Cobalt therapy

Cobalt therapy or cobalt-60 therapy is the medical use of gamma rays from the radioisotope cobalt-60 to treat conditions such as cancer. Beginning in the 1950s cobalt-60 was widely used in external beam radiotherapy (teletherapy) machines, which produced a beam of gamma rays which was directed into the patient's body to kill tumor tissue. Because these "cobalt machines" were expensive and required specialist support, they were often housed in cobalt units. Cobalt therapy was a revolutionary advance in radiotherapy in the post-World War II period but is now being replaced by other technologies such as linear accelerators.

Before the development of medical linear accelerators in the 1970s, the only artificial radiation source used for teletherapy was the x-ray tube. Researchers found ordinary x-ray tubes, which used voltages of 50-150 keV, could treat superficial tumors, but did not have the energy to reach tumors deep in the body. To have the penetrating ability to reach deep-seated tumors without subjecting healthy tissue to dangerous radiation doses required rays with energy around 1 MeV, called "mega electron volt" radiation. To produce a significant amount of MeV x-rays required potentials on the tube of 3-5 million volts (3-5 megavolts), necessitating huge, expensive x-ray machines. By the late 30s these were being built, but they were available at only a few hospitals.

Radioisotopes produced gamma rays in the mega electron volt range, but prior to World War II virtually the only radioisotope available for radiotherapy was naturally occurring radium (producing 1-2 MeV gamma rays), which was extremely expensive due to its low occurrence in ores. In 1937 the total worldwide supply of radium available for beam radiotherapy (teletherapy) was 50 grams, valued at $50 million in 2005 dollars.

The invention of the nuclear reactor in the Manhattan Project during the World War II made possible the creation of artificial radioisotopes for radiotherapy. Cobalt-60, produced by neutron irradiation of ordinary cobalt metal in a reactor, is a high activity gamma ray emitter, emitting 1.17 and 1.33 MeV gamma rays with an activity of 44 TBq/g (about 1100 Ci/g). The main reason for its wide use in radiotherapy is that it has a longer half-life, 5.27 years, than many other gamma emitters. However the half life still requires cobalt sources to be replaced about every 5 years.

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