CyberKnife®

Robotic Radiosurgery System

The present design of the CyberKnife® Robotic Radiosurgery System consists of three key components: 1) an advanced, lightweight linear accelerator (LINAC) (this device is used to produce a high energy (6MV) “killing beam” of radiation), 2) a robot which can point the linear accelerator from a wide variety of angles, and 3) several x-ray cameras (imaging devices) that are combined with powerful software to track patient and tumor position. The x-ray cameras obtain frequent pictures of the patient during treatment, and use this information to target the radiation beam emitted by the linear accelerator.

The robot is instrumental in precisely aiming this device. When a patient moves during treatment, the change in position is detected by the x-ray cameras, and the robot compensates by re-targeting the linear accelerator before administering the radiation beam. This process of continually detecting and correcting ensures accurate radiation targeting throughout treatment.

The accuracy of treatment is probably the single most important defining feature of radiosurgery. As a consequence, the method of targeting utilized by the CyberKnife represents a major advance over older frame-based radiosurgical instruments.

By using digital radiographic images of anatomical features or implanted fiducials to localize the position of a tumor, the CyberKnife eliminates the need for a stereotactic frame. Although frameless and therefore non-invasive, the accuracy of the CyberKnife is comparable if not superior to frame-based systems.

Once CyberKnife is determined to be the best option for a particular condition, there are three basic steps involved in the treatment process:

• 1. Treatment setup
• 2. Treatment planning,
• 3. Treatment delivery.

These steps can be performed on the same day or separate days, depending on the combined schedule preferences of the CyberKnife team and patient.

Unlike frame-based procedures, where the entire process must be performed in a single day, a CyberKnife patient does not need to wait in the hospital while the treatment plan is being developed. As a result, a patient can go home after setup and return on a separate day for treatment delivery.

Treatment setup

Setup is the initial process that allows a physician to plan and deliver a CyberKnife treatment. For a typical cranial tumor, a custom-fit plastic mask is made for each patient. This mask, unlike the conventional metal head frame, is noninvasive and painless. With the mask in place, the patient undergoes a CT scan with contrast (iodinated dye), which is then used to precisely plan delivery of radiation to the tumor. In some instances, a MRI scan may also be necessary in order to fully visualize the tumor and adjacent critical anatomy.

When tumors are treated in other areas of the body, a body cradle is custom-fit for the individual patient instead of the mask. Some patients require placement of small metal fiducials prior to treatment set-up. These implanted metal fiducials are 3 to 4 mm long and are used to accurately target radiation from the CyberKnife. These markers must be implanted during a short 10 to 15 minute outpatient procedure prior to the CT scan.

Treatment planning

Treatment planning is the process through which physicians and the physics team plan the details of radiation delivery to a tumor or other lesion. The CyberKnife treatment planning system harnesses the massive computing power of high-speed computers to develop an optimal pattern of radiation. Once the volume and dose of radiation is determined, the CyberKnife computer performs millions of calculations to determine the best radiation delivery plan.

The CyberKnife’s treatment planning system exploits the robot’s high degree of maneuverability to allow a more even delivery of radiation throughout a tumor than can be achieved by frame-based radiosurgical systems.

Treatment Delivery

At some point after planning, the patient returns for treatment delivery. The CyberKnife treatment follows the following basic format. The patient is fitted with the custom plastic mask (for cranial tumors) or a cradle (for body tumors) and lies on the treatment table. Prior to beginning the actual radiation treatment, the imaging system acquires digital x-rays of the patient position.

This information is used to move the linear accelerator on the robot to the appropriate position. Subsequently, the robot moves and re-targets the linear accelerator at a large number of positions around the patient. At each position or “node”, a small radiation beam is delivered.

This process is repeated at 50 to 300 different positions around the patient to complete the treatment. At various intervals, the linear accelerator stops and additional pictures are obtained of the patient, thereby allowing the CyberKnife to track and compensate for small amounts of patient movement. These intervals can be as frequent as every node.

The entire process is painless, and it typically takes between 30 to 90 minutes to deliver all radiation beams. Usually a patient can go home immediately upon completion and return to normal activities. If the treatment prescription is for staged (fractionated) radiosurgery, the patient will return on a separate visit and repeat the above process for treatment delivery.

Benefits

This technology makes it possible for neurosurgeons to reach the deepest recesses of the brain and correct disorders not treatable with conventional surgery. Since there is no incision, surgical risks such as infection are not an issue, and there is little discomfort. Adult patients may be lightly sedated but are awake throughout the procedure. Hospitalization is short and at most, requires an overnight stay. The majority of patients are treated on an outpatient basis. As a result, patients have less discomfort and much shorter recovery periods.

Recovery

Following stereotactic radiosurgery, bandages are usually placed over the pin sites from the stereotactic frame, which should be removed the following day. Patients may be observed for a specified time after the treatment before they go home, or they may be kept in the hospital overnight for observation. Some people experience minimal tenderness around the pin sites. Occasionally, swelling may also occur around the pin sites. Most patients can return to their usual activities the following day if swelling is not bothersome.

Follow-up

Customarily, the neurosurgeon will want to see the patient in the office about one month after the procedure. A neurological examination will be performed. Often, a diagnostic test such as a CT scan or MRI will be performed about six months after the procedure to check on the status of the radiated area. These changes may take from one to three years to take effect.