Iodine-131

Coordinates	41°52′55″N87°37′40″N
Alternate names	Radioiodine

Iodine-131 (¹³¹I), also sometimes loosely called radioiodine even though there are many other radioactive isotopes of this element, is an important radioisotope of iodine. 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. This is because I-131 is a major uranium fission product, comprising nearly 3% of the total products of fission.

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. Thus, iodine-131 is increasingly not used in small doses in medical use, but increasingly only in large and maximal treatment doses, as a way of killing targeted tissues. This is known as "therapeutic use."

Although the isotope can be "seen" by nuclear medicine imaging techniques (i.e., gamma cameras) whenever given for therapeutic use, other safer radioisotopes of iodine are coming to be preferred in situations when only nuclear imaging is required. I-131 is still occasionally used for diagnostic work, due to its low expense compared to other isotopes. This availability, in turn, is due to the relative ease of creating the nuclide by neutron bombardment of natural tellurium in a nuclear reactor, and then separating it by various simple methods. (Other iodine radioisotopes are usually created by far more expensive techniques starting with reactor radiation of xenon gas).

Much smaller incidental doses of iodine-131 than are used in medical treatment, are thought to be the major cause of increased thyroid cancers after accidental nuclear contamination.

Production

Most I-131 production is from nuclear reactor neutron-irradiation of a natural tellurium target. Irradiation of natural tellurium produces almost entirely I-131 as the only radionuclide with a half-life longer than hours, since most lighter isotopes of tellurium become heavier stable isotopes, or else stable iodine or xenon. However, the heaviest naturally-occurring tellurium nuclide, Te-130 (34% of natural Te) absorbs a neutron and emits a beta to become tellurium-131, which decays to I-131 with a half life of 25 minutes.

A tellurium compound can be irradiated while bound as an oxide to an ion exchange column, and evolved I-131 then eluted into an alkaline solution. More commonly, powdered elemental tellurium is irradiated and then I-131 separated from it by dry distillation of the iodine, which has a far lower vapor pressure. The element is then dissolved in a mildly alkaline solution in the standard manner, to produce I-131 as iodide and hypoiodate (which is soon oxidized to iodide).

Radioactive decay

¹³¹I decays with a half-life of 8.02 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:

$\{^\{131\}\_\{53\}\backslash mathrm\{I\}\}\; \backslash rightarrow\; \backslash beta\; +\; \{^\{131\}\_\{54\}\backslash mathrm\{Xe\}\}$

The primary emissions of ¹³¹I decay are 364 keV gamma rays (81% abundance) and beta particles with a maximal energy of 606 keV (89% abundance).

The beta particles, due to their high mean energy (190 keV; 606 kev is the maximum, but a typical beta-decay spectrum is present) have a tissue penetration of 0.6 to 2 mm.

¹³¹I is a fission product with a yield of 2.8336% from uranium-235, and can be released in nuclear weapons tests and nuclear accidents. However, the short half-life means it is not present in cooled spent nuclear fuel, unlike iodine-129 whose halflife is nearly a billion times that of I-131.

Effects of exposure

Iodine in food is absorbed by the body and preferentially concentrated in the thyroid where it is needed for the functioning of that gland. When ¹³¹I is present in high levels in the environment from radioactive fallout, it can be absorbed through contaminated food, and will also accumulate in the thyroid. As it decays, it may cause damage to the thyroid. The primary risk from exposure to high levels of ¹³¹I is the chance occurrence of radiogenic thyroid cancer in later life. Other risks include the possibility of non-cancerous growths and thyroiditis.

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 risk 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. Unfortunately, such supplements were not distributed to the population living nearest to the Chernobyl nuclear power plant after the disaster, though they were widely distributed to children in Poland.

