Cyberknife

CyberKnife is the name of a frameless robotic radiosurgery system invented by John R. Adler, a Stanford University Professor of Neurosurgery and Radiation Oncology. The CyberKnife system is sold by the company Accuray, located in Sunnyvale, California. The CyberKnife system is used for treating benign tumors, malignant cancers and other medical conditions located anywhere in the body.^[1][2]

Numerous CyberKnife systems are installed around the world, for example in China, Japan, Italy and the USA. Many of the American University Medical Centers operate a CyberKnife system. Examples are the Stanford Blake Wilbur CyberKnife Center and at the Comprehensive Cancer Center at Stanford University, Georgetown University Hospital, UCSF Medical Center and the University of Pittsburgh.^{[citation needed]} Stanford University has treated over 3,500 patients using the CyberKnife system, and worldwide over 40,000 patients have been treated.^[3]

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Main features

  Several generations of the CyberKnife system have been developed since its initial inception in 1990. There are two essential features of the CyberKnife system that set it apart from other stereotactic therapy methods.

Robotic mounting

The Cyberknife system uses a radiation source mounted on a precisely controlled industrial robot. The original CyberKnife used a Fanuc robot^[4], however the more modern systems use a KUKA 240.^[5] 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. The radiation is collimated using tungsten collimators (also referred to as “cones”) which produce circular radiation fields. At present the radiation field sizes are 5mm, 7.5mm, 10mm, 12.5mm, 15mm, 20mm, 25mm, 30mm, 35mm, 40mm, 50mm and 60mm. Mounting the radiation source on the robot allows complete freedom to position the radiation within a sphere about the patient. The robotic mounting allows very fast repositioning of the source, which allows the system to deliver radiation from many different directions, which is impractical using a conventional gantry based linear accelerator system due to the mechanical limitations of the gantry as compared to a 6 degree of freedom robot.

Image guidance

The image guidance system is the other essential item in the CyberKnife system. X-ray imaging cameras are located on supports around the patient allowing instantaneous x-ray images to be obtained.

6D Skull

  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 patients skull.^[6] 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.

Xsight

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^[7] 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.

A recent enhancement to Xsight is Xsight-Lung^[8] which allows tracking of some lung tumors without the need to implant fiducials.

Fiducial

For soft tissue tumors, a method known as fiducial tracking can be utilized.^[9] 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.^[10][11]

Synchrony

The final technology of image guidance that the CyberKnife system can use is called the Synchrony system. The Synchrony system is utilized primarily for tumors that are in motion while being treated, such as lung tumors and pancreatic tumors.^[12] The synchrony system uses a combination of surgically placed internal fiducials, and light emitting optical fibers (markers) mounted on the patient skin. 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 patients skin. The Synchrony system overcomes this by periodically taking images of the internal fiducials, and predicting their location at a future time using the motion of the markers that are located on the patients skin. The light from the markers can be tracked continuously using a CCD camera, and are placed so that their motion is correlated with the motion of the tumor. A computer algorithm creates a correlation model that represents how the internal fiducial markers are moving compared to the external markers. The Synchrony system is therefore continuously predicting the motion of the internal fiducials, and therefore the tumor, based on the motion of the markers. The correlation model can be updated at any time if the patient breathing becomes in any way irregular. The advantage of the Synchrony system is that no assumptions about the regularity or reproducibility of the patient breathing have to be made. To function properly Synchrony 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.

RoboCouch

Some version of the Cyberknife system are equipped with the RoboCouch ^[13], which allows additional flexibility in positioning the patient.

Frameless

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. 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 Neurosurgeon, 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.

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.^[14] 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.^[15] This can allow a larger dose to be delivered to the tumor compared to a single treatment.

Clinical uses

The CyberKnife system has FDA clearance for treatment of tumors in any location of the body. Some of the tumors treated include: pancreas,^[16][17] liver,^[18] prostate,^[19] Spinal Lesions,^[20] head and neck cancers,^[21] and benign tumors.^[22]

See also

• Gamma knife
• Horsley-Clarke apparatus
• Robotic surgery
• Medical Physics

References

Example treatment centers

• Stanford CyberKnife Center, first site, second site
• University of Pittsburgh
• UCSF Cyberknife
• (http://http://www.lifebridgehealth.org/) Sinai CyberKnife
• Cyberknife Miami
• Georgetown University Hospital
• Hematology/Oncology Associates Of Central New York

Competitors

• Brainlab
• Elekta