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http://en.wikipedia.org/wiki/Gamma_knife
Radiosurgery is a medical procedure which allows non-invasive brain surgery, i.e., without actually opening the skull, by means of directed beams of ionizing radiation. It is a relatively recent technique (1951), which is used to destroy, by means of a precise dosage of radiation, intracranial tumors and other lesions that could be otherwise inaccessible or inadequate for open surgery. There are many nervous diseases for which conventional surgical treatment is difficult or has many deleterious consequences for the patient, due to arteries, nerves, and other vital structures being damaged.
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Radiosurgery started with Dr. Lars Leksell from the Karolinska Institute of Stockholm, Sweden, in 1949, in a joint development with Bjorn Larsson, a radiobiologist from Uppsala University. Leksell initially used heavy particles, protons from a to irradiate brain tumor lesions.
In 1968, they developed the "gamma knife", a new device exclusively for radiosurgery, which consisted of radioactive sources of Cobalt-60 placed in a kind of helmet with central channels for irradiation, using gamma rays. In the last version of this device, 201 sources of radioactive cobalt direct gamma radiation to the center of a helmet, where the patient's head is inserted.
In order to achieve high precision in the positioning of the patient's head, it is placed first on a rigid frame of reference called a stereotactic device. It uses a geometrical coordinate system for each structure of the brain, so the surgeon knows, by using a published atlas, precisely the point where the gamma rays must converge. The stereotactic frame then fits into the helmet.
The "Gamma-knife" is used nowadays in four continents for carrying out what is called functional stereotactic neurosurgery, and for the superselective irradiation of tumors and brain arteriovenous malformations.
Another type of radiosurgery which has enjoyed great dissemination in neurosurgery was introduced by Betti and Colombo, in the mid 80's, utilizing commercial medical linear accelerators available for radiation therapy in oncology (the so-called LinAc). High energy, narrowly focused beams of x-rays are employed.
This system differs from the Gamma Knife in the way the radiation beams are delivered to the patient's head. In a similar way, a stereotactic device is used to provide a geometric coordinates reference, but the radiation beams are emitted by a single source, which rotates slowly around the patient's head.
Finally, at some medical centers such as in Boston and in California, particle accelerators built for doing research in high energy physics have been used since the 1960's for the treatment of brain tumors and brain arteriovenous malformations in humans. A still experimental type of radiosurgery, that utilizes a nuclear reactor for the nuclear fission of uranium, is the Neutron Capture Therapy (NCT) which was started in the United States at the nuclear reactor of the Massachusetts Institute of Technology (MIT) in the 60's, with promising results. Nowadays it is carried out as a promising advanced clinical research in several countries, due to the progress and to the results obtained in Japan by Dr. Hiroshi Hatanaka. He used NCT in more than 100 cases in the treatment of malignant tumors and of gigantic arterio-venous malformations.
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How it works
The fundamental principle of radiosurgery is that of selective ionization of the tissue to be operated upon, by means of high-energy beams of radiation. Ionization is the production of inorganic ions which are usually deleterious to the cells, by forming free radicals that are harmful to the cellular and nuclear membranes, and even to the RNA and DNA chains of the cells, producing an irreparable damage to these structures and then the cell's death. Thus, biological inactivation is carried out in a volume of tissue to be treated, with a precise destructive effect. The radiation dose absorbed by the treated mass of tissue is what defines the degree of biological inactivation. It usually is measured in grays, where one gray (Gy) is the absorption of one joule per kilogram of mass.
In order to perform optimal therapy, the neurosurgeon, assisted by physicists specialized in radiation therapy and often in conjunction with a radiation oncologist, chooses the best type of radiation to be used and how it will be delivered. Usually, the total dose of radiation required to kill a tumor, for example, is not delivered in a single, massive section, because this would cause undesirable effects on the patient. Instead, it is divided into several sessions of smaller duration and energy dose, in a procedure called dose fractioning. The aim of dose fractioning is to minimize the undesirable damage to healthy tissues, as healthy tissue cells are better than cancerous cells at repairing radiation induced damage between irradiations. In order to plan the radiation incidence and dosage, the physicists calculate a map portraying the lines of equal absorbed dose of radiation upon the patient's head (this is called a isodose map). Information about the tumor's location is obtained from a series of computerized tomograms, which are then feed to special planning computer software.
There are five types of irradiation currently used in radiosurgery: electromagnetical waves (gamma rays and x-rays), subatomic particles (protons and neutrons), and carbon ions.
The first type of radiation is gamma rays, which are beams of high energy photons]that interact with the corona of electrons of the atoms that compose the irradiated tissue, ionizing them. Gamma radiation is used in the "gamma knife" device, where they are produced by fixed sources of radioactive cobalt.
The second type of radiation, X-rays, are also high energy photons that are identical to gamma rays except for the way they are produced. Radiosurgery can be performed using a linear accelerator, the source being now a commercial medical device of universal use in radiotherapy. The Linac consists of an emitting tube of X radiation, with an energy of 4, 6, or 18 megaelectronvolts (MeV).
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