precision-guided surgical procedure, he introduced ionizing radiation to substitute for the knife or needle. He began with photons from a 300 kV X-ray tube mounted on his stereotactic apparatus. In Berkeley, California, in 1950, Tobias had begun cross-firing the sella turcica with charged particles to suppress pituitary function, and work with proton beams was taken up at the Gustaf Werner Institute, Uppsala, in 1954, tested clinically from 1958 onwards, and subsequently in 1961 by Kjellberg, in Boston. In the late 1950s and 1960s, Leksell and Larsson explored the practicability of protons from the Uppsala synchrocyclotron, as well as the theoretical advantages of other particles, before deciding that gamma rays, from a heavily shielded static array of Co60 sources directed towards a central point by narrow collimators, provided the simplest and most practical system for daily clinical use. The first Leksell "gamma knife" was completed in 1967 and became operational in Stockholm in 1968. It was designed for treating functional targets, and thus intractable pain and movement disorders were the first indications. The introduction of stereotactic radiosurgery was initially received with skepticism. However, since that time, the gamma knife (as the later model became known owing to its ease of use and precision) found its way into the neurosurgical armamentarium in many centers around the world. Indeed, some purists are of the view that the phrase "gamma knife surgery" should be applied in order to emphasize the differences from other delivery methods of radiosurgery on one hand, and the crucial role of neurosurgeons in this intervention on the other. After the initial experience became better known, further "gamma units" were installed in 1984 and 1985 in Buenos Aires, Argentina and Sheffield, England, respectively. The next unit, installed in 1987 in Pittsburgh heralded an increasingly broad acceptance worldwide. According to data held at the manufacturer, by September 2001 more than 170,000 patients had been treated with a gamma knife worldwide.

Colombo and others modified the radiotherapy linear accelerator to provide a similar treatment technique. By virtue of the different technical properties, linear accelerator (Linac) radiosurgery has developed in many ways very differently from gamma knife surgery. In particular, at least in part due to the lesser degree of precision achievable with a Linac, a fractionated delivery of radiation was favored, and thus "stereotactic radiotherapy" (SRT) was born. This technique is a multi-fraction, stereotacti-cally guided radiation therapy. Although it is possible to deliver SRT with the gamma knife, it is usually performed using a Linac.

In parallel with the developments in the radiation delivery technique, the imaging localization of the target has also undergone dramatic developments over the last five decades. The initial treatments were based on plain X-rays and pneumo-encephalography. With the introduction of computed tomography in the 1970s and magnetic resonance imaging in the 1980s, it became possible to properly delineate the pathological substrate of the treatment. Moreover, the precise identification of adjacent normal structures further dramatically improved the safety and efficacy of the technique.

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