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Particle therapy is a form of external beam radiotherapy using beams of energetic protons, neutrons, or positive ions for cancer treatment. The most common type of particle therapy as of 2012 is proton therapy. Although a photon, used in x-ray or gamma ray therapy, can also be considered a particle, photon therapy is not considered here. Additionally, electron therapy is generally put into its own category. Because of this, particle therapy is sometimes referred to, more correctly, as hadron therapy (that is, therapy with particles that are made of quarks). Neutron capture therapy might be considered a type of particle therapy, but it is not discussed here, as the damage it does to tumors is mostly from energetic ions produced by the secondary nuclear reaction after the neutrons in the external beam are absorbed into boron-10 (or occasionally some other nuclide), and not due primarily to the neutrons themselves. It is therefore a type of secondary particle therapy. Muon therapy, a rare type of particle therapy not within the categories above, has occasionally been attempted. == Method == Particle therapy works by aiming energetic ionizing particles at the target tumor.〔U. Amaldi and G. Kraft, "Radiotherapy with beams of carbon ions" in ''Rep. Progr. Physics'' 68 (2005) () 1861, 1861–1882.〕〔O. Jäkel, "State of the art in hadron therapy" in ''AIP Conference Proceedings'' vol. 958, no.1, 2007, pp. 70-77.〕 These particles damage the DNA of tissue cells, ultimately causing their death. Because of their reduced ability to repair damaged DNA, cancerous cells are particularly vulnerable to attack. The figure shows how beams of electrons, X-rays or protons of different energies (expressed in MeV) penetrate human tissue. Electrons have a short range and are therefore only of interest close to the skin (see electron therapy). Bremsstrahlung X-rays penetrate more deeply, but the dose absorbed by the tissue then shows the typical exponential decay with increasing thickness. For protons and heavier ions, on the other hand, the dose increases while the particle penetrates the tissue and loses energy continuously. Hence the dose increases with increasing thickness up to the Bragg peak that occurs near the end of the particle's range. Beyond the Bragg peak, the dose drops to zero (for protons) or almost zero (for heavier ions). The advantage of this energy deposition profile is that less energy is deposited into the healthy tissue surrounding the target tissue. The ions are first accelerated by means of a cyclotron or synchrotron. The final energy of the emerging particle beam defines the depth of penetration, and hence, the location of the maximum energy deposition. Since it is easy to deflect the beam by means of electro-magnets in a transverse direction, it is possible to employ a raster scan method, i.e., to scan the target area quickly like the electron beam scans a TV tube. If, in addition, the beam energy and hence, the depth of penetration is varied, an entire target volume can be covered in three dimensions, providing an irradiation exactly following the shape of the tumor. This is one of the great advantages compared to conventional X-ray therapy. At the end of 2008, 28 treatment facilities were in operation worldwide and over 70,000 patients had been treated by means of pions, protons and heavier ions. Most of this therapy has been conducted using protons.〔(PTCOG: Particle Therapy Co-Operative Group )〕 At the end of 2013, 105 000 patients had been treated with proton beams, and approximately 13, 000 patients had received carbon-ion therapy. As of April 1, 2015, for proton beam therapy, there are 49 facilities in the world, including 14 in the USA. with another 29 facilities are under construction. For Carbon-ion therapy, there are eight centers operating and four under construction.〔 Carbon-ion therapy centers exist in Japan, Germany, Italy, and China. Two USA federal agencies are hoping to stimulate the establishment of at least one US heavy-ion therapy center.〔 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「particle therapy」の詳細全文を読む スポンサード リンク
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