IODINE, RADIOACTIVE: Nuclear Power Plant Emissions
There are 36 isotopes of iodine
having masses between 108 and 143. Only one isotope is stable (iodine-127); the
remaining are radioactive. Most of these have radioactive half-lives of minutes
or less. Twelve have half-lives that exceed 1 hour, and six have half-lives that
exceed 12 hours (iodine-123, iodine-124, iodine-125, iodine-126, iodine-129, and
iodine-131). Four isotopes (iodine-123, iodine-125, iodine-129, and iodine-131)
are of particular interest with respect to human exposures because iodine-123
and iodine-131 are used medically and all four are sufficiently long-lived to be
transported to human receptors after their release into the environment. The
U.S. population has been exposed to radioiodine in the general environment as a
result of atmospheric fallout of radioiodine released from uncontained and/or
uncontrolled nuclear reactions. Historically, this has
resulted from surface or atmospheric detonation of nuclear
bombs, from routine and accidental releases from nuclear
power plants and nuclear
fuel reprocessing facilities, and from hospitals and medical research
facilities. Estimates have been made of radiation doses to the U.S. population
attributable to nuclear bomb tests conducted during the
1950s and 1960s at the Nevada Test Site; however, dose estimates for global
fallout have not been completed. Geographic-specific geometric mean lifetime
doses are estimated to have ranged from 0.19 to 43 cGy (rad) for a hypothetical
individual born on January 1, 1952 who consumed milk only from commercial retail
sources, 0.7-55 cGy (rad) for people who consumed milk only from home-reared
cows, and 6.4-330 cGy (rad) for people who consumed milk only from home-reared
goats. Additional information is available on global doses from nuclear
bomb tests and doses from nuclear fuel processing and
medical uses can be found in United Nations Scientific Committee on the Effects
of Atomic Radiations. Individuals in the United States can also be exposed to
radioiodine, primarily iodine-123 and iodine-131, as a result of clinical
procedures in which radioiodine compounds are administered to detect
abnormalities of the thyroid gland or to destroy the thyroid gland to treat
thyrotoxicosis or thyroid gland tumors. Diagnostic uses of radioiodine typically
involve administration, by the oral or intravenous routes, of 0.1-0.4 mCi (4-15
MBq) of iodine-123 or 0.005-0.01 mCi (0.2-0.4 MBq) of iodine-131. These
correspond to approximate thyroid radiation doses of 1-5 rad (cGy) and 6-13 rad
(cGy) for iodine-123 and iodine-131, respectively. Cytotoxic doses of iodine-131
are delivered for ablative treatment of hyperthyroidism or thyrotoxicosis;
administered activities typically range from 10 to 30 mCi (370-1,110 MBq).
Higher activities are administered if complete ablation of the thyroid is the
objective; this usually requires 100-250 mCi (3,700-9,250 MBq). Thyroid gland
doses of approximately 10,000-30,000 rad (300 Gy) can completely ablate the
thyroid gland. An administered activity of 5-15 mCi (185-555 MBq) yields a
radiation dose to the thyroid gland of approximately 5,000-10,000 rad (50-100 Gy).
The health effects of exposure to radioiodine derive from the emission
of beta and gamma radiation. Radioiodine that is absorbed into the body quickly
distributes to the thyroid gland and, as a result, the tissues that receive the
highest radiation doses are the thyroid gland and surrounding tissues (e.g.,
parathyroid gland). Tissues other than the thyroid gland can accumulate
radioiodine, including salivary glands, gastric mucosa, choroid plexus, mammary
glands, placenta, and sweat glands. Although these tissues may also receive a
radiation dose from internal radioiodine, the thyroid gland receives a far
higher radiation dose. The radiation dose to the thyroid gland from absorbed
radioiodine varies with the radiation emission
properties of the isotope (e.g., type of radiation, energy of emission,
effective radioactive half-life). The highest total doses are achieved with
iodine-131 which has an effective half-life in the thyroid gland of 177 hours.
Radioiodine is cytotoxic to the thyroid gland at high radiation doses and
produces hypothyroidism when doses to the thyroid gland exceed 25 Gy (2,500 rad).
Thyroid gland doses of approximately 300 Gy (30,000 rad) can completely destroy
the thyroid gland. This dose can be achieved with an acute exposure to
approximately 25-250 mCi (0.9-9 GBq) of iodine-131. Although, a rare outcome,
cytotoxic doses of iodine-131 can also produce dysfunction of the parathyroid
gland, which can receive a radiation dose from emission
of iodine-131 in the adjacent thyroid gland. Congenital hypothyroidism can occur
in newborn infants after maternal exposures to high amounts of iodine-131 (11 to
77 mCi; 407-2,850 MBq) for treatment of thyroid cancer during pregnancy.
Exposures to radioiodine may increase risk of thyroid cancer. In studies of
relatively high exposures (3-27 mCi, 111-999 MBq) and cytotoxic thyroid gland
doses (6,000 rad, cGy), achieved in the treatment of thyroid gland disorders,
significant risks for cancers in organs other than the thyroid gland have not
been consistently detected when the study designs control for other treatments
administered to the patients. However, a small increased thyroid cancer risk may
be associated with iodine-131 treatment for hyperthyroidism. Studies of
diagnostic doses of radioiodine (40-70 uCi, 1.5-2.6 MBq; 80-130 rad, cGy) have
not consistently revealed significant risks of thyroid or other cancers; those
that have, however, found significantly elevated risks only in patients who were
administered the radioiodine for diagnosing a suspected thyroid gland tumor and
the cancer may have predated the administration of iodine-131 or the patients
may have had previous external radiation exposure. However, in general, studies
of the outcomes of medical uses of radioiodine involve subjects who were exposed
as adults. Studies of thyroid cancers and external radiation exposure have found
a strong age-dependence between thyroid radiation dose and thyroid cancer. Risk
is substantially greater for radiation doses received prior to age 15 years when
compared to risks for doses received at older ages, although the excess thyroid
cancer risk is not limited to that age group. This same general trend in
age-dependence would be expected for internal exposures to radioiodine; thus,
studies of adult exposures to radioiodine may not be directly applicable to
predicting outcomes from exposures to children. Health outcomes in populations
that were exposed to environmental releases of radioiodine have been extensively
studied. These include (1) releases from explosions of nuclear
bombs such as the Marshall Islands BRAVO test, the largest U.S. detonation (15
megatons), and from the Nevada Test Site; (2) releases from nuclear
fuel production facilities such as the Hanford Nuclear
Site; and (3) accidental releases from nuclear power
plants such as the Chernobyl explosion and fire. In
general, releases of these types result in mixed exposures to a variety of
radioisotopes and to radiation doses from both external and internal exposure.
