Iodine-131

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Radioactive decay

¹³¹I decays with a half-life of 8.0197 days with beta and gamma emissions. This nuclide of iodine atom has 78 neutrons in nucleus, the stable nuclide ¹²⁷I has 74 neutrons. On decaying, ¹³¹I transforms into ¹³¹Xe:

¹³¹I is a fission product with a yield of 2.8336%, and was released in nuclear weapons tests and the Chernobyl accident. However, the short half-life means it is not present in cooled spent nuclear fuel, unlike iodine-129.

Effects of adverse exposure

When ¹³¹I is present in high levels in the environment from radioactive fallout, it is absorbed by the body and may cause damage to the thyroid. This can be mitigated by taking iodine supplements, raising the total amount of iodine in the body and therefore reducing uptake and retention in tissues and lowering the relative proportion of radioactive iodine. Such supplements were distributed to the population living nearest to the Chernobyl nuclear power plant after the disaster.

The primary risk of concern associated with exposure to ¹³¹I in fallout is the chance occurrence of radiogenic thyroid cancer in later life. Other risks include the possibility of non-cancerous growths and thyroiditis. Within the USA, the highest ¹³¹I fallout doses occurred during the 1950's and early 1960's to children who consumed fresh sources of milk contaminated as the result of above ground testing of nuclear weapons.^[1]

The risk of thyroid cancer in later life appears to diminish with increasing age at time of exposure. Most risk estimates are based on studies in which radiation exposures occurred in children or teenagers. When adults are exposed, it has been difficult for epidemiologists to detect a statistically significant difference in the rates of thyroid disease above that of a similar but otherwise unexposed group.

The National Cancer Institute provides additional information on the health effects from exposure to ¹³¹I in fallout,^[2] as well as individualized estimates, for those born before 1971, for each of the 3070 counties in the USA from the nuclear weapons tests conducted at the Nevada Test Site.^[3]

Medical and pharmaceutical uses

It is used in nuclear medicine both diagnostically and therapeutically. Examples of its use in radiation therapy include the treatment of thyrotoxicosis and thyroid cancer. Diagnostic tests exploit the mechanism of absorption of iodine by the normal cells of the thyroid gland. As an example iodine-131 is one of the radioactive isotopes of iodine that can be used to test how well the thyroid gland is functioning.

¹³¹I is also used as a radioactive label for radiopharmaceuticals that can be used for imaging and therapy e.g. ¹³¹I-metaiodobenzylguanidine (¹³¹I-MIBG) for imaging and treating phaeochromocytoma and neuroblastoma.

Post-treatment isolation

Patients receiving radioiodine treatment are warned not to have sexual intercourse for one month (or shorter, depending on dose given), and women are told not to become pregnant for six months afterwards. These guidelines vary from hospital to hospital and will depend also on the dose of radiation given. One also advises not to hug or hold children when the radiation is still high, and a one or two metre distance to others may be recommended.

Many airports now have radiation detectors in order to detect the smuggling of radioactive materials that may be used in nuclear weapons manufacture. Patients should be warned that if they choose to travel by air, they may set off radiation detectors at airports up to 12 weeks after their treatment with ¹³¹I. A physician's letter does not exempt one from interrogation by airport security personnel, because these letters are easily forged. For security reasons, there is no information available in the public domain on which airports use radiation detectors.

References