by John S. Yu, Anne Luptrawan, Robert E. Wallace, and Behrooz Hakimian
Radiosurgery of Intracranial Lesions
In 1951 Swedish neurosurgeon Lars Leksell coined the term radiosurgery to denote a noninvasive technique that precisely delivers a single high dose of radiation to a targeted area of brain through an intact skull. The desired biologicaleffect of radiosurgery is the destruction of a targeted area in the brain while avoiding nearby normal tissue and critical structures. Leksell, along with biophysicist Dr. Borje Larsson, introduced the first gamma knife in Europe in 1968. Radiosurgery can be performed using two devices: gamma knife and linear accelerator. Photon and proton beam radiation are two forms of radiation sources used toperform stereotactic radiosurgery.
N Modified Linear Accelerator
The linear accelerator (linac) is another radiosurgery tool used to effectively treat brain lesions. Unlike a natural emission of gamma ray photons produced by the gamma knife cobalt-60 sources, photons are created via the linac by accelerating electrons along a linear path and colliding with a metal target. The singlestream of photon radiation simulates multiple stationary beams by using multiple noncoplanar arcs around the patient’s head while the patient rotates on a turntable (couch) in each of four positions. Multiple beams of radiation can also be shaped with multileaved collimators to treat complex-shaped lesions. Linac radiosurgery delivers very precise and uniform irradiation, but unlike the gammaknife, it allows for fractionation of treatment. Fractionation of treatment divides treatments into multiple sessions using smaller doses, or fractions, of radiation. This treatment strategy is referred to as stereotactic radiotherapy. Fractionation allows for treatment of larger lesions and lesions that are intrinsically part of a critical structure while minimizing effects on surrounding normalbrain as compared with the gamma knife. Radiosurgery using the linac device can be more cost-effective as compared with gamma knife, particularly if institutions already use linacs for other applications.
N Gamma Knife Radiosurgery
Gamma knife is a multisource photon-based device that houses 201 fixed cobalt-60 sources. Cobalt-60 emits gamma ray photons. These photons travel as high-energy beamsand are delivered at a predictable and easily quantifiable rate. The gamma knife device allows precise delivery of radiation to a target. The cobalt-60 sources deliver 201 separate beams of radiation, which converge onto a predetermined central target. Only at the point where these beams cross is radiation delivered high enough to effectively destroy the cells of the abnormal brain lesion. Theamplitude of radiation at this point of convergence is so high that it allows for “scalpel-like” precision. The targeted tissue absorbs the radiation, leading to cell death. This process of cell death occurs over time, usually weeks to months. The end result of treatment is typically shrinkage of the lesion, halting further growth of the lesion or causing total obliteration of the lesion. When usedwith a stereotactic head frame, the precision of radiation delivery is 0.3 mm. The radiobiological effect of gamma knife radiosurgery is different from that of conventional fractionated radiotherapy. Conventional radiotherapy usually involves the delivery of large volumes of irradiation, which may deliver radiation to normal brain tissue. Conventional radiotherapy also includes fractionatedradiotherapy, which involves fractionation or dividing radiation treatment into multiple smaller daily doses. Normal brain tissue can tolerate fractionated radiation but it is not tolerated by the brain tumor, which results in the control of tumor growth. Gamma knife radiosurgery, on the other hand, delivers the entire dose of radiation in a single sitting. In fact, a single given dose with the gamma...