However, doses from radioiodine that are significant to health derive largely
from internal exposure as a result of uptake of relatively short-lived
radioiodine isotopes into the thyroid gland. Thus, effects on the thyroid
attributable to radioiodine that were subsequently observed, in some cases,
years after the event, derived from exposures to the relatively high levels of
radioiodine found immediately after the event, rather than from sustained
exposures. Results of these studies suggest that environmental exposures that
resulted in high doses of radiation to the thyroid gland, which appear to have
occurred in association with the BRAVO detonation (300-2,000 rad, cGy) and the
Chernobyl explosion and fire (1-200 rad, cGy), may have contributed to thyroid
gland abnormalities, including cancers. However, evidence is inconclusive for
thyroid effects in association with the much lower estimated exposures that have
resulted from the Nevada Test Site (1-30 rad, cGy) and Hanford Nuclear
Site (mean 17 rad, cGy; range: 0.0003-282 rad, cGy). (SRC)
Human Toxicity Excerpts :
/EPIDEMIOLOGY STUDIES/ /THYROID CANCER/ The thyroid gland receives the highest
radiation dose of any organ or tissue following an internal exposure to
radioiodine and, therefore, cancer of the thyroid gland is the major health
concern associated with radioiodine exposures. Children, in particular, are
highly vulnerable to radioiodine toxicity. Cancer morbidity and mortality among
populations that received exposures to radioiodine have been examined in several
large-scale epidemiology studies. In general, these studies fall into several
categories that can be distinguished by the sources of exposure and estimated
radiation doses to the thyroid gland and include: (1) exposure to high doses (10
to 20 mCi, 370 to 740 MBq; >10,000 rad, >100 Gy) achieved when iodine-131
is administered to treat hyperthyroidism ...; (2) exposures to moderately high
doses (40 to 70 ?Ci, 1.5-2.6 MBq; 80 to 130 rad, cGy) associated with clinical
administration of iodine-131 for diagnosis of thyroid gland disorders; (3) low
doses from exposures to fallout from nuclear bomb tests
(BRAVO test, 300 to 2,000 rad, cGy; Nevada Test Site, 1 to 40 rad, cGy); (4) low
to high doses from exposures to releases from nuclear power
plant accidents (Chernobyl, 10 to 500 rad, cGy); and
(5) low to high environmental exposures from operational releases from nuclear
fuel processing plants (Hanford Nuclear
Site, 0.0001 to 284 rad, cGy). As a point of reference, the dose-response
relationship for thyroid cancer and external radiation appears to extend down to
thyroid doses of 0.1 Gy (10 rad) and predicts an excess relative risk (ERR) of
7/Gy for ages <15 years at exposure. Studies of thyroid cancers and external
radiation exposure have found a strong dependence of thyroid cancer risk on age
at exposure ? and this increased risk persists, possibly for the lifetime. This
same general trend in age-dependence would be expected for internal exposures to
radioiodine? . Breast cancer is also a concern with exposures to high levels of
radioiodine after ablative therapy for hyperthyroidism because breast expresses
NIS and can transport and accumulate iodide. However, the epidemiological
literature to date has not implicated such exposures as a significant risk
factor for breast cancer. /Radioactive iodine from medical tests and fallout/
Environmental Fate/Exposure Summary :
Iodine-129 (half-life = 16 million years) is the only naturally occurring
radioisotope of iodine and is produced as a fission product of uranium and
thorium in soils and oceans. Iodine-129 is also produced through the reaction of
xenon with high energy particles in the upper atmosphere. Artificial sources of
radioiodine include: exposure when 131-I is administered to treat
hyperthyroidism; exposures associated with clinical administration of 131-I for
diagnosis of thyroid gland disorders; exposures to fallout from nuclear
bomb tests; low to high doses from exposures to releases from nuclear
power plant accidents; and low
to high environmental exposures from operational releases from nuclear
fuel processing plants. The estimated global inventory
of iodine-129 is approximately 9,600 Ci (5.4X10+7 grams of iodine-129).
Iodine-125 (half-life = 60 days) and iodine-131 (half-life = 8.04 days) are
produced in the fission of uranium and plutonium by neutron bombardment in
reactors and particle accelerators. Since these isotopes of iodine have short
half-lives, they do not have long residency times in the environment. (SRC)
Artificial Pollution Sources :
There are 36 isotopes of iodine having masses between 108 and 143(1); 14 of
these yield significant radiation(2). These iodine radioisotopes are of
particular interest with respect to human exposures because iodine-123 and
iodine-131 are used medically and all six are sufficiently long-lived to be
transported to human receptors after their release into the environment(2).
Artificial sources of radioiodine include: exposure when 131-I is administered
to treat hyperthyroidism; exposures associated with clinical administration of
131-I for diagnosis of thyroid gland disorders; exposures to fallout from nuclear
bomb tests; low to high doses from exposures to releases from nuclear
power plant accidents; and low
to high environmental exposures from operational releases from nuclear
fuel processing plants(2).
General Manufacturing Information :
Both iodine-129 and iodine-131 are produced by the fission of uranium atoms
during operation of nuclear reactors and by plutonium
(or uranium) in the detonation of nuclear weapons.
Iodine reacts easily with other chemicals, and isotopes of iodine are found as
compounds rather than as a pure elemental nuclide. However, iodine released to
the environment from nuclear power
plants is usually a gas.
Drug Warnings :
Chromosome aberrations (breakages, dicentrics, micronuclei) have been found in
peripheral blood cells of patients who received iodine-131 ablative therapy for
hyperthyroidism, in infants born to mothers who received such therapy during
pregnancy, and in children exposed to radioiodine released from the Chernobyl nuclear
power plant. The range of
iodine-131 exposures in these cases was 15 to 200 mCi (0.6 to 7.4 GBq).
/Iodine-131/
Disposal Methods :
Low-level radioactive waste (LLW) is a general term for a wide range of wastes.
Industries, hospitals and medical, educational, or research institutions;
private or government laboratories; and nuclear fuel
cycle facilities (e.g., nuclear power
reactors and fuel fabrication plants) using radioactive
materials generate low-level wastes as part of their normal operations. These
wastes are generated in many physical and chemical forms and levels of
contamination.
Human Toxicity Excerpts :
/EPIDEMIOLOGY STUDIES/ /THYROID/ Subsequent to the release of radioactive
materials from the Chernobyl power plant
in 1986, an increased prevalence of thyroid nodules in children of the Belarus
region was reported. An analysis of the results of ultrasound screening of
20,785 people in Belarus conducted during the period 1990-1995 revealed a
prevalence of thyroid gland nodules that ranged from 4 to 22 per 1,000.
Prevalence was highest (16 to 22 per 1,000) among residents from districts in
which thyroid radiation doses were estimated to have been above 1 Gy (1.3 to 1.6
Gy, 130 to 160 rad). Verified diagnoses from patients who were referred for
further examination as a result of ultrasound results revealed a prevalence of
thyroid cancer of 2.5 to 6.2 per 1,000, or approximately 13 to 50% of nodule
cases, among cases from districts where thyroid radiation doses were estimated
to have been above 1 Gy (1.3 to 1.6 Gy, 130 to160 rad). Adenoma was diagnosed in
7 to 12% of thyroid nodule cases, nodular goiter was diagnosed in 5 to 22% of
the thyroid nodule cases, and 7 to 64% of the nodule cases were diagnosed as
benign cysts. ? The results of other thyroid screening programs (e.g., the
Chernobyl Sasakawa Health and Medical Cooperation Project) also suggest a high
prevalence of goiter among people born in Belarus between the years 1976 and
1986, which would be consistent with a high prevalence of iodine deficiency in
the population. Therefore, iodine deficiency may have contributed to the
observed thyroid nodularity and also may be a confounding variable in
susceptibility to thyroid cancer. /Radioactive iodine and thyroid deficiencies/
Effluent Concentrations :
The total iodine-131 concentration measured in liquid waste released on July 1,
1992 from the nuclear plant at
Bugey on the River Rhone, France was 6.5 Bq/L, corresponding to total
radioactivity of 2.5 MBq(1).
Plant Concentrations :
Aquatic moss (Cinclidotus riparius) were collected from downstream following
discharges from a nuclear plant
at Bugey on the River Rhone, France(1). Sampled from 1986 to 1990, the mosses
were shown to contain negligible levels of iodine-131, presumably due to loss
volatilization during dessication and incineration of samples, and its short
physical half-life(8 days)(1). Fontinalis antipyretica from the Sorgue River,
upstream from any human activity, sampled on July 1 and 2, 1992, were used to
determine background concentrations(1).
Prior History of Accidents :
/Windscale, United Kingdow Radiation Incident/ In October 1957, the first
substantially publicized release of radioactive material from a nuclear
reactor accident occurred at the Windscale nuclear
weapons plant at Sellafield in the United Kingdom.
During a routine release of stored energy from the graphite core of a carbon
dioxide-cooled, graphite-moderated reactor, operator error allowed the fuel to
overheat. This led to uranium oxidation and a subsequent graphite fire. Attempts
to extinguish the fire with carbon dioxide were ineffective. In the end, water
was applied directly to the fuel channels but not before the fire had burned for
3 days, resulting in the release of iodine-131 (740 terrabecquerel; 20 kCi),
cesium-137 (22 terrabecquerel; 0.6 kCi), polonium- 210 (8.8 terrabecquerel; 0.2
kCi), ruthenium-106 (3 terrabecquerel; 0.08 kCi), and xenon-133 (1.2
petabecquerel; 32.4 kCi). The fire consumed much of the uranium fuel, and some
of the resulting fallout was in the form of flake-like uranium oxide varying in
size from 1 to 25 cm. The contamination of pastureland was widespread; for those
in close proximity to the accident, the greatest threat of exposure was
considered to be from iodine-131 via contaminated cow's milk. Those living
farther from the accident were exposed to significant amounts of iodine-131 via
milk consumption and air inhalation. The consumption of cow's milk was quickly
banned; this lessened the exposure to iodine-131. The highest individual doses
(approximately 100 milligray) were to the thyroids of children living near the
accident site. The collective dose equivalent received in the United Kingdom and
the rest of Europe was estimated to be 2,000 man-sieverts, of which 900 man-sieverts
was from inhalation, 800 man-sieverts was from ingestion, and 300 man-sieverts
was from external exposure. The main radionuclides contributing to the exposures
were iodine-131 (37%), polonium-210 (37%), and cesium-137 (15%). There has been
no detected impact on the health of the public from this accident.
