiotherapy. The two main elements of the CyberKnife are:
- the radiation produced from a small linear particle accelerator (linac)
- a robotic arm which allows the energy to be directed at any part of the body from any direction
Several generations of the CyberKnife system have been developed since its initial inception in 1990. There are two major features of the CyberKnife system that are different from other stereotactic therapy methods.
The first is that the radiation source is mounted on a general purpose industrial robot. The original CyberKnife used a Japanese Fanuc robot, however the more modern systems use a German KUKA KR 240. Mounted on the Robot is a compact X-band linac that produces 6MV X-ray radiation. The linac is capable of delivering approximately 600 cGy of radiation each minute – a new 800 cGy / minute model was announced at ASTRO 2007. The radiation is collimated using fixed tungsten collimators (also referred to as "cones") which produce circular radiation fields. At present the radiation field sizes are: 5, 7.5, 10, 12.5, 15, 20, 25, 30, 35, 40, 50 and 60 mm. ASTRO 2007 also saw the launch of the IRIS variable-aperture collimator which uses two offset banks of six prismatic tungsten segments to form a blurred regular dodecagon field of variable size which eliminates the need for changing the fixed collimators. Mounting the radiation source on the robot allows near-complete freedom to position the source within a space about the patient. The robotic mounting allows very fast repositioning of the source, which enables the system to deliver radiation from many different directions without the need to move both the patient and source as required by current gantry configurations.
The second is that the CyberKnife system uses an image guidance system. X-ray imaging cameras are located on supports around the patient allowing instantaneous X-ray images to be obtained.
The original (and still utilized) method is called 6D or skull based tracking. The X-ray camera images are compared to a library of computer generated images of the patient anatomy. Digitally Reconstructed Radiographs (or DRR's) and a computer algorithm determines what motion corrections have to be given to the robot because of patient movement. This imaging system allows the CyberKnife to deliver radiation with an accuracy of 0.5mm without using mechanical clamps attached to the patient's skull. The use of the image-guided technique is referred to as frameless stereotactic radiosurgery. This method is referred to as 6D because corrections are made for the 3 translational motions (X,Y and Z) and three rotational motions. It should be noted that it is necessary to use some anatomical or artificial feature to orient the robot to deliver X-ray radiation, since the tumor is never sufficiently well defined (if visible at all) on the X-ray camera images.
Additional image guidance methods are available for spinal tumors and for tumors located in the lung. For a tumor located in the spine, a variant of the image guidance called Xsight-Spine is used. The major difference here is that instead of taking images of the skull, images of the spinal processes are used. Whereas the skull is effectively rigid and non-deforming, the spinal vertebrae can move relative to each other, this means that image warping algorithms must be used to correct for the distortion of the X-ray camera images.
For soft tissue tumors, a method known as fiducial tracking can be utilized. Small metal markers (fiducials) made out of gold for bio-compatibility and high density to give good contrast on X-ray images are surgically implanted in the patient. This is carried out by an interventional radiologist, or neurosurgeon. The placement of the fiducials is a critical step if the fiducial tracking is to be used. If the fiducials are too far from the location of the tumor, or are not sufficiently spread out from each other it will not be possible to accurately deliver the radiation. Once these markers have been placed, they are located on a CT scan and the image guidance system is programmed with their position. When X-ray camera images are taken, the location of the tumor relative to the fiducials is determined, and the radiation can be delivered to any part of the body. Thus the fiducial tracking does not require any bony anatomy to position the radiation. Fiducials are known however to migrate and this can limit the accuracy of the treatment if sufficient time is not allowed between implantation and treatment for the fiducials to stabilize.
Another technology of image guidance that the CyberKnife system can use is called the Synchrony system or Synchrony method. This method uses a combination of surgically placed internal fiducials (typically small gold markers, well visible in x-ray imaging), and light emitting optical fibers (LED markers) mounted on the patient skin. LED markers are tracked by an infrared tracking camera. Since the tumor is moving continuously, to continuously image its location using X-ray cameras would require prohibitive amounts of radiation to be delivered to the patient's skin. The Synchrony system overcomes this by periodically taking images of the internal fiducials, and computing a correlation model between the motion of the external LED markers and the internal fiducials. Time stamps from the two sensors (x-ray and infrared LED) are needed to synchronize the two data streams, hence the name Synchrony.
