|
External beam radiotherapy (EBRT) or teletherapy is the most common form of radiotherapy. The patient sits or lies on a couch and an external source of radiation is pointed at a particular part of the body. In contrast to internal radiotherapy (brachytherapy), in which the radiation source is inside the body, external beam radiotherapy directs the radiation at the tumour from outside the body. Kilovoltage ("superficial") X-rays are used for treating skin cancer and superficial structures. Megavoltage ("deep") X-rays are used to treat deep-seated tumours (e.g. bladder, bowel, prostate, lung, or brain). While X-ray and electron beams are by far the most widely used sources for external beam radiotherapy, a small number of centers operate experimental and pilot programs employing heavier particle beams, particularly proton sources. ==Photons== Conventionally, the energy of diagnostic and therapeutic gamma- and X-rays is expressed in kilovolts or megavolts (kV or MV), whilst the energy of therapeutic electrons is expressed in terms of megaelectronvolts (MeV). In the first case, this voltage is the maximum electric potential used by a linear accelerator to produce the photon beam. The beam is made up of a spectrum of energies: the ''maximum'' energy is approximately equal to the beam's maximum electric potential times the electron charge. Thus a 1 MV beam will produce photons of no more than about 1 MeV. The ''mean'' X-ray energy is only about 1/3 of the maximum energy. Beam quality and hardness may be improved by special filters, which improve the homogeneity of the X-ray spectrum. In the medical field, useful X-rays are produced when electrons are accelerated to a high energy. Some examples of X-ray energies used in medicine are: *diagnostic X-rays - 20 to 150 keV *superficial X-rays - 50 to 200 keV *orthovoltage X-rays - 200 to 500 keV *supervoltage X-rays - 500 to 1000 keV *megavoltage X-rays - 1 to 25 MeV Megavoltage X-rays are by far most common in radiotherapy for treatment of a wide range of cancers. Superficial and orthovoltage X-rays have application for the treatment of cancers at or close to the skin surface.〔Advances in kilovoltage x-ray beam dosimetry in http://iopscience.iop.org/0031-9155/59/6/R183/article〕 Medically useful photon beams can also be derived from a radioactive source such as iridium-192, caesium-137 or radium-226 (which is no longer used clinically), or cobalt-60. Such photon beams, derived from radioactive decay, are more or less monochromatic and are properly termed gamma rays. The usual energy range is between 300 keV to 1.5 MeV, and is specific to the isotope. Therapeutic radiation is mainly generated in the radiotherapy department using the following equipment: # Orthovoltage units. These are also known as "deep" and "superficial" machines depending on their energy range. Orthovoltage units have essentially the same design as diagnostic X-ray machines. These machines are generally limited to less than 600 kV. # Linear accelerators ("linacs") which produce megavoltage X-rays. The first use of a linac for medical radiotherapy was in 1953 (see also radiotherapy). Commercially available medical linacs produce X-rays and electrons with an energy range from 4 MeV up to around 25 MeV. The X-rays themselves are produced by the rapid deceleration of electrons in a target material, typically a tungsten alloy, which produces an X-ray spectrum via bremsstrahlung radiation. The shape and intensity of the beam produced by a linac may be modified or collimated by a variety of means. Thus, conventional, conformal, intensity-modulated, tomographic, and stereotactic radiotherapy are all produced by specially-modified linear accelerators. # Cobalt units which produce stable, dichromatic beams of 1.17 and 1.33 MeV, resulting in an average beam energy of 1.25 MeV. The role of the cobalt unit has partly been replaced by the linear accelerator, which can generate higher energy radiation. Cobalt treatment still has a useful role to play in certain applications (for example the Gamma Knife) and is still in widespread use worldwide, since the machinery is relatively reliable and simple to maintain compared to the modern linear accelerator. 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「external beam radiotherapy」の詳細全文を読む スポンサード リンク
|