Prior History of Accidents :
/Chernobyl Radiation Incident/ The accident at the Chernobyl reactor happened
during an experimental test of the electrical control system as the reactor was
being shut down for routine maintenance. The operators, in violation of safety
regulations, had switched off important control systems and allowed the reactor
to reach unstable, low-power conditions. A sudden power
surge caused a steam explosion that ruptured the reactor vessel, allowing
further violent fuel-steam interactions that destroyed the reactor core and
severely damaged the reactor building. The radioactive gases and particles
released in the accident were initially carried by the wind in westerly and
northerly directions. On subsequent days, the winds came from all directions.
The deposition of radionuclides was governed primarily by precipitation
occurring during the passage of the radioactive cloud, leading to a complex and
variable exposure pattern throughout the affected region. The radionuclides
released from the reactor that caused exposure of individual were mainly
iodine-131, cesium-134 and cesium-137. Iodine-131 has a short radioactive
half-life, but it can be transferred to humans relatively rapidly from the air
and through milk and leafy vegetables. Iodine becomes localized in the thyroid
gland. The isotopes of cesium have relatively longer half-lives. These
radionuclides cause longer-term exposures through the ingestion pathway and
through external exposure from their deposition on the ground Average doses to
those persons most affected by the accident were about 100 mSv for 240,000
recovery operation workers, 30 mSv for 116,000 evacuated persons and 10 mSv
during the first decade after the accident to those who continued to reside in
contaminated areas. Outside Belarus, the Russian Federation and Ukraine, other
European countries were affected by the accident. Doses there were at most 1 mSv
in the first year after the accident with the dose over a lifetime estimated to
be 2-5 times the first year doses. The exposures were much higher for those
involved in mitigating the effects of the accident and those who resided nearby.
The Chernobyl accident caused many severe radiation effects almost immediately.
Of 600 workers present on the site, 134 suffered from radiation sickness. Of
these, 28 died in the first three months and another 2 soon afterwards. In
addition, during 1986 and 1987, about 200,000 recovery operation workers
received doses of between 0.01 Gy and 0.5 Gy. That cohort is at potential risk
of late consequences such as cancer and other diseases The Chernobyl accident
also resulted in widespread radioactive contamination in areas of Belarus, the
Russian Federation and Ukraine inhabited by several million people. In addition
to causing radiation exposure, the accident caused long-term changes in the
lives of the people living in the contaminated districts For the last 14 years,
attention has been focused on investigating the association between exposure
caused by the radionuclides released in the Chernobyl accident and late effects,
in particular thyroid cancer in children. The number of thyroid cancers (about
1,800) in individuals exposed in childhood is considerably greater than expected
based on previous knowledge. Apart from the increase in thyroid cancer after
childhood exposure, no increases in overall cancer incidence or mortality have
been observed that could be attributed to ionizing radiation.
Other Chemical/Physical Properties :
DECAY PATHWAY: Iodine-124, half-life 4.1760 days, 76.9% decays via electron
capture (3170 keV) and 22.8% via beta(+) emission
(10.8% 2137.6 keV; 11.7% 1534.9 keV; gamma emission of
511.0 keV from annihilation of beta(+)) and 62.9% gamma emission
(abs intensities: 62.9% 602.72 keV; 10.9% 1691 keV; 10.4% 722.8 keV) to
tellurium-124, half-life stable
Radiation Limits & Potential :
DECAY PATHWAY: Iodine-124, half-life 4.1760 days, 76.9% decays via electron
capture (3170 keV) and 22.8% via beta(+) emission
(10.8% 2137.6 keV; 11.7% 1534.9 keV; gamma emission of
511.0 keV from annihilation of beta(+)) and 62.9% gamma emission
(abs intensities: 62.9% 602.72 keV; 10.9% 1691 keV; 10.4% 722.8 keV) to
tellurium-124, half-life stable
Formulations/Preparations :
There are 23 isotopes of iodine/ 127-I is the only stable one. 125-I ...decays
by gamma emissions. 131-I... decays by beta activity
and gamma emissions.
Other Chemical/Physical Properties :
DECAY PATHWAY: Iodine-129, half-life 15,700,000 years, decays via beta(-) emission
(100%, 154.4 keV maximum; 40.9 keV average energy) and gamma emission
(abs intensity: 7.51% 39.6 keV) to xenon-129, half-life stable
Other Chemical/Physical Properties :
DECAY PATHWAY: Iodine-131, half-life 8.0207 days, decays via beta(-) emission
(89.9%, 606.3 keV maximum; 191.6 keV average energy; 7.3%, 333.8 keV maximum;
96.6 keV average energy) and gamma emission (abs
intensity: 81.7% 364.5 keV) to xenon-131, half-life stable
Radiation Limits & Potential :
DECAY PATHWAY: Iodine-129, half-life 15,700,000 years, decays via beta(-) emission
(100%, 154.4 keV maximum; 40.9 keV average energy) and gamma emission
(abs intensity: 7.51% 39.6 keV) to xenon-129, half-life stable
Radiation Limits & Potential :
DECAY PATHWAY: Iodine-131, half-life 8.0207 days, decays via beta(-) emission
(89.9%, 606.3 keV maximum; 191.6 keV average energy; 7.3%, 333.8 keV maximum;
96.6 keV average energy) and gamma emission (abs
intensity: 81.7% 364.5 keV) to xenon-131, half-life stable
Therapeutic Uses :
In contrast to the smaller amount of radioactivity utilized in diagnostic nuclear
medicine, larger amounts of radioactivity are intentionally chosen for use in
therapeutic nuclear medicine. Therapy in nuclear
medicine involves oral, intravenous, or intracavitary delivery of radionuclides
in liquid form (sometimes called "unsealed" radionuclides). The
radionuclide is chosen with the aim of ensuring that subsequent physiological
redistribution will concentrate the radioactivity in the target tissue and, at
the same time, reduce the radioactivity in surrounding normal tissues.
Radionuclides suitable for use in therapeutic nuclear
medicine must either localize in their elemental form (such as iodine uptake in
the thyroid gland) or be bound to an appropriate pharmaceutical or antibody.
Radionuclides commonly used for therapeutic nuclear
medicine include: colloidal gold-198; iodine-131 as sodium iodine, meta-iodobenzylguanidine,
monoclonal antibodies; colloidal P-32; and Sr-89 chloride /from table/.
/Iodine-131, colloidal phosphorus-32, strontium-89, colloidal gold-198/
Plant Concentrations :
Once radioiodine deposits on foliage, it is removed by weathering and death of plant
parts at a rate of about 5% per day, leaving an effective half-life of removal
from grass of around 5 days(1).
Human Toxicity Excerpts :
/EPIDEMIOLOGY STUDIES/ /Thyroid Cancer/ Only a few studies have evaluated the
effects of environmental exposure to radioactive iodine. In contrast to the
medical exposures..., which were due exclusively to iodine-131, environmental
exposures have generally contained mixtures of iodine-131, external radiation,
and short-lived radioiodines. Initial studies of thyroid disease incidence in
Utah school children exposed to fallout from atmospheric nuclear
weapons testing at the Nevada Test Site appeared to show no difference in
thyroid disease outcomes compared to children from unexposed areas. However, a
follow-up study reported a slight excess risk of thyroid neoplasms associated
with radioiodine exposure. Although positive dose-response trends were also
noted for total nodules and thyroid cancer (when analyzed separately), they were
not statistically significant. The study was limited by small numbers of exposed
individuals and a low incidence of thyroid neoplasms and by the fact that the
examiners were not blinded to exposure. In contrast, a follow-up study of 3,440
persons exposed as young children to atmospheric releases of primarily
iodine-131 from the Hanford Nuclear Site found no
increased risk of thyroid cancer associated with individual radiation dose to
the thyroid. The explanation for the apparent difference in results in the Utah
study and the Hanford study is not clear. One possibility is that the exposures
were substantially different in terms of the mix of radionuclides and the dose
rate. Thyroid dose at Hanford was due almost entirely to iodine-131, whereas in
Utah there was greater contribution form other radioiodines as well as external
sources. Exposures in Utah were also more concentrated and episodic than at
Hanford, corresponding to specific nuclear tests. This
likely resulted in doses being delivered at substantially higher dose rates.
