Iodine-131 (131I), also called radioiodine (though many other radioactive isotopes of this element are known), is an important radioisotope of iodine. It has a radioactive decay half life of about eight days. Its uses are mostly medical and pharmaceutical.
It also plays a role as a major radioactive hazard present in nuclear fission products, and was a significant contributor to the health effects from open-air atomic bomb testing in the 1950s, and from the Chernobyl disaster, as well as being a threatening presence today in the Japanese nuclear crisis. This is because I-131 is a major uranium, plutonium and indirectly thorium fission product, comprising nearly 3% of the total products of fission (by weight).
Due to its mode of beta decay, iodine-131 is notable for causing mutation and death in cells which it penetrates, and other cells up to several millimeters away. For this reason, high doses of the isotope are sometimes paradoxically less dangerous than low doses, since they tend to kill thyroid tissues which would otherwise become cancerous as a result of the radiation.
For example, children treated with moderate dose of I-131 for thyroid adenomas had a detectable increase in thyroid cancer, but children treated with a much higher dose did not.
Similarly most studies of very high dose I-131 for treatment of Graves disease have failed to find any increase in thyroid cancer, even though there is linear increase in thyroid cancer risk with I-131 absorption at moderate doses. Thus, iodine-131 is increasingly less employed in small doses in medical use (especially in children), but increasingly is used only in large and maximal treatment doses, as a way of killing targeted tissues. This is known as "therapeutic use."
Iodine-131 can be "seen" by nuclear medicine imaging techniques (i.e., gamma cameras) whenever it is given for therapeutic use, since about 10% of its energy and radiation dose is via gamma radiation. However, since the other 90% of radiation (beta radiation) causes tissue damage without contributing to any ability to see or "image" the isotope, other less-damaging radioisotopes of iodine are preferred in situations when only nuclear imaging is required.
The isotope I-131 is still occasionally used for purely diagnostic (i.e., imaging) work, due to its low expense compared to other iodine radioisotopes. Very small medical imaging doses of I-131 have not shown any increase in thyroid cancer.
The low-cost availability of I-131, in turn, is due to the relative ease of creating I-131 by neutron bombardment of natural tellurium in a nuclear reactor, then separating I-131 out by various simple methods (i.e., heating to drive off the volatile iodine). By contrast, other iodine radioisotopes are usually created by far more expensive techniques, starting with reactor radiation of expensive capsules of pressurized xenon gas.
Much smaller incidental doses of iodine-131 than are used in medical therapeutic uses, are thought to be the major cause of increased thyroid cancers after accidental nuclear contamination. These cancers happen from residual tissue radiation damage caused by the I-131, and usually appear years after exposure, long after the I-131 has decayed.
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