P&S Journal: Fall 1995, Vol.15, No.3
Clinical Advances: By Lynne Christensen
Experts: Stereotactic Radiosurgery
For patients with small tumors or cerebral arterio-venous malformations (AVMs), the possibility of eradicating the lesion without brain surgery is attractive. Ever since stereotactic radiosurgery ("knifeless" brain surgery) was introduced in the United St ates in the mid 1980s, hundreds of clinics have rushed to embrace this seemingly miraculous technique. Physicians at P&S have performed the procedure since 1989 using a linear accelerator to deliver the necessary radiation.
With the growing popularity of the procedure, P&S experts caution that not all patients with AVMs or small tumors are candidates for the procedure. Patients must be carefully chosen and the technical accuracy of the procedure must be tightly controlled.
In stereotactic radiosurgery, computers and imaging equipment are used to create a 3-D map of the brain and to precisely locate the site of the tumor or AVM. The patient's head is completely immobilized in a head fixation frame, and four or five beams of high-energy radiation are delivered to the target tissue. At CPMC a linear accelerator is used to deliver the radiation; some other institutions use a Gamma Knife. At the focal point, where all the beams converge, the concentrated energy destroys tissue. The procedure requires no anesthesia, and patients are sent home the same day. Since it is not necessary to cut through the skull or brain tissue, damage to healthy tissue is minimized when the procedure is used appropriately. Dr. Steven Isaacson, associa te professor of clinical radiation oncology and of otolaryngology, recommends using the procedure only for small radiographically distinct lesions, no larger than 3.5 to 4 cm.
Depending on the type and location of the abnormality, treatment is administered in a single dose or fractionated (multiple) doses. Fractionated doses (called radiotherapy) are given as sin-gle low doses administered on successive days or every other day. Radiotherapy is used when the target is adjacent to vital structures, such as the retina or speech areas of the brain, to avoid collateral damage. Fractionated stereotactic radiotherapy must be performed with a linear accelerator. A Gamma Knife is capable of only performing single-dose radiosurgery.
Dr. Michael Sisti, assistant professor of clinical neurological surgery, and Dr. Isaacson caution that candidates for stereotactic radiosurgery or radiotherapy must be carefully selected. Most tumors and AVMs are too large to be safely treated in this way and require conventional surgery. Attempting to treat too large an area can result in damage to healthy tissue. Inadvertently delivering radiation to peripheral tissue also can cause neurotoxicity. Stereotactic radiosurgery requires targeting accuracy on the order of 1 millimeter. Even a minor error in setting the coordinates can result in neurological damage that may not be detected until after a patient is discharged. These two factors (lesion size and peripheral dose) correlate with much of the toxicity seen with single, high-dose radiosurgery. Immediate side effects are seizures or nausea and vomiting, experienced by 3 percent to 16 percent of patients. Between tw o and 31 months after treatment, new neurologic deficits develop in about 18 percent of patients. Corticosteroid treatment provides clinical improvement for almost all of these patients. Reoperation for radionecrosis is required in 3 percent to 35 percent of patients with brain metastases or high-grade gliomas, two to 14 months after radiosurgery.
"The kinds of things we can effectively treat with radiosurgery are few and far between," says Dr. Sisti. The target must be small, regularly shaped, and far enough away from vital structures. "That doesn't leave you with a lot of patients.
"The dark side of radiosurgery is that it appears deceptively simple," says Dr. Sisti. Patients can be treated and released in a matter of hours, feeling fine (although complications may arise later). Because the procedure seems so benign, doctors and pat ients are tempted to give it a try, especially when the alternative is conventional brain surgery.
In reality, radiosurgery requires a high degree of quality assurance, extensive operator training, careful patient selection, regulatory review, and an experienced multidisciplinary team. At CPMC, each treatment involves a team of six specialists working for a total of 20 hours after the neurosurgeon places the head frame on the patient. Additionally, the equipment-which
can cost $2 million-is maintenance- and operator-dependent.
In making a decision about undergoing radiosurgery, patients should look for a program with a seasoned team of neurosurgeons, neurologists, neu- roradiologists, and radiation oncologists, advises
Dr. Isaacson. Equally important, the program should offer both stereotactic radiosurgery and conventional microsurgery. "There are few places that have all of those pieces of the puzzle, perhaps only a dozen medical centers in the nation"