(although total dose among 3,545 study participants for whom thyroid doses could
be estimated [mean 98 mGy] was similar to Hanford doses). A second possibility
is that the Utah study's estimated dose-response could have been biased in the
direction of finding an association because the collections of dietary
consumption data took place after thyroid disease classification was known for
each participant. /Iodine-131 in fallout/
Methods of Manufacturing :
A total of 72% of uranium fissions and 75% of plutonium fissions lead directly
or indirectly, by beta decay of precursors, to iodine isotopes. For example,
2.89% of uranium-235 and 3.86% of plutonium-239 fission atoms lead to the
formation of a series of isobar 131 isotopes, including indium-131, tin-131,
antimony-131, tellurium-131, iodine-131, and xenon-131. Each isotope can be
formed as an initial fission product and, once formed, each isotope decays by
beta-ray emission to the right on the sequence, through
iodine-131, and with stable xenon-131.
Other Chemical/Physical Properties :
DECAY PATHWAY: Iodine-125, half-life 59.408 days, decays via electron capture
(149 keV) and gamma emission (abs intensity: 6.69% 35.5
keV) to tellurium-125, half-life stable
Radiation Limits & Potential :
DECAY PATHWAY: Iodine-125, half-life 59.408 days, decays via electron capture
(149 keV) and gamma emission (abs intensity: 6.69% 35.5
keV) to tellurium-125, half-life stable
Medical Surveillance :
/WORKER SURVEILLANCE/ In vivo measurements and a routine monitoring program for
radioiodine isotopes are the main bioassay considerations. ... Radioiodine
isotopes can be easily detected by in vivo measurements. Iodine-131 can be
readily detected using the NaI-detectorbased preview counter or the coaxial
germanium detector system. Because of their low-energy photon emissions,
iodine-125 and iodine-129 can only be measured using the intrinsic germanium (IG)
detector systems in the thyroid-counting configuration.
Human Toxicity Excerpts :
/OTHER TOXICITY INFORMATION/ /Parathyroid Gland/ Cases of hypo- and
hyperparathyroidism are rare outcomes in patients who receive iodine-131
treatments for ablative therapy of thyroid cancer or hyperthyroidism. The
parathyroid gland is in close proximity to the thyroid gland. Although in most
people, the parathyroid and thyroid glands are separated by more than 1 cm, in
approximately 20% of people, the parathyroid gland is located within the thyroid
gland capsule. The latter configuration would result in irradiation of the
parathyroid gland with beta emission from iodine-131 ?
. Cases of parathyroid dysfunction have been reported after exposures to
iodine-131 ranging from 4 to 30 mCi (0.15 to 1.1 GBq). /Iodine-131/
Other Occupational Permissible Levels :
The following values may be used for determining if facilities are in compliance
with the national emission standards for hazardous air
pollutants for iodine-123, iodine-124, iodine-125, iodine-126, iodine-129, and
iodine-131 in the gaseous form: 4.90X10-1, 9.30X10-3, 6.20X10-3, 3.70X10-3,
2.60X10-4, and 6.70X10-3 Ci/yr, respectively; in the liquid/powder form:
4.90X10+2, 9.30X10+0, 6.20X10+0, 3.70X10+0, 2.60X10-1, and 6.70X10+0 Ci/yr,
respectively; and in the solid form: 4.90X10+5, 9.30X10+3, 6.20X10+3, 3.70X10+3,
2.60X10+2, and 6.70X10+3 Ci/yr, respectively. Radionuclides with a boiling point
at 100 degrees Celsius or less, or exposed to a temperature of 100 degrees
Celsius, must be considered a gas. Capsules containing radionuclides in liquid
or powder form can be considered to be solids.
General Manufacturing Information :
Nearly all of the iodine-129 and iodine-131 generated in the United States is
present in spent nuclear reactor fuel rods. These fuel
rods are currently located at commercial reactor facilities or at Department of
Energy (DOE) facilities across the United States. The cumulative yield of
iodine-129 is about 1% of all fission products. Thus, iodine-129 represents only
a very small fraction of the total fission product inventory in the nuclear
fuel cycle.
Therapeutic Uses :
The radiation safety hazard--both external (from gamma rays emitted from the
patient) and internal (from radioactive material in the patient's
excretions)--can be calculated according to guides issued by the US Nuclear
Regulatory Commission. The protocol aimed to produce an occupancy factor of
0.125 or less for the critical person (person caring most for the patient) for
the entire 72-hour period after administration of iodine-131. This is the
equivalent of 3 hours per day at a distance of 1 m from the patient. ... Each
patient's suitability for outpatient therapy is determined on the basis of the
patient's home environment, ability to understand the risks involved and
likelihood of compliance, by the referring physician's opinion, through a
self-report questionnaire and through a patient interview with the radiation
safety officer and the nuclear medicine physician. This
protocol has been approved by the Atomic Energy Control Board /of Canada/ and
has been used to screen 8 patients to date, with 1 patient being denied
outpatient treatment. /The authors concluded that/ outpatient therapies with
relatively high doses of iodine-131 can be performed safely. Care must be taken
to ensure that the patient's home environment is suitable and that the patient
can understand and comply with precautions. If external exposure can be
minimized, only basic precautions are needed to ensure that internal
contamination does not lead to excessive doses to members of the public.
/Iodine-131/
Other Occupational Permissible Levels :
Monitoring frequency and compliance requirements for radionuclides in community
water systems (both surface and ground water) designated as utilizing waters
contaminated by effluents from nuclear facilities must
sample for beta particle and photon radioactivity. Systems must collect
quarterly samples for beta emitters and iodine-131 and annual samples for
tritium and strontium-90 at each entry point to the distribution system,
beginning within one quarter after being notified by the State. Systems already
designated by the State as systems using waters contaminated by effluents from nuclear
facilities must continue to sample until the State reviews and either reaffirms
or removes the designation. For iodine-131, a composite of five consecutive
daily samples shall be analyzed once each quarter. As ordered by the State, more
frequent monitoring shall be conducted when iodine-131 is identified in the
finished water.
Methods of Manufacturing :
The only naturally-occurring radioisotope of iodine is iodine-129. Isotopes of
mass less than 127 are produced in particle accelerators (common examples are
iodine-123 and iodine-125), while those greater than 127 are formed in neutron
generators such as nuclear reactors and atomic bombs
(common examples are iodine-129 and iodine-131).
Methods of Manufacturing :
Derivation: By pile irradiation of tellurium and from the fission products of nuclear
reactor fuels /Iodine-131/
Major Uses :
Nuclear reactors and bomb tests are the most
significant sources of these radioisotopes with the exception of iodine-131.
That isotope is routinely produced for use in medical imaging and diagnosis. The
isotopes released from the other sources represent a short-term environmental
health hazard should there be an abnormal release from a reactor or testing of
bombs. /Iodine radionuclides/
Disposal Methods :
Nuclear Regulatory Commission regulations separate
low-level waste into three classes: A, B and C. The classification of the waste
depends on the concentration, half-life and types of the various radionuclides
it contains. The NRC sets requirements for packaging and disposal of each class
of waste. Class A low-level waste contains radionuclides with the lowest
concentrations and the shortest half-lives. About 95 percent of all low-level
waste is categorized as Class A.
Disposal Methods :
/MEDICAL USES/ The same regulatory apparatus that applies to radiation used in
brachytherapy applies to radiation used for therapeutic nuclear
medicine. For the latter, regulations address the fact that with unsealed source
administration, the patient becomes a source of radiation and radioactive
contamination. Regulations allow patient excreta to be exempt from treatment as
radioactive waste. Hence, disposal of I-131-contaminated urine down the sanitary
sewer system is allowed.
