Gamma Knife

What is this equipment used for?

The gamma knife and its associated computerized treatment planning software enable physicians to locate and irradiate relatively small targets in the head (mostly inside the brain) with extremely high precision. Intense doses of radiation can be given to the targeted area(s) while largely sparing the surrounding tissues. The gamma knife can be used for a wide variety of problems. For example, it can be used to treat selected malignant tumors that arise in or spread to the brain (primary brain tumors or metastatic tumors), benign brain tumors (meningiomas, pituitary adenomas, acoustic neuromas), blood vessel defects (arterio-venous malformations) and functional problems (trigeminal neuralgia). Possible future uses are being evaluated for epilepsy and Parkinson's disease.

The gamma knife loses its ability to spare surrounding normal tissues as the number of targets increases, is not suitable for large targets (larger than three to four centimeters in size) and is not used for targets outside of the head.

How does it work?

The gamma knife works by a process called stereotactic radiosurgery, which uses multiple beams of radiation converging in three dimensions to focus precisely on a small volume, such as a tumor, permitting intense doses of radiation to be delivered to that volume safely. Treatment that is fractionated over several days may be referred to as stereotactic radiotherapy as opposed to stereotactic radiosurgery.

The patient is placed under local anesthesia while a special headframe that has three-dimensional coordinates built into it is attached to the skull with four screws. Then, magnetic resonance imaging (MRI) and/or a computed tomography (CT) scan and/or a catheter angiogram are obtained and the results are sent to the gamma knife's planning computer system. Physicians (radiation oncologists, neurosurgeons and/or neuroradiologists) and medical radiation physicists use the planning computer to determine the exact relationship between the target lesions and the frame and calculate how to set the controls of the gamma knife to treat the targets optimally. Targets often are best treated by combinations of several aimings, commonly known as "shots." The physicians and physicists routinely consider numerous fine-tuning adjustments until an optimal plan is created. Simultaneously, an optimal dose is selected.

Using the three-dimensional coordinates determined in the planning process, the frame is then precisely attached to the gamma knife unit to guarantee that when the unit is activated, the target is placed exactly in the center of approximately 200 precision-aimed, converging beams of (Cobalt-60 generated) gamma radiation. Treatment takes anywhere from several minutes to a few hours to complete depending on the shape of the target and the dose required. Patients do not feel the radiation. Following treatment the headframe is removed. Each target generally requires only one treatment session.

See the Stereotactic Radiosurgery page for additional information.

Who operates this equipment?

A multidisciplinary team approach provides patients with the greatest safety. The team is most commonly comprised of a radiation oncologist, a medical radiation physicist and a neurosurgeon—all specially trained in the use of the gamma knife—with support from nursing staff, anesthesiologists (for patients who are unable to cooperate, such as children) and radiation therapists, who work together to provide patients with the high-quality care they deserve.

How is safety ensured?

Because placement accuracy of the shots is critical to localization of the radiation (to the fraction of a millimeter) anything that would degrade this precision is unacceptable. Rigid attachment of the headframe, geographic targeting accuracy of the MRI, shaping of the volume of tissue to be treated (selection of the number, size and relative intensity of the shots) and accuracy of attachment of the frame to the gamma knife unit are all critical. As is true of all radiation therapy, correct selection and calculation of the amount of radiation to deliver are essential. A qualified medical physicist assures that the imaging and treatment planning computers and software are correct and acceptable. The mechanical functions of the machine are tested on a regular basis to ensure the safety of patients and medical staff.