The principle of Gamma Knife treatment is the delivery of high-dose ionizing radiation through 201 cobalt-60 sources that focus the radiation to converge simultaneously at the target tumor.
The basic steps of the procedure are localization of the target, dose planning, and treatment. From start to finish, the Gamma Knife procedure typically lasts three to five hours.
Stereotactic Head Frame
Localization begins with the fitting of a stereotactic frame onto the patient's head. The frame attaches at four points around the patient's head with pins that penetrate about 2 millimeters. To minimize discomfort when the frame is attached, the patient is given local anesthesia (usually general for young children) and, as needed, a mild sedative. Frame fitting usually takes about 20 minutes.
With the frame in place, the patient undergoes MRI or other imaging studies to precisely determine the position of the lesion within a coordinate field.These localization coordinates are entered into the Gamma Knife's three-dimensional computer planning program.
The image-integrated software allows the physician team to fit the isodose curves to conform to the tumor's shape with an accuracy of within 0.1 millimeter. They then calculate the dose and exposure time. By employing multiple isocenters and advanced beam channel plugging patterns, this unique planning program enables the physician team to customize a treatment plan even in the event of geometrically complex lesions. While the team is planning treatment, the patient may visit with family and friends.
Once planning is complete, the patient is moved to the treatment room that houses the Gamma Unit. The apparatus is composed of a moveable couch and a hemispherical radiation unit containing 201 sources of cobalt-60. For patient and team safety, the unit is heavily shielded. The Gamma Unit is FDA approved and meets the safety standards of the U.S. Nuclear Regulatory Commission.
The patient is placed on the couch and positioned into the collimator helmet. The stereotactic frame is still in place, immobilizing the patient's head to ensure a safe, accurate procedure. After the patient is properly aligned on the couch, the team enters the adjacent control room to administer the treatment, observing the procedure via video monitors. Two-way microphone communication between the team and the patient is ongoing.
The couch slides the patient into the unit for exposure. During treatment, the helmet's 201 ports will focus the unit's carefully collimated gamma rays to converge simultaneously and precisely at the target tumor, with accuracy within 0.1 millimeter. The arrangement of radiation sources and the exacting collimation ensure minimal exposure to surrounding tissue. The total length of treatment is related to the size and location of the lesion.
At the end of the treatment, the couch automatically slides the patient from the unit and the stereotactic frame is removed. No special recovery is required; the patient is returned immediately to his or her hospital room. Treatment-day side effects are rare; some mild nausea may be experienced immediately after treatment, and some patients feel discomfort or headache due to removal of the frame. Observation continues for a short period, and the patient usually goes home the same day. Most Patients are able to return to their usual daily routine immediately upon discharge.
As with any medical procedure, some risks are involved. Possible delayed complications (6 months or longer) include injury to surrounding brain and edema, which is reversible with proper treatment. Immediate side effects are extremely rare. The risks of traditional brain surgery, such as hemorrhage, infection, and stroke, are avoided with Gamma Knife radiosurgery.
Because the positive effects of Gamma Knife treatment are realized over time, periodic imaging studies are conducted to confirm that growth of the tumor has been arrested and to ascertain whether shrinkage has occurred. In the case of AVMs, imaging will confirm that occlusion has taken place or that the malformation has disappeared. As with all stages of Gamma Knife treatment, this post-procedure monitoring will involve collaboration of the Gamma Knife team and the patient's primary physician.
The team meets at least weekly, so they can ensure that each patient receives an immediate evaluation.
Some images courtesy of Elekta, Inc. www.elekta.com.