Radiation Limits & Potential :
Environmental radiation protection standards for management and disposal of
spent nuclear fuel, high-level, and transuranic
radioactive wastes include release limits for containment requirements
(cumulative releases to the accessible environment for 10,000 years after
disposal) per 1,000 metric tons of heavy metal or other unit of waste for
iodine-129 is 100 curies(1).
Evidence for Carcinogenicity :
There is sufficient evidence in humans that exposure during childhood to
short-lived radioisotopes of iodine, including iodine-131, in fall-out from
reactor accidents and nuclear weapons detonations
causes thyroid cancer. /Iodine-131/
Medical Surveillance :
/GENERAL SURVEILLANCE/ An exposure history includes previous childhood head,
neck, and upper mediastinum radiation exposure; previous residences (downwind
from or proximity to nuclear testing or release sites);
dietary habits since childhood; source of drinking water; occupational history;
and hobbies. Milk consumption and source are important risk factors (for
example, fresh versus processed milk; milk from a cow, sheep, or goat). The
patient should be asked about symptoms consistent with hypothyroidism,
hyperthyroidism, and disorders of calcium metabolism. Exposure to iodine-131
could be indicated by the patient's answers to questions in the exposure history
relating to the following: previous childhood head, neck, and upper mediastinum
radiation exposure previous residences dietary habits since childhood milk
consumption and source. /Iodine-131/
Human Toxicity Excerpts :
/EPIDEMIOLOGY STUDIES/ /THYROID CANCER/ The health effects of exposure to
iodine-131 fallout from atmospheric nuclear tests
conducted at the Nevada test site in the 1950s have been studied for the last
four decades. ... 2,500 children were examined and individual doses to the
thyroid reconstructed. Nineteen neoplasms, of which eight were malignant, were
diagnosed. /Iodine-131 in fallout/
Human Toxicity Excerpts :
/EPIDEMIOLOGY STUDIES/ /Thyroid Cancer/... A follow up study of 3,440 persons
exposed as young children to atmospheric releases of primarily iodine-131 from
the Hanford Nuclear Site in eastern Washington State
/was conducted/. No increased risk of thyroid cancer was found associated with
individual radiation dose to the thyroid. /Iodine-131 fallout/
Human Toxicity Excerpts :
/EPIDEMIOLOGY STUDIES/ Approximately 740,000 Ci (2.73x10+16 Bq) of iodine-131
were released to the atmosphere from the Hanford Nuclear
Site in Washington State from 1944 through 1957. ... The Hanford Thyroid Disease
Study (HTDS) was conducted to determine if thyroid disease is increased among
persons exposed as children to atmospheric releases of iodine-131 from Hanford.
... Retrospective cohort study: Exposure could have occurred from December 1944
through 1957. Follow-up occurred until the time of the HTDS examination
(December 1992-September 1997). Participants' thyroid radiation doses from
Hanford's iodine-131 releases were estimated from interview data regarding
residence and dietary histories. ... The cohort included a sample of all births
from 1940 through 1946 to mothers with usual residence in 1 of 7 counties in
eastern Washington State. ... Of 5,199 individuals identified, 4,350 were
located alive and 3,440 were evaluable; ie, had sufficient data for dose
estimation and received an HTDS evaluation for thyroid disease, including a
thyroid ultrasound, physical examination, and fine needle biopsy if required to
evaluate thyroid nodularity. ./The main outcome measures were/ thyroid cancer,
benign thyroid nodules, total neoplasia, any thyroid nodules, autoimmune
thyroiditis, and hypothyroidism. ... There was no evidence of a relationship
between Hanford radiation dose and the cumulative incidence of any of the
outcomes. These results remained unchanged after taking into account several
factors that might confound the relationship between radiation dose and the
outcomes of interest. /The authors concluded that/ these results do not support
the hypothesis that exposure during infancy and childhood to iodine-131 at the
dose levels (median, 97 mGy; mean, 174 mGy) and exposure circumstances
experienced by our study participants increases the risk of the forms of thyroid
disease evaluated in this study. /Iodine-131/
Interactions :
Many hormones are potent growth stimulators. ... Thyroid stimulating hormone is
increased during puberty and pregnancy as a result of increased levels of female
sex hormones. There is epidemiological evidence suggesting that the development
of thyroid cancer after high-dose radiation exposure in females can be
potentiated by subsequent child bearing. Marshall Islanders who were exposed to
radioactive fallout from a nuclear weapons test in 1954
received high thyroid doses from radioiodines. Women who later became pregnant
were at higher risk of thyroid cancer than exposed women who remained
nulliparous. The numbers, however, were small. The same effect was found in a
population-based case-control study in Connecticut in the United States
involving 159 subjects with thyroid cancer and 285 controls; 12% of the cases
but only 4% of the controls reported prior radiotherapy to the head and neck.
Among women, this risk appeared to be potentiated by subsequent live births.
Artificial Pollution Sources :
Radioisotopes of mass less than 127 are produced in particle accelerators
(common examples are iodine-123 and iodine-125), while those greater than 127
are formed in neutron generators such as nuclear
reactors and atomic bombs (common examples are iodine-129 and iodine-131)(1).
Effluent Concentrations :
Medical institutions can contribute surprising amounts of iodine-131 to sewage
and consequnetly to rivers, due primarily to excretia elimination of
therapy-level doses(1). In the United States, radionuclides in patient excretia
are specifically excluded from the sewage disposal requirements that do apply to
research laboratories, etc(1). Therefore, measurable radioactivity has been
noted in sewage from some medical institutions, notably those with intensive nuclear
medicine pracitces(1). Radioactivity has also been reported in the solid sludge
resulting from treatment of sewage, however rapid decay and dilution prevent any
large dose to the public from this practice(1).
Atmospheric Concentrations :
SOURCE DOMINATED: Iodine-127, iodine-129, and iodine-131 have been identified in
2, 1, and 5 air samples, respectively, collected from the 1,636 NPL hazardous
waste sites where they were detected in some environmental media(1). Iodine-131
was detected in surface air of Tsukuba, Japan on May 5, 1986, with a maximum
concentration in airborne particulates of 494 mBq/cu m(2). Inherent barriers to
iodine-121 transport through a boiling water nuclear
reactor system (BWR) function with a high degree of efficiency, thereby reducing
this nuclide in gaseous releases from this type of reactor(3).
Food Survey Values :
Iodine-131, -132, and -135 were identified, not quantified in imported foods
entering the United States following the Chernobyl Nuclear
accident in 1986(1). Two cheeses collected shortly after the accident had
iodine-131 concentrations above the level of concern (LOC), 56 pCi/kg for infant
foods and 300 pCi/kg for other foods(1). By the end of 1990, nearly all
contamination was below the limit of detection of 2 Bq/kg(2).
Probable Routes of Human Exposure :
Medical personnel that are exposed to patients receiving iodine-131 therapy for
thyroid oblation or to treat thyroid carcinoma or thyrotoxicosis receive
radiation doses of 1.43X10-4 rem/hour per MBq of iodine-131 in the patient at a
distance of 0.1 meters from the patient. At increasing distances from the
patient the dose decreases to 0.18X10-4 and 0.07X10-4 rem/hour per MBq at 0.5
meters and 1.0 meters, respectively. Typical thyroid burdens of iodine-131 in
medical personnel range from 35-18,131 pCi with an average of 2,400 pCi.
Radiochemists and their support staff can have activities of 0.03-200 nCi per
year from exposures to iodine-131. Workers at a nuclear
fuel reprocessing facility were found to receive doses, via inhalation, as high
as 47.4-68.1 rem to the thyroid and 0.024-0.047 rem whole body. Laboratory
personnel that work directly with iodine-125 were found to have thyroid burdens
of iodine-125 at levels ranging between 9-560 nCi(1).
Acceptable Daily Intakes :
The US Nuclear Regulatory Commission has set annual
limits of inhalation intake (ALI) for iodine-123, iodine-125 and iodine-131 at
6,000 uCi, 60 uCi and 50 uCi, respectively.
