Kenneth Ott, M.D., F.A.C.S.
San Diego Gamma Knife Center
Gamma Knife Radiosurgery
Last updated 4/23/02
Radiosurgery is a surgical procedure in which narrow beams of radiation are targeted to a volume of tissue within the brain. This highly focused and destructive dose of radiation is given in a single session and avoids potentially harmful radiation to surrounding brain structures. Stereotaxis refers to an accurate targeting technique for intracranial structures (such as brain tumors) using an external reference frame fixed to the head. Modern imaging by CT and MR technology and computer advances has made stereotaxis a very potent aid in brain tumor diagnosis and treatment. Since 1968, non-invasive Gamma Knife radiosurgery for the treatment of brain tumors and vascular malformations has enjoyed incredible success. More than 65,000 patients have been safely treated with focused gamma rays worldwide.
Radiosurgery differs from conventional radiation therapy in several respects. With standard external beam radiation therapy techniques, tumors and much or all of the surrounding brain are treated to the same dose of radiation. The radiation dose is given in small increments over several weeks to allow normal brain tissue to recover from its effect, while tumor tissue is less likely to recover. Ultimately, the brain can absorb only a maximal dose of radiation, beyond which no further treatment is advisable. There is increasing evidence that over long periods of time, high doses of radiation are harmful to normally functioning brain. The technique of Gamma Knife radiosurgery, however, treats only the abnormal tissue. This treatment occurs in a single session, without significant radiation to adjacent brain. There is no evidence that radiosurgery has led to the development of other malignant tumors since the introduction of the GK more than 25 years ago.
Stereotactic techniques can also be used to accurately aim fractionated doses of gamma rays or x-rays to a target; administering the treatment in small doses over days to weeks. This technique is a compromise between radiosurgery and conventional radiotherapy and is termed stereotactic radiotherapy.
What Brain Disorders Can Gamma Knife Radiosurgery Treat ?
Modern brain imaging with CT and MR techniques and sophisticated computers allow GK radiosurgery for many tumors, vascular abnormalities, and pain problems which are now treated by open surgery. The results of treatment are very beneficial in most cases, optimistic in others, and under going continuing evaluation in all cases. Here is a partial list of some intracranial diseases treated by the Gamma Knife:
- Anaplastic astrocytomas
- Gliomas/ Astrocytomas
- Pilocytic astrocytomas
- Pituitary tumors
- Pineal region tumors
- Acoustic Neuromas
- Neuromas of the cranial nerves
- Glomus jugulare tumors
- Metastatic brain tumors
Skull base tumors
- Invasive squamous and basal cell carcinomas
- Arteriovenous malformations
- Cavernous malformations
- Trigeminal neuralgia
- Parkinson’s disease
- Uveal melamona
- Orbital metastases
- Optic nerve sheath meningioma
How is Gamma Knife Radiosurgery Performed ?
Radiosurgery is carried out through the cooperative efforts of a neurosurgeon, radiation oncologist and physicist. Your initial consultation will help you determine if GK radiosurgery is appropriate, effective and safe for your problem.
Early in the morning a lightweight aluminum frame is fixed to the head
using local anesthesia and intravenous, conscious sedation. This procedure
is rapid and well tolerated. In fact, most patients
have no recall of frame placement! The frame remains in
place until the end of treatment later in the day.
After frame placement patients undergo CT or MR imaging. Patients with vascular abnormalities may undergo an angiogram. These images are used for treatment planning purposes.
The greatest advances since the first Gamma Knife treatment in 1968
have been the advent of CT and MR imaging as well as high speed data processing
which allows surgeons to treat intracranial disease with computer techniques. The
CT/ MR images are displayed by software designed for conformal treatment planning.
This allows the Gamma Knife’s highly focused energy to accumulate within the
target volume while minimizing radiation to sensitive adjacent brain tissue.
The neurosurgeon, radiation oncologist and physicist develop the conformal treatment plan.
Gamma Plan 4.12 software used to plan GK treatment of a frontal lobe meningioma. Note abrupt fall off of radiation at tumor edge. The tumor volume is red and the yellow lines represent the 50% isodose curves.
The Gamma Knife:
||The Gamma Knife contains 201 small cobalt sources of gamma rays arrayed in a hemisphere within a thickly shielded structure. This radiant energy is focused into overlapping beams by collimators. The focal point of the collimators is extremely intense. The treatment planning software allows the focal point to be accurately placed on the target volume.
||The computer software reduces the treatment plan to a list of simple
instructions to guide the gamma rays to the target. The patient’s stereotactic
head frame is fixed within the collimator for each treatment. Usually several
shots are used to cover the entire target volume. Total treatment time varies
from 45 minutes to 1½ hours. Following treatment, the frame is removed and
patients are observed overnight or are discharged home.
There are almost no initial effects of radiosurgery. A very few patients have experienced seizures; almost always these are individuals with established seizure disorders. Care is taken to adjust anticonvulsant levels prior to treatment to avoid this event. Local pain in the scalp responds to simple, oral pain medication. Long range effects, after many months, include swelling within the adjacent brain that can cause symptoms such as headache and neurological disturbances. Almost always this swelling is treated with oral steroids and is self-limiting. Permanent cranial nerve dysfunction causing double vision, facial numbness, weakness, hearing loss, visual loss (depending on the site treated) is rare with modern gamma ray doses. Usually your neurosurgeon will follow treatment with MR/CT imaging every 3 months to every year to assure control of the tumor. Arteriovenous malformations may be followed by interval MR angiograms each year. These follow-up protocols vary from center to center.