Motion prediction is used to overcome the motion latency of the robot and the latency of image acquisition. Before treatment, a computer algorithm creates a correlation model that represents how the internal fiducial markers are moving compared to the external markers. During treatment, the system continuously infers the motion of the internal fiducials, and therefore the tumor, based on the motion of the skin markers. The correlation model is updated at fixed time steps during treatment. Thus, the Synchrony tracking method makes no assumptions about the regularity or reproducibility of the patient breathing pattern.
To function properly, the system requires that for any given correlation model there is a functional relationship between the markers and the internal fiducials. The external marker placement is also important, and the markers are usually placed on the patient abdomen so that their motion will reflect the internal motion of the diaphragm and the lungs. This method was invented in 1998. The first patients were treated at Cleveland Clinic in 2002. Synchrony is utilized primarily for tumors that are in motion while being treated, such as lung tumors and pancreatic tumors.
A robotic six degree of freedom patient treatment couch called RoboCouch improves patient positioning options for treatment.
The frameless nature of the CyberKnife also increases the clinical efficiency. In conventional frame-based radiosurgery, the accuracy of treatment delivery is determined solely by connecting a rigid frame to the patient which is anchored to the patient’s skull with invasive aluminum or titanium screws. The CyberKnife is the only radiosurgery device that does not require such a frame for precise targeting. Once the frame is connected, the relative position of the patient anatomy must be determined by making a CT or MRI scan. After the CT or MRI scan has been made, a radiation oncologist must plan the delivery of the radiation using a dedicated computer program, after which the treatment can be delivered, and the frame removed. The use of the frame therefore requires a linear sequence of events that must be carried out sequentially before another patient can be treated. Staged CyberKnife radiosurgery is of particular benefit to patients who have previously received large doses of conventional radiation therapy and patients with gliomas located near critical areas of the brain. Unlike whole brain radiotherapy, which must be administered daily over several weeks, radiosurgery treatment can usually be completed in 1–5 treatment sessions. Radiosurgery can be used alone to treat brain metastases, or in conjunction with surgery or whole brain radiotherapy, depending on the specific clinical circumstances.
By comparison, using a frameless system, a CT scan can be carried out on any day prior to treatment that is convenient. The treatment planning can also be carried out at any time prior to treatment. During the treatment the patient need only be positioned on a treatment table and the predetermined plan delivered. This allows the clinical staff to plan many patients at the same time, devoting as much time as is necessary for complicated cases without slowing down the treatment delivery. While a patient is being treated, another clinician can be considering treatment options and plans, and another can be conducting CT scans.
In addition, very young patients (pediatric cases) or patients with fragile heads because of prior brain surgery cannot be treated using a frame based system. Also, by being frameless the CyberKnife can efficiently re-treat the same patient without repeating the preparation steps that a frame-based system would require.
The delivery of a radiation treatment over several days or even weeks (referred to as fractionation) can also be beneficial from a therapeutic point of view. Tumor cells typically have poor repair mechanisms compared to healthy tissue, so by dividing the radiation dose into fractions the healthy tissue has time to repair itself between treatments. This can allow a larger dose to be delivered to the tumor compared to a single treatment.
Since August 2001, the CyberKnife system has FDA clearance for treatment of tumors in any location of the body. Some of the tumors treated include: pancreas, liver, prostate, spinal lesions, head and neck cancers, and benign tumors.
None of these studies have shown any general survival benefit over conventional treatment methods. By increasing the accuracy with which treatment is delivered there is a potential for dose escalation, and potentially a subsequent increase in effectiveness, particularly in local control rates. However the studies cited are so far limited in scope, and more extensive research will need to be completed in order to show any effects on survival.
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Stanford University has treated over 2,500 patients using the Cyberknife system, and worldwide over 40,000 patients have been treated.
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