Threshold Limit Values :
The Physical Agents TLV Committee accepts the occupational exposure guidance of
the International Commission on Radiological Protection (ICRP). Ionizing
radiation includes particulate radiation (e.g., alpha particles and beta
particles emitted from radioactive materials, and neutrons from nuclear
reactors and accelerators) and electromagnetic radiation (e.g., gamma rays
emitted from radioactive materials and x-rays from electron accelerators and
x-ray machines) with energy greater than 12.4 electron-volts (eV) ... The
guiding principle of radiation protection is to avoid all unnecessary exposures.
ICRP has established principles of radiological protection. There are (1) the
justification of a work practice: No work practice involving exposure to
ionizing radiation should be adopted unless it produces sufficient benefit to
the exposed individuals or the society to offset the detriment it causes. (2)
The optimization of a workpractice: All radiation exposures must be kept as low
as reasonably achievable (ALARA), economic and social factors being taken into
account. (3) The individual dose limits: The radiation dose from all relevant
sources should not exceed the /ICRP/ prescribed dose limits.
Other Occupational Permissible Levels :
TITLE 10--ENERGY CHAPTER I--NUCLEAR REGULATORY
COMMISSION PART 35_MEDICAL USE OF BYPRODUCT MATERIAL--Table of Contents Subpart
J_Training and Experience Requirements Sec. 35.932. Training for treatment of
hyperthyroidism. Except as provided in Sec. 35.57, the licensee shall require
the authorized user of only iodine-131 for the treatment of hyperthyroidism to
be a physician with special experience in thyroid disease who has had classroom
and laboratory training in basic radioisotope handling techniques applicable to
the use of iodine-131 for treating hyperthyroidism, and supervised clinical
experience as follows-- (a) 80 hours of classroom and laboratory training that
includes-- (1) Radiation physics and instrumentation; (2) Radiation protection,
(3) Mathematics pertaining to the use and measurement of radioactivity; and (4)
Radiation biology; and (b) Supervised clinical experience under the supervision
of an authorized user that includes the use of iodine-131 for diagnosis of
thyroid function, and the treatment of hyperthyroidism in 10 individuals.
/Iodine-131/
Other Occupational Permissible Levels :
The recommendations in the American National Standards Institute standard, ANSI
Z88.2-1992, "American National Standard For Respiratory Protection,"
are endorsed by the U.S. Nuclear Regulatory Commission
and may be used by licensees in establishing a respiratory protection program
with the /several/exceptions /including limitations that do not permit or
greatly restrict the use of quarter-facepiece respirators and supplied air
respirators and self-contained breathing apparatus (SCBA) that operate in the
demand mode./
Special Reports :
Thompson MA; Radiation safety precautions in the management of the hospitalized
131-I therapy patient. J Nucl Med Technol 29 (2): 61-6 (2001) The patient who
has been dosed with therapeutic activities of iodine-131 for thyroid carcinoma
poses a unique set of problems for nuclear medicine
technologists in their efforts to reduce personnel exposure and control
contamination spread. It is the objective of this article to: (a) review
practical radiation safety concerns associated with hospitalized iodine-131
therapy patients; (b) propose preventative measures that can be taken to
minimize potential exposure and contamination problems; and (c) review pertinent
federal regulations that apply to patients containing therapeutic levels of
radionuclides.
Special Reports :
U.S. Nuclear Regulatory Commission; Regulatory Guide
8.34 - Monitoring Criteria and Methods to Calculate Occupational Radiation
Doses. 1992/ Available at http://www.nrc.gov/reading-rm/doc-collections/reg-guides/occupational-health/active/8-34/index.html
as of September 25, 2006
Prior History of Accidents :
The releases of radiation from the accident at the Three Mile Island reactor in
Pennsylvania, USA, in March 1979 were caused by failure to close a pressure
relief valve, which led to melting of the uncooled fuel. The large release of
radioactive material was dispersed to only a minor extent outside the
containment building; however, xenon-133 (370x10+15 Bq) and iodine-131 (550x10+9
Bq) were released into the environment, leading to a total collective dose of 40
person-Sv and an average individual dose from external gamma-radiation of 15 uSv.
No individual was considered to have received doses to the thyroid of > 850
uSv ...The nuclear reactor accident at Three-Mile
Island, Pennsylvania (USA), released little radioactivity into the environment
and resulted in doses to the population that were much lower than those received
from the natural background. Any increase in the incidence of cancer would thus
be expected to be negligible and undetectable.
Prior History of Accidents :
Twenty years ago on 26 April 2006, the Chernobyl nuclear
reactor accident occurred (or, more precisely, the explosion that marked the
start of the accident occurred - the resultant fire lasted several days). This
is by far the largest unintentional release of radioactive material into the
environment and caused widespread contamination in Europe, which was
sufficiently great in the vicinity of Chernobyl to require evacuation of the
population. Much attention has been paid to the possible effects on health of
the resulting exposure to radiation, but the rapidity of the appearance and the
magnitude of the excess cases of childhood thyroid cancer in the heavily
contaminated areas of Belarus, Ukraine and Russia took most people by surprise.
The striking excess of thyroid cancer among those exposed as children is
undoubtedly attributable to the high thyroid doses received from radioiodine
released during the accident; but many of these cases might have been prevented
by the swift administration of stable iodine to block the uptake of the
short-lived radioisotopes of iodine (as occurred in the town of Pripyat, close
to Chernobyl) and especially by the prevention of the consumption of
contaminated foodstuffs, in particular milk. It was a milk ban in the most
affected parts of West Cumbria in the immediate aftermath of the Windscale
reactor accident in October 1957 that significantly reduced thyroid doses and
the risk of thyroid cancer (and the dose and risk calculations upon which this
milk ban was based were largely conducted by John Dunster, whose death has
recently been announced). However, the Chernobyl accident and the consequent
contamination was on a much larger scale than occurred after the Windscale fire,
and the logistics of determining the areas upon which the greatest attention
should have been focused should not be underestimated. Nonetheless, the
authorities in the former USSR did obtain, with some rapidity, information on
the extent of the contamination from experts that were dispatched to Chernobyl
immediately after the accident, and at least some action could have been taken
to alleviate the risk of thyroid cancer due to radioiodine intake. The high
radiation levels that were found in some locations close to Chernobyl show how
fortunate it was that population centers were not worse affected, or the
consequences of the accident could have been much more serious, and with the
relatively swift evacuation of people from the area surrounding the reactor,
doses that would have posed a risk of acute health effects were avoided.
MORE ABOUT HEALTH EFFECTS
IODINE,
RADIOACTIVE
CASRN: NO CAS RN
Toxicity Summary:
There are 36 isotopes of iodine having masses between 108 and 143. Only one
isotope is stable (iodine-127); the remaining are radioactive. Most of these
have radioactive half-lives of minutes or less. Twelve have half-lives that
exceed 1 hour, and six have half-lives that exceed 12 hours (iodine-123,
iodine-124, iodine-125, iodine-126, iodine-129, and iodine-131). Four isotopes
(iodine-123, iodine-125, iodine-129, and iodine-131) are of particular interest
with respect to human exposures because iodine-123 and iodine-131 are used
medically and all four are sufficiently long-lived to be transported to human
receptors after their release into the environment. The U.S. population has been
exposed to radioiodine in the general environment as a result of atmospheric
fallout of radioiodine released from uncontained and/or uncontrolled nuclear
reactions. Historically, this has resulted from surface or atmospheric
detonation of nuclear bombs, from routine and accidental releases from nuclear
power plants and nuclear fuel reprocessing facilities, and from hospitals and
medical research facilities. Estimates have been made of radiation doses to the
U.S. population attributable to nuclear bomb tests conducted during the 1950s
and 1960s at the Nevada Test Site; however, dose estimates for global fallout
have not been completed. Geographic-specific geometric mean lifetime doses are
estimated to have ranged from 0.19 to 43 cGy (rad) for a hypothetical individual
born on January 1, 1952 who consumed milk only from commercial retail sources,
0.7-55 cGy (rad) for people who consumed milk only from home-reared cows, and
6.4-330 cGy (rad) for people who consumed milk only from home-reared goats.