What are the Results of Gamma Knife Radiosurgery?
Gamma Knife "Cure"
The cure of a brain tumor by radiosurgery means that the tumor loses its ability to grow and remains the same size, never growing again. Its growth is controlled. The intensely focused gamma rays destroy the ability of the cells to divide. Sometimes benign tumors actually shrink over time and malignant tumors may completely disappear. Arteriovenous malformations usually occlude after focused radiosurgery. This curative process occurs over months to years.
Metastatic Brain Tumors
Controlled studies have shown that surgical removal of single brain metastases followed
by radiation therapy to the brain benefits patients’ quality of life and survival
when compared to treatment by brain radiotherapy alone. To achieve
this benefit, usually there must be control of the patient’s primary tumor.
Experience has also shown that Gamma Knife radiosurgery is as effective as
open surgery in the control of metastatic brain tumors when combined with
radiotherapy of the brain. Control rates of over 85% are expected.
In selected individuals, we no longer carry out whole brain radiotherapy following
Gamma Knife radiosurgery. Some individuals with multiple brain metastases are
also candidates for GK radiosurgery. Usually we request follow-
up by frequent MR images of the brain to ensure control. Recurrent or new tumor
deposits can be retreated by radiosurgery.
|Metastatic carcinoma of the lung
||Before Gamma Knife radiosurgery
|| Follow-up CT scans 3 to 4 months after Gamma Knife radiosurgery
Radiosurgery is quite useful in the treatment of meningiomas. This can be the primary treatment for difficult to operate tumors or the treatment of tumors recurring after open surgery. Skull- based meningiomas often recur after operation and conventional surgery may occasionally lead to increased cranial nerve dysfunction or other complication. Tumors arising from the cavernous sinus and petroclival tumors of the posterior fossa are typical candidates for GK radiosurgery as the side effects of complex procedures for skull-based tumors are avoided. There is an expectation that more than 90% of tumors treated by Gamma Knife will be controlled.
Acoustic neuromas within the base of the skull and slowly expand into the
skull cavity. Destruction of hearing in the affected ear, a sense of imbalance,
ringing in the ear, weakness of facial movement, and facial numbness occurs
progressively in patients. Thousands of patients with acoustic neuromas have been
treated over the past 25 years by means of the Gamma Knife and the results compare
favorably with the published results of microsurgery. Reports of reoperation on
individuals treated by Gamma Knife being more difficult or dangerous are unsubstantiated.
Reoperation is quite rare and failure of control may be retreated by radiosurgery.
There are no reports of cancer being caused by radiosurgery. Patients should expect a
94% cure rate, with rare loss of facial movement or sensation in the face.
Most patients will retain some hearing and possibly ½ of those with useful hearing before radiosurgery will continue with useful hearing.
|Small acoustic neuroma
||Seven months after Gamma Knife radiosurgery showing a slightly smaller tumor with central necrosis
In general, we treat glioblastoma and anaplastic astrocytoma with an attempt to remove a maximal volume of abnormal tissue aided by frameless stereotactic surgery (Sofamor-Danek Stealth system). A MR scan is performed within 48 hours of surgery and residual, enhancing tissue is boosted with radiosurgery followed by conventional radiation therapy. Alternately, recurrence is treated with GK radiosurgery as long as the tumor nidus is small. Our reoperation rate for necrosis after radiosurgery is low. Controlled studies need to be completed to conclusively demonstrate the role of radiosurgery in the treatment of malignant gliomas. Recent studies indicate radiosurgery is useful in extending survival in patients with recurrent glioblastoma.
The aim of Gamma Knife treatment for AVM is total obliteration of the abnormal collection of blood vessels to reduce the chance of spontaneous hemorrhage. Probably 2% to 4% of AVM’s bleed spontaneously each year. The risk of hemorrhage may increase with age. Radiosurgery causes proliferation of the blood vessel lining, gradually occluding the AVM over time. Approximately 80% of avms under 3 cm in diameter will occlude by 2 years after treatment and perhaps 90 % three years following radiosurgery. During this time there is no increased risk of hemorrhage. Radiosurgery is sometimes the only curative treatment available in high-risk arteriovenous malformations. An example is an AVM in the brain stem, basal ganglia or eloquent brain, which bring a high risk of neurological deficit following surgery.
Tic pain in the face may be successfully treated in a non-invasive fashion by Gamma Knife radiosurgery. Treatment involves placing a single, 4mm "shot" of gamma rays on the trigeminal nerve and takes only a few minutes. Over weeks 60 to 80 % of tic sufferers achieve good to excellent pain relief. Results tend to be better in individuals who have not had other surgical remedies. GK radiosurgery rarely results in any sensory loss and does not interfere with additional treatment. It is a suitable alternate to all surgical treatment modalities and is in view of its ease of treatment, low complication rate, and non-invasive nature is an alternate to carbamazepine (Tegretol).
FOR FURTHER INFORMATION REGARDING GAMMA KNIFE RADIOSURGERY OR STEREOTACTIC RADIOTHERAPY CONTACT:
San Diego Gamma Knife Center|
Kenneth Ott, M.D., F.A.C.S|
501 Washington Street
San Diego, Ca 92103
Tel: (858) 297-4481
Fax: ( 858) 291-5536
David Hodgens, M.D.|
Director of Radiation Oncology
Scripps Memorial Hospital
PO Box 28
9888 Genesee Avenue
La Jolla, CA 92037
Tel (858) 626-6864
Fax (858) 626-6815