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Gamma Knife Radiosurgery
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INTRODUCTION

Defining Radiosurgery

Radiosurgery is defined as a single high-dose of focused radiation delivered to a stereotactically localized target. Multiple beams of radiation are directed to a central focal point. Each individual beam is relatively weak and does not injure the brain, however, at the site of convergence of the beaming, the total radiation dose is high enough to produce the desired effect. With Gamma Knife surgery (GKS), gamma rays are precisely directed to fully include the selected target (i.e. conformal) while minimizing radiation exposure to the immediately surrounding brain structures because of the small beams. Small and medium sized targets (i.e. less than 3.5 centimeters) are safely treated with a single GKS treatment. GKS patients experience few of the side effects associated with conventional radiation therapy, such as nausea, vomiting, hair loss and headaches.

Radiosurgery differs from conventional fractionated radiotherapy, in which the total dose of radiation is higher and focused. Fractionated radiotherapy exposes normal brain, surrounding the target, to potentially dangerous levels of radiation. To minimize the risks, radiation therapy is delivered over several treatment days, allowing the brain time to recover after each treatment. The radiated tumor, however, does not have the same ability to recover from each radiation treatment, and therefore is damaged. Conversely, radiosurgery delivers a sufficiently high dose of radiation in a single treatment to destroy or arrest the growth of a tumor, while minimizing injury to the surrounding brain. It does this by using 201 individual beams, each one of which is a small diameter.

There are other radiosurgery techniques, using modified linear accelerators (LINAC) or proton beams generated by a cyclotron. However, GKS is the only radiosurgery procedure specifically designed, by a neurosurgeon, for the treatment of intracranial targets. The Gamma Knife is the most well established and frequently used instrument to perform a radiosurgery, with over 210,000 patients treated over the last 35 years. Currently, over 25,000 patients are treated with GKS each year, at over 175 units around the world.

The Gamma Knife's extensive record of effectiveness and safety has not been matched by other techniques. Alternative radiotherapy protocols called "stereotactic radiotherapy" or "fractionated radiosurgery" have been developed in an attempt to mimic the conformality and sensitivity of Gamma Knife Surgery. These LINAC based techniques make use of less conformal, or less focused, treatment plans. This exposes the brain structures surrounding the target to more radiation, and therefore the radiation dose must be split up over several days or weeks to reduce radiation complications. "Stereotactic radiotherapy" or "fractionated radiosurgery" cannot be considered radiosurgery because it is not a single dose treatment, and it does not take full advantage of the stereotactic precision that can be accomplished with true radiosurgery. Nevertheless, fractionated stereotactic radiotherapy is sometimes used as an alternative to radiosurgery, and may be well suited to treat large lesions beyond the size range of GKS (i.e. over 3.5 centimeters) or rare malignant tumors adjacent to radiation sensitive structures such as cranial nerves.


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