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Radiosurgery is surgery using radiation, that is, the destruction of precisely selected areas of tissue using ionizing radiation rather than excision with a blade. Like other forms of radiation therapy, it is usually used to treat cancer. Radiosurgery was originally defined by the Swedish neurosurgeon Lars Leksell as “a single high dose fraction of radiation, stereotactically directed to an intracranial region of interest”. In stereotactic radiosurgery (SRS), the word ''stereotactic'' refers to a three-dimensional coordinate system that enables accurate correlation of a virtual target seen in the patient's diagnostic images with the actual target position in the patient anatomy. Technological improvements in medical imaging and computing have led to increased clinical adoption of stereotactic radiosurgery and have broadened its scope in recent years. Notwithstanding these improvements, the localization accuracy and precision that are implicit in the word “stereotactic” remain of utmost importance for radiosurgical interventions today. Stereotactic accuracy and precision are significantly increased by using a device known as the N-localizer that was invented by the American physician and computer scientist Russell Brown and that has achieved widespread clinical use in several stereotactic surgical and radiosurgical systems. Recently, the original concept of radiosurgery has been expanded to include treatments comprising up to five fractions, and stereotactic radiosurgery has been redefined as a distinct neurosurgical discipline that utilizes externally generated ionizing radiation to inactivate or eradicate defined targets in the head or spine without the need for a surgical incision. Irrespective of the similarities between the concepts of stereotactic radiosurgery and fractionated radiotherapy, and although both treatment modalities are reported to have identical outcomes for certain indications, the intent of both approaches is fundamentally different. The aim of stereotactic radiosurgery is to destroy target tissue while preserving adjacent normal tissue, where fractionated radiotherapy relies on a different sensitivity of the target and the surrounding normal tissue to the total accumulated radiation dose.〔 Historically, the field of fractionated radiotherapy evolved from the original concept of stereotactic radiosurgery following discovery of the principles of radiobiology: repair, reassortment, repopulation, and reoxygenation. Today, both treatment techniques are complementary as tumors that may be resistant to fractionated radiotherapy may respond well to radiosurgery and tumors that are too large or too close to critical organs for safe radiosurgery may be suitable candidates for fractionated radiotherapy.〔 ==History== Stereotactic radiosurgery was first developed in 1949 by the Swedish neurosurgeon Lars Leksell to treat small targets in the brain that were not amenable to conventional surgery. The initial stereotactic instrument he conceived used probes and electrodes. The first attempt to supplant the electrodes with radiation was made in the early fifties, with x-rays. The principle of this instrument was to crossfire the intra-cranial target from multiple directions with narrow beams of radiation. The beam paths converge in the target volume, delivering a lethal cumulative dose of radiation, while limiting the dose to the adjacent healthy tissue. Ten years later significant progress had been made, due in considerable measure to the contribution of the physicists Kurt Liden and Borje Larsson. At this time, stereotactic proton beams had replaced the x-rays. The heavy particle beam presented as an excellent replacement for the surgical knife but the synchrocyclotron was too clumsy. Dr. Leksell set his mind on the development of a practical, compact, precise and simple tool which could be handled by the surgeon himself. In 1968, this resulted in the Gamma Knife, which was installed at the Karolinska Institute and consisted of several radioactive sources of cobalt-60 placed in a kind of helmet with central channels for irradiation with gamma rays. This prototype was designed to produce slit-like radiation lesions for functional neurosurgical procedures to treat pain, movement disorders, or behavioral disorders that did not respond to conventional treatment. The success of this first unit led to the construction of a second device, containing 179 cobalt-60 sources. This second gamma knife unit was designed to produce spherical lesions to treat brain tumors and intracranial arteriovenous malformations AVMs. In the 1980s the third and fourth units (with 201 cobalt-60 sources) were installed in Buenos Aires, Argentina, and Sheffield, England. The fifth gamma knife was installed at the University of Pittsburgh Medical Center in Pittsburgh in 1987. In parallel to these developments, a similar approach was designed for a linear particle accelerator or Linac. Installation of the first 4 mega electronvolt (MeV) clinical linear accelerator began in June 1952 in the Medical Research Council (MRC) Radiotherapeutic Research Unit at the Hammersmith Hospital, London. The system was handed over for physics and other testing in February 1953 and began to treat patients on 7 September that year. Meanwhile, work by at the Stanford Microwave Laboratory led to the development of a 6-MV accelerator, which was installed at Stanford University Hospital, California, in 1956. Linac units quickly became favored devices for conventional fractionated radiotherapy but it lasted until the eighties of last century before dedicated Linac radiosurgery became a reality. In 1982, the Spanish neurosurgeon J. Barcia-Salorio began to evaluate the role of cobalt-generated and then Linac-based photon radiosurgery for the treatment of AVMs and epilepsy. In 1984, Betti and Derechinsky described a Linac-based radiosurgical system. Winston and Lutz further advanced Linac-based radiosurgical prototype technologies by incorporating an improved stereotactic positioning device and a method to measure the accuracy of various components. Using a modified Linac, the first patient in the United States was treated at in Boston Brigham and Women's Hospital in February 1986. Today, both Gamma Knife and Linac radiosurgery programs are commercially available worldwide. While the Gamma Knife is dedicated to radiosurgery, most Linacs are build for conventional fractionated radiotherapy and require additional technology and expertise to become dedicated radiosurgery tools. This is exemplified by the (Novalis Radiosurgery Program ), designed to complement conventional Linacs with sophisticated beam shaping technology, treatment planning solutions and image-guidance tools to warrant highest treatment accuracy. An example of a dedicated radiosurgery Linac is the CyberKnife, a compact Linac mounted onto a robotic arm that moves around the patient and irradiates the tumor from a large set of fixed positions, thereby mimicking the Gamma Knife concept. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Radiosurgery」の詳細全文を読む スポンサード リンク
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