Within the USA, the highest ¹³¹I fallout doses occurred during the 1950s and early 1960s to children who consumed fresh sources of milk contaminated as the result of above ground testing of nuclear weapons. The National Cancer Institute provides additional information on the health effects from exposure to ¹³¹I in fallout, as well as individualized estimates, for those born before 1971, for each of the 3070 counties in the USA. The calculations are taken from data collected regarding fallout from the nuclear weapons tests conducted at the Nevada Test Site.

Treatment and Prevention

A common treatment method for preventing iodine-131 exposure is by saturating the thyroid with regular, non-radioactive iodine-127. This prevents the thyroid from absorbing the radioactive iodine-131, thereby avoiding the damage caused by radiation to the body. This treatment method is most commonly accomplished by administering potassium iodide to those at risk.

Medical and pharmaceutical uses

It is used in nuclear medicine therapeutically and can also be seen with diagnostic scanners if it has been used therapeutically. Use of the ¹³¹I as iodide salt exploits the mechanism of absorption of iodine by the normal cells of the thyroid gland. Examples of its use in radiation therapy are those where tissue destruction is desired after iodine uptake by the tissue. Major uses of ¹³¹I include the treatment of thyrotoxicosis and some types of thyroid cancer that absorb iodine. The ¹³¹I isotope is also used as a radioactive label for certain radiopharmaceuticals that can be used for therapy, e.g. ¹³¹I-metaiodobenzylguanidine (¹³¹I-MIBG) for imaging and treating pheochromocytoma and neuroblastoma. In all of these therapeutic uses, ¹³¹I destroys tissue by short-range beta radiation. It can be seen in diagnostic scans after its use as therapy, because it is also a gamma-emitter.

Because of the carcinogenicity of its beta radiation in the thyroid in small doses, I-131 is rarely used primarily or solely for diagnosis (although in the past this was more common due to this isotope's relative ease of production and low expense). Instead the more purely gamma-emitting radioiodine Iodine-123 is used in diagnostic testing (nuclear medicine scan of the thyroid). The longer half-lived iodine-125 is also occasionally used when a longer half-life radioiodine is needed for diagnosis, and in brachytherapy treatment (isotope confined in small seed-like metal capsules), where its low-energy gamma radiation without a beta component, makes it useful.

The use of ¹³¹I as a medical isotope has been blamed for a routine shipment of biosolids being rejected from crossing the Canada—U.S. border. Such material can enter the sewers directly from the medical facilities, or by being excreted by patients after a treatment.

Post-treatment isolation

Patients receiving I-131 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. "This is because a theoretical risk to a developing fetus exists, even though the amount of radioactivity retained may be small and there is no medical proof of an actual risk from radioiodine treatment. Such a precaution would essentially eliminate direct fetal exposure to radioactivity and markedly reduce the possibility of conception with sperm that might theoretically have been damaged by exposure to radioiodine." 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.

I-131 will be eliminated from the body by the action of its radioactive decay. During the course of its decay, small amounts may also be eliminated through sweat and waste removal (urination). For this reason, it may be advisable to regularly clean toilets, sinks, bed sheets and clothing used by the person who received the treatment. This will help minimize accidental exposure by family members, especially children. Use of a decontaminant specially made for radioactive iodine removal may be advised. Two common products used by institutions are Bind-It Decontaminant (from Laboratory Technologies, Inc.) and I-Bind. Avoid using general purpose radioactive decontamination products, as these may only spread or volatilize it.

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 95 days after their treatment with ¹³¹I.

See also

Isotopes of iodine

Iodine in biology

References

External links

ANL factsheet

Patient brochure on radioactive iodine treatment From the American Thyroid Association

RadiologyInfo - The radiology information resource for patients: Radioiodine (I -131) Therapy

Case Studies in Environmental Medicine: Radiation Exposure from Iodine 131

Sensitivity of Personal Homeland Security Radiation Detectors to Medical Radionuclides and Implications for Counseling of Nuclear Medicine Patients

NLM Hazardous Substances Databank – Iodine, Radioactive

Category:Isotopes of iodine Category:Antithyroid drugs Category:Fission products