What is stereotactic radiosurgery?
Stereotactic radiosurgery (also called SRS) is a means of treating brain disorders with a precise delivery of a single, high dose of radiation in a one-day session. Focused radiation beams are delivered to a specific area of the brain to treat abnormalities, tumors or functional disorders. Stereotactic radiosurgery is limited to the head and neck, because these areas can be immobilized with a device that completely restricts the head's movement, permitting the most precise and accurate treatment. One-session treatments without a head immobilization device is not usually recommended because of the high potential for damage to healthy brain tissue, cranial nerves (optic, hearing, etc.) and the brain stem.
Radiosurgery (a one-session treatment) has such a dramatic effect in the targeted treatment area that the changes are considered "surgical." Through the use of three-dimensional computer-aided planning devices and the high degree of head immobilization, the treatment can minimize the amount of radiation that passes through healthy brain tissue. Stereotactic radiosurgery is routinely used to treat brain tumors and lesions. It may be the primary treatment, used when a tumor is inaccessible by surgical means or as an adjunct to other treatments for a recurring or malignant tumor. In some cases, it may be an inappropriate treatment option.
How does stereotactic radiosurgery work?
Stereotactic radiosurgery is based on the same principles as other forms of radiation treatment. It does not remove the tumor or lesion, but the radiation beams distort the DNA of the tumor cells. The cells then lose their ability to reproduce and retain fluids. The tumor reduction occurs at the rate of normal growth for the specific tumor cell. In lesions such as AVMs (a tangle of blood vessels in the brain), radiosurgery causes the blood vessels to thicken and close off. The shrinking of a tumor or closing off of a vessel occurs over a period of time. For benign tumors and vessels, this will usually be 18 months to two years. For malignant or metastatic tumors, because these cells are very fast-growing, results may be seen in a few months.
- Swelling: As with all radiation treatments, the tumor cells lose their ability to regulate fluids, and edema or swelling may occur. This does not happen in all treatments. If swelling does occur, and it causes symptoms that are unpleasant, then a mild course of steroid medication may be given to reduce the fluid within the tumor cavity.
- Necrosis: The tumor tissue that remains after the radiation treatment will typically shrink. On rare occasions this necrotic or dead tissue can cause further problems and may require removal. This occurs in a very small percentage of cases.
- Other Effects: Other side effects may occur dependent upon the target site and the dose of radiation received. This should be discussed thoroughly with your physician.
Is radiation treatment right for me?
Because all forms of radiation treatments work over time, this course of treatment may be inappropriate if symptoms are severe or life-threatening. For example, if relief of acute symptoms is urgent, the first treatment choice may be traditional open skull surgery or medication to relieve symptoms affecting quality of life. Secondary treatment may then be radiosurgery. In other cases where cells are extremely fast growing (with or without severe symptoms), such as in brain metastases, radiosurgery can quickly control the brain tumors to allow time to treat the primary cancer site. Medication can be given for the side effects (such as edema), and radiation therapy may be used over a period of time to help eliminate stray cancer cells from the brain.
Stereotactic radiosurgery can be used in patients where standard radiation techniques have failed, or in patients where the maximum radiation dose permissible has been administered. There is little literature on radiation-induced new tumors caused by stereotactic radiosurgery. It is expected that the possibility of developing a tumor is 1 in 10,000 cases. This may be attributed to the precision of the treatment and the sparing of healthy nerves and tissues. A patient who has had stereotactic radiosurgery for a brain tumor or another condition may have open skull surgery later without problems. In many cases, stereotactic radiosurgery can be performed again if necessary.
Types of Radiosurgery
There are three basic forms of stereotactic radiosurgery represented by three different technological instruments. Each instrument operates differently, has a different source of radiation and may be more effective under different circumstances. The three are:
- Particle beam (proton)
- Cobalt-60 based (photon)
- Linear accelerator based (linac)
The particle/proton beam exists in a handful of centers in the United States. In addition to brain tumors, it treats body cancers in a fractionated manner. Due to the cost of the particle beam facility (greater than $100 million), little research is currently available, however, more is expected in the future. Many of the costs of the particle beam are funded by endowments and public research centers.
The cobalt-60 based machines are located at dedicated neuroscience centers such as the Riverside and University of Virginia Radiosurgery Center. Centers exist in the United States and throughout the world. These machines provide extremely accurate targeting and precise treatment for brain cancers and other conditions. This technology is used to treat only brain tumors and dysfunctions and is usually completed in one singe session on one day. The most well-known machine is the Gamma Knife®. The Gamma Knife® does not move during treatment, providing a high degree of precision within the brain. It has been available for over 40 years, and utilizes multiple sources of radiation which causes less damage to healthy tissue and results in better targeting. Over 350,000 patients have received treatment with the Gamma Knife. The Gamma Knife is ideal for smaller tumors (less than 3.5 cm) and functional disorders of the brain.
The linear accelerator based radiosurgery machines are also prevalent throughout the world to treat body tumors not in the brain. One benefit of this technology is its ability to easily treat large tumor volumes (over 3.5 cm) by treating them over several sessions. Treatments over a period of time are called fractionated stereotactic radiotherapy and not stereotactic radiosurgery (a one-session treatment). The machines are made by multiple manufacturers with common brand names: Novalis Tx®, CyberKnife®, and others. The Novalis Tx® is the most common machine available. Linear accelerator based machines are not dedicated to treatments only within the brain. They can be used throughout the body, as well as the head and neck. The linear accelerator based machines utilize one large intense radiation beam that is redirected in many "arcs" to lessen the adverse effects on healthy tissue. Since the linear accelerator moves during treatment, the degree of precision is less than that of cobalt-60 machines. These machines can perform radiosurgery on larger tumors and can fractionate treatments over several days, yielding a flexibility that is not available with other machines. Treatments that are given over time are referred to as fractionated stereotactic radiotherapy (FSR) or stereotactic radiotherapy (SRT).
Because multiple manufacturers produce linear accelerator units, data and results are usually not comparable between different units and treating sites, as each type of unit requires different calibrations and adjustments. As a result, there may appear to be a lack of peer-reviewed research available about a diagnosis and treatment. Click here to see a comparison of Gamma Knife to non-fixed linear accelerator units.
Stereotactic radiosurgery may or may not be appropriate for a condition. It may be used as the primary treatment or recommended in addition to other treatments that are needed. Only a treating neurosurgeon who operates radiosurgery equipment can make the evaluation as to whether someone can be treated. A neurosurgeon must always be present during treatment and should work with a radiation oncologist when the brain is being targeted.