Additional information is available on global doses from nuclear bomb tests and
doses from nuclear fuel processing and medical uses can be found in United
Nations Scientific Committee on the Effects of Atomic Radiations. Individuals in
the United States can also be exposed to radioiodine, primarily iodine-123 and
iodine-131, as a result of clinical procedures in which radioiodine compounds
are administered to detect abnormalities of the thyroid gland or to destroy the
thyroid gland to treat thyrotoxicosis or thyroid gland tumors. Diagnostic uses
of radioiodine typically involve administration, by the oral or intravenous
routes, of 0.1-0.4 mCi (4-15 MBq) of iodine-123 or 0.005-0.01 mCi (0.2-0.4 MBq)
of iodine-131. These correspond to approximate thyroid radiation doses of 1-5
rad (cGy) and 6-13 rad (cGy) for iodine-123 and iodine-131, respectively.
Cytotoxic doses of iodine-131 are delivered for ablative treatment of
hyperthyroidism or thyrotoxicosis; administered activities typically range from
10 to 30 mCi (370-1,110 MBq). Higher activities are administered if complete
ablation of the thyroid is the objective; this usually requires 100-250 mCi
(3,700-9,250 MBq). Thyroid gland doses of approximately 10,000-30,000 rad (300
Gy) can completely ablate the thyroid gland. An administered activity of 5-15
mCi (185-555 MBq) yields a radiation dose to the thyroid gland of approximately
5,000-10,000 rad (50-100 Gy). The health effects of exposure to radioiodine
derive from the emission of beta and gamma radiation. Radioiodine that is
absorbed into the body quickly distributes to the thyroid gland and, as a
result, the tissues that receive the highest radiation doses are the thyroid
gland and surrounding tissues (e.g., parathyroid gland). Tissues other than the
thyroid gland can accumulate radioiodine, including salivary glands, gastric
mucosa, choroid plexus, mammary glands, placenta, and sweat glands. Although
these tissues may also receive a radiation dose from internal radioiodine, the
thyroid gland receives a far higher radiation dose. The radiation dose to the
thyroid gland from absorbed radioiodine varies with the radiation emission
properties of the isotope (e.g., type of radiation, energy of emission,
effective radioactive half-life). The highest total doses are achieved with
iodine-131 which has an effective half-life in the thyroid gland of 177 hours.
Radioiodine is cytotoxic to the thyroid gland at high radiation doses and
produces hypothyroidism when doses to the thyroid gland exceed 25 Gy (2,500 rad).
Thyroid gland doses of approximately 300 Gy (30,000 rad) can completely destroy
the thyroid gland. This dose can be achieved with an acute exposure to
approximately 25-250 mCi (0.9-9 GBq) of iodine-131. Although, a rare outcome,
cytotoxic doses of iodine-131 can also produce dysfunction of the parathyroid
gland, which can receive a radiation dose from emission of iodine-131 in the
adjacent thyroid gland. Congenital hypothyroidism can occur in newborn infants
after maternal exposures to high amounts of iodine-131 (11 to 77 mCi; 407-2,850
MBq) for treatment of thyroid cancer during pregnancy. Exposures to radioiodine
may increase risk of thyroid cancer. In studies of relatively high exposures
(3-27 mCi, 111-999 MBq) and cytotoxic thyroid gland doses (6,000 rad, cGy),
achieved in the treatment of thyroid gland disorders, significant risks for
cancers in organs other than the thyroid gland have not been consistently
detected when the study designs control for other treatments administered to the
patients. However, a small increased thyroid cancer risk may be associated with
iodine-131 treatment for hyperthyroidism. Studies of diagnostic doses of
radioiodine (40-70 uCi, 1.5-2.6 MBq; 80-130 rad, cGy) have not consistently
revealed significant risks of thyroid or other cancers; those that have,
however, found significantly elevated risks only in patients who were
administered the radioiodine for diagnosing a suspected thyroid gland tumor and
the cancer may have predated the administration of iodine-131 or the patients
may have had previous external radiation exposure. However, in general, studies
of the outcomes of medical uses of radioiodine involve subjects who were exposed
as adults. Studies of thyroid cancers and external radiation exposure have found
a strong age-dependence between thyroid radiation dose and thyroid cancer. Risk
is substantially greater for radiation doses received prior to age 15 years when
compared to risks for doses received at older ages, although the excess thyroid
cancer risk is not limited to that age group. This same general trend in
age-dependence would be expected for internal exposures to radioiodine; thus,
studies of adult exposures to radioiodine may not be directly applicable to
predicting outcomes from exposures to children. Health outcomes in populations
that were exposed to environmental releases of radioiodine have been extensively
studied. These include (1) releases from explosions of nuclear bombs such as the
Marshall Islands BRAVO test, the largest U.S. detonation (15 megatons), and from
the Nevada Test Site; (2) releases from nuclear fuel production facilities such
as the Hanford Nuclear Site; and (3) accidental releases from nuclear power
plants such as the Chernobyl explosion and fire. In general, releases of these
types result in mixed exposures to a variety of radioisotopes and to radiation
doses from both external and internal exposure. However, doses from radioiodine
that are significant to health derive largely from internal exposure as a result
of uptake of relatively short-lived radioiodine isotopes into the thyroid gland.
Thus, effects on the thyroid attributable to radioiodine that were subsequently
observed, in some cases, years after the event, derived from exposures to the
relatively high levels of radioiodine found immediately after the event, rather
than from sustained exposures. Results of these studies suggest that
environmental exposures that resulted in high doses of radiation to the thyroid
gland, which appear to have occurred in association with the BRAVO detonation
(300-2,000 rad, cGy) and the Chernobyl explosion and fire (1-200 rad, cGy), may
have contributed to thyroid gland abnormalities, including cancers. However,
evidence is inconclusive for thyroid effects in association with the much lower
estimated exposures that have resulted from the Nevada Test Site (1-30 rad, cGy)
and Hanford Nuclear Site (mean 17 rad, cGy; range: 0.0003-282 rad, cGy). (SRC)
Evidence for Carcinogenicity:
There is sufficient evidence in humans that exposure during childhood to
short-lived radioisotopes of iodine, including iodine-131, in fall-out from
reactor accidents and nuclear weapons detonations causes thyroid cancer.
/Iodine-131/
There is sufficient evidence in experimental animals for the carcinogenicity of
mixed beta-particle emitters (iodine-131, cesium-137, cerium-144 and
radium-228). /Iodine, Cesium, Cerium, Radium/
Human Toxicity Excerpts:
/CASE REPORTS/... /Iodine-131/ given in millicurie doses can damage or ablate
the developing thyroid of the human fetus. Hypothyroidism, either congenital or
of late onset, has been reported in at least 5 children whose mothers were
treated with iodine-131 during pregnancy. /Iodine-131/
/EPIDEMIOLOGY STUDIES/ Cancer/ .../A study/ based on 107 cases /of thyroid
cancer/ diagnosed in Belarus /was conducted/. Although a strong relationship
between estimated radiation dose and thyroid cancer was found, thyroid doses
were inferred for children from estimates for adults who lived in the same
villages. The second /study/ is based on confirmed cases of thyroid cancer in
children and adolescents aged 0-19 years at the times of the accident, residing
in the more highly contaminated areas of the Bryansk Oblast of Russia. Based on
26 cases and 52 controls /(randomly selected from the same region as the case,
matched on age, sex, and type of settlement)/ and using a log-linear
dose-response model treating estimated individual thyroid radiation dose as a
continuous variable, the trend of increasing risk with increasing dose was
statistically significant (one-sided p=0.009). /The major contributor to thyroid
dose from the Chernobyl fallout is iodine-131. Lesser contributions from
iodine-132 and iodine-133 external radiation. Individual accumulated dose to the
thyroid estimated based on environmental measurements and individual dosimetry
interviews./ The third /study/ is a population-based case-control study of
thyroid cancer carried out in contaminated regions of Belarus and the Russian
Federation. The study included 276 cases and 1300 /age and sex/ matched controls
aged <15 years at the time of the accident. Individual doses were calculated
for each subject. A very strong dose-response relationship was observed in this
study (p<0.0001). At one Gy, the odds ratio (OR) varied form 5.5 (95%
confidence interval 3.1, 9.5) to 8.4 (95% confidence interval 4.1, 17.3)
depending on the form of the risk model used. A clear linear dose-response
relationship was observed up to about one Gy, followed by a marked flattening.
The risk appeared to be mainly related to exposure to iodine-131. Collectively,
data from these studies suggest that exposure to radiation from Chernobyl is
associated with an increased risk of thyroid cancer, and that the relationship
is dose-dependent. These findings are consistent with descriptive reports from
contaminated areas of Ukraine and Belarus, and the quantitative estimate of
thyroid cancer risk is generally consistent with estimates from other
radiation-exposed populations. /Iodine-131 fallout/
/EPIDEMIOLOGY STUDIES/ /THYROID CANCER/ The health effects of exposure to
iodine-131 fallout from atmospheric nuclear tests conducted at the Nevada test
site in the 1950s have been studied for the last four decades. ... 2,500
children were examined and individual doses to the thyroid reconstructed.
Nineteen neoplasms, of which eight were malignant, were diagnosed. /Iodine-131
in fallout/
/EPIDEMIOLOGY STUDIES/ /Thyroid Cancer/ Only a few studies have evaluated the
effects of environmental exposure to radioactive iodine. In contrast to the
medical exposures..., which were due exclusively to iodine-131, environmental
exposures have generally contained mixtures of iodine-131, external radiation,
and short-lived radioiodines. Initial studies of thyroid disease incidence in
Utah school children exposed to fallout from atmospheric nuclear weapons testing
at the Nevada Test Site appeared to show no difference in thyroid disease
outcomes compared to children from unexposed areas. However, a follow-up study
reported a slight excess risk of thyroid neoplasms associated with radioiodine
exposure. Although positive dose-response trends were also noted for total
nodules and thyroid cancer (when analyzed separately), they were not
statistically significant. The study was limited by small numbers of exposed
individuals and a low incidence of thyroid neoplasms and by the fact that the
examiners were not blinded to exposure. In contrast, a follow-up study of 3,440
persons exposed as young children to atmospheric releases of primarily
iodine-131 from the Hanford Nuclear Site found no increased risk of thyroid
cancer associated with individual radiation dose to the thyroid. The explanation
for the apparent difference in results in the Utah study and the Hanford study
is not clear. One possibility is that the exposures were substantially different
in terms of the mix of radionuclides and the dose rate. Thyroid dose at Hanford
was due almost entirely to iodine-131, whereas in Utah there was greater
contribution form other radioiodines as well as external sources. Exposures in
Utah were also more concentrated and episodic than at Hanford, corresponding to
specific nuclear tests. This likely resulted in doses being delivered at
substantially higher dose rates. (although total dose among 3,545 study
participants for whom thyroid doses could be estimated [mean 98 mGy] was similar
to Hanford doses). A second possibility is that the Utah study's estimated
dose-response could have been biased in the direction of finding an association
because the collections of dietary consumption data took place after thyroid
disease classification was known for each participant. /Iodine-131 in fallout/
/EPIDEMIOLOGY STUDIES/ /Thyroid Cancer/... A follow up study of 3,440 persons
exposed as young children to atmospheric releases of primarily iodine-131 from
the Hanford Nuclear Site in eastern Washington State /was conducted/. No
increased risk of thyroid cancer was found associated with individual radiation
dose to the thyroid. /Iodine-131 fallout/
/EPIDEMIOLOGY STUDIES/ /THYROID/ Extensive evaluation of the population of the
Marshall Islands has shown an increase in benign and malignant thyroid nodules
in residents of the northern atolls of Rongelap and Utirik. In addition, a
retrospective cohort study of over 7,000 Marshall Islanders showed that the
prevalence of palpable thyroid nodularity (>/ =1.0 cm) decreased linearly
with increased distance from the Bikini test site. /Radioactive fallout
containing iodine/
/EPIDEMIOLOGY STUDIES/ Approximately 740,000 Ci (2.73x10+16 Bq) of iodine-131
were released to the atmosphere from the Hanford Nuclear Site in Washington
State from 1944 through 1957. ... The Hanford Thyroid Disease Study (HTDS) was
conducted to determine if thyroid disease is increased among persons exposed as
children to atmospheric releases of iodine-131 from Hanford. ... Retrospective
cohort study: Exposure could have occurred from December 1944 through 1957.
Follow-up occurred until the time of the HTDS examination (December
1992-September 1997). Participants' thyroid radiation doses from Hanford's
iodine-131 releases were estimated from interview data regarding residence and
dietary histories. ... The cohort included a sample of all births from 1940
through 1946 to mothers with usual residence in 1 of 7 counties in eastern
Washington State. ... Of 5,199 individuals identified, 4,350 were located alive
and 3,440 were evaluable; ie, had sufficient data for dose estimation and
received an HTDS evaluation for thyroid disease, including a thyroid ultrasound,
physical examination, and fine needle biopsy if required to evaluate thyroid
nodularity. ./The main outcome measures were/ thyroid cancer, benign thyroid
nodules, total neoplasia, any thyroid nodules, autoimmune thyroiditis, and
hypothyroidism. ... There was no evidence of a relationship between Hanford
radiation dose and the cumulative incidence of any of the outcomes. These
results remained unchanged after taking into account several factors that might
confound the relationship between radiation dose and the outcomes of interest.
/The authors concluded that/ these results do not support the hypothesis that
exposure during infancy and childhood to iodine-131 at the dose levels (median,
97 mGy; mean, 174 mGy) and exposure circumstances experienced by our study
participants increases the risk of the forms of thyroid disease evaluated in
this study. /Iodine-131/
/EPIDEMIOLOGY STUDIES/ 72,073 person-years of follow-up after iodine-131
treatment for hyperthyroidism /was found to produce/ significantly increased
incidence and mortality rates for cancer of the small bowel (SIR, 4.81; 95% CI,
2.16-10.7; six observed cancers; SMR, 7.03; 95% CI, 3.16-15.7; six fatal
cancers). /In/ ? 1,771 patients who had been treated for thyroid cancer (?846
had received iodine-131for therapy, 651 had received iodine-131 for diagnosis
and 274 had not received iodine-131.) ? 80 patients developed a second solid
malignancy, of which 13 were colorectal cancers, and the risk for this cancer in
11 cases was significantly related to the cumulative dose of iodine-131
administered ? /Iodine-131/
/EPIDEMIOLOGY STUDIES/ /Acute Radiation Syndrome and Thyroid Effects/ Marshall
Islands BRAVO Test. Shortly after the BRAVO test, residents on three of the
Marshall Islands were identified as having been exposed to external gamma
radiation during the 2 days prior to their evacuation : 64 residents of Rongelap
(1.90 Gy, 190 rad), 18 residents of Ailingnae (1.10 Gy, 110 rad) and 150
residents of Utrik (0.11 Gy, 11 rad) ? .Estimated total absorbed doses to the
thyroid gland (external and internal) were 3.3 to 20 Gy (330 to 2,000 rad) on
Rongelap (highest doses in children), 1.3 to 4.5 Gy (130 to 450 rad) on
Ailingnae, and 0.3 to 0.95 Gy (30-95 rad) on Utrik. As part of a medical
evaluation program, these individuals, the so-called BRAVO cohort, were
evaluated periodically for health consequences of their exposures. Evidence of
acute radiation sickness was prevalent early after exposures, including nausea
and vomiting, hematological suppression, and dermal radiation burns. Cases of
thyroid gland disorders began to be detected in the exposed population in 1964,
10 years after the exposure, particularly in exposed children; these included
cases of apparent growth retardation, myxedema, and thyroid gland neoplasms. In
1981, when the children from Rongelap island were screened, it was discovered
that 83% of the children who were <1 year of age at the time of the BRAVO
test were found to have evidence of hypothyroidism (i.e., a serum concentration
of TSH >5 mU/L). This group of children had received an estimated thyroid
dose exceeding 1,500 rad (15 Gy). Prevalence of hypothyroidism and thyroid
radiation dose decreased with exposure age: 25% for ages 2 to 10 years (800 to
1,500 rad, 8 to15 Gy) and 9% for ages > or = to 10 years (335 to 800 rad,
3.35 to 8.00 Gy). Prevalences in the exposed groups from Ailignae were 8% for
exposure ages >10 years (135 to 190 rad, 1.35 to 1.90 Gy) and 1% on Utrik (30
to