- Gamma Knife
- Steps of Gamma Knife Procedure
- What to Expect/FAQ
Gamma Knife Radiosurgery for Acoustic Neuromas and Glomus Tumors
Gamma Knife radiosurgery is a highly specialized technique used to treat tumors and lesions of the brain and surrounding structures with radiation. As used by the physicians of the Ear Institute of Chicago, the Gamma Knife is used to treat certain tumors in and around the ear (specifically, acoustic neuromas, meningiomas and glomus jugulare tumors). Despite its name, Gamma Knife radiosurgery does not require an incision; the skull is never opened. The "blades" of the Gamma Knife are the beams of gamma radiation computer programmed to target the lesion at the point where the beams intersect. In a single treatment session, 201 beams of gamma radiation focus precisely on the lesion. Each individual radiation beam is too weak to harm the brain tissue it passes through. The damage occurs only at the spot where all the beams overlap. With the help of a computer, this spot can be accurately plotted to within a fraction of a millimeter.
Gamma rays entering target (side view)
Gamma rays entering target (top view)
Gamma Knife Radiosurgery is not designed to remove an acoustic neuroma or glomus jugulare tumor. Instead, Gamma Knife Radiosurgery is used to control the growth of these tumors. Specifically, the Gamma Knife is designed to prevent tumor growth and, at times, to decrease the size of the tumor. A small percentage of tumors treated with Gamma Knife Radiosurgery will continue to grow and may require traditional, open surgical treatment.
The physicians of the Ear Institute of Chicago perform Gamma Knife Radiosurgery at the Illinois Gamma Knife Center located at Alexian Brothers Medical Center in Elk Grove, Illinois. The Illinois Gamma Knife Center uses the Leksell ® Gamma Knife 4C, which is considered by many to be the gold standard in stereotactic radiosurgical treatment for lesions in the skull.
History of the Gamma Knife
In the early 1950’s Professor Lars Leksell, a Swedish neurosurgeon, first developed the concept of radiosurgery along with stereotactic devices (used to guide the gamma rays). Professor Leksell worked with a physicist, Borje Larsson, to build the first Gamma Knife unit in Sweden in 1968. Since that time, this non-invasive technique for the treatment of brain tumors and vascular malformations has enjoyed incredible success. Worldwide, the Leksell ® Gamma Knife has been used to treat more than 330,000 malignant and benign tumors and tens of thousands of other brain conditions. Since the development of the Gamma Knife, over 2,500 peer-reviewed medical research articles support the use of Gamma Knife.
(Leksell Gamma Knife is a registered trademark of Elekta Instruments, Inc.)
Steps of the Gamma Knife Radiosurgery Procedure
Stereotactic Radiosurgery using the Gamma Knife consists of five main steps
- 1. Frame placement. A lightweight frame is fitted over the patient’s head and attached with screws using local anesthesia. The frame serves two purposes: On the side of the frame are markers that allow the treatment team to pinpoint areas that will receive treatment. The frame also keeps the patient’s head immobile during the radiation procedure.
Head frame with CT/MRI box
- 2. Imaging Study. Imaging studies are obtained using MRI, CT scan, angiography, or a combination of these technologies. The imaging studies produce a three-dimensional map of the tumor and the surrounding brain structures.
3-D image of an acoustic neuroma (in yellow)
3-D image of an acoustic neuroma (in yellow)
- 3. Treatment planning. Based on the imaging studies, your physician works with a radiation oncologist and a medical physicist to develop the treatment plan that accurately targets the tumor with exact precision, sparing surrounding tissue as much as possible (i.e. a plan than conforms closely to the shape of the lesion).
Screenshot of computer workstation view of left-sided 1.8-cm acoustic neuroma prior to planning. (Clockwise from top left: axial, coronal, axial, sagittal views)
Left-sided 1.8-cm acoustic neuroma prior to treatment plan. Yellow=outline of tumor; red circles=planned radiation shots
Above left-sided 1.8-cm acoustic neuroma after treatment plan (close up view). Yellow=outline of tumor; red circles=planned radiation shots
- 4. Radiation treatment. Treatment begins by first having the patient lie on the Gamma Knife bed (Left Figure Below). The head frame is fitted into a helmet with 201 holes in the helmet (Right Figure Below). The helmet helps to guide 201 sources of cobalt 60 based (photon) radiation beams to accurately target the tumor. After the helmet is placed, the bed is precisely moved into the domed unit and treatment begins. Depending on the size and location of the tumor, the actual time of radiation delivery could range from a few minutes to over an hour.
Leksell 4C gamma knife unitPatient inside helmet
- 5. Removal of the head frame. Removal of the frame is a painless procedure. Most patients are able to go home shortly after the head frame is removed.
Frequently Asked Questions Regarding Gamma Knife Radiosurgery
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After patients arrive at the Gamma Knife Center they are given a mild sedative (children are often completely anesthetized). Shortly after that time, a box-shaped head frame is attached to the head with four screws (two in front and two in back). The key to the gamma-knife's precision lies in this box-shaped frame. The frame serves two purposes: 1. It holds the patient's head perfectly still when radiation is given. 2. The frame acts as a reference point in determining exactly where the beams of radiation should converge.
The four spots on the scalp where the screws enter are numbed first with injections containing an anesthetic similar to that used by dentists. Hair will not be shaved, but it may be tied back into a ponytail if it is long. The head-frame is lightweight, so patients are able to move their head around after the frame is attached to the skull.
Once the head-frame is attached, an imaging scan (MRI or CT) is performed to locate the exact area inside the skull that needs treatment. Even though a scan may have been done before, these scans must be repeated with the head-frame in place.
A transparent plastic box is attached to the head-frame for the imaging scans. This box has special material in it that acts as localizer when the physicians plan the necessary radiation configuration used for treatment.
After the imaging scans are taken, it may take an hour or more for the targeting plan to be computed. During this waiting period, patients are taken to an area in the Gamma Knife Center where they can relax.
When it is time for the treatment, the patient is asked to lie down in the radiation machine so that the patient's head is put into a helmet. The helmet attaches to the head-frame, which keeps the head perfectly still. This type of setup ensures that there is no head movement. The lack of head movement allows the radiation beams to converge on the target and not on the healthy tissue surrounding the target area(s).The helmet does not cover the face and the radiation is not felt by the patient. There is no noise during the treatment.
The number of minutes that each radiation dose lasts is determined during the dose planning. Treatment may include multiple doses. Sometimes, more than one type of helmet and head position is used to deliver the radiation.
The head-frame stays on the head through the entire procedure. When the frame is removed, the places on the scalp where the screws entered may be a little tender, but the pin sites typically do not scar. Some patients may have a headache or feel nauseated for a few hours after the procedure.
Adult patients will be given a pill to take prior to placement of the head frame. The pill causes a sensation of relaxation, but patients are still awake. The scalp is numbed (anesthetized) with an anesthetic similar to that used by dentists.
Young children are given a general anesthetic for the entire procedure including placement of the head frame.
Headache and nausea my occur within the first 24 hours after treatment. These side effects are often short term, uncommon, and often effectively treated if brought to the attention of the physician or nurse.
Long term side effects are related to both the type of tumor that is treated, as well as the size of the tumor. For a detailed discussion of long term risks of the Gamma Knife, please consult your doctor.
With standard external beam radiation therapy, tumors and much or all of the surrounding brain are treated with the same dose of radiation. Standard external beam radiation is given in small increments over several weeks to allow normal brain tissue to recover from the harmful effects of the external beam radiation. The brain can only absorb a certain dose of radiation, beyond which no further treatment is advisable.
Gamma Knife Radiosurgery, on the other hand, allows radiation to be largely confined to the target (e.g. a brain tumor) itself and little radiation reaches the surrounding brain. The treatment is given in one session, not in fractions over a period of days or weeks, as is the case for external beam radiation. Finally, if the tumor re-grows, or new tumors appear, patients can be retreated with radiosurgery. Usually patients cannot be retreated by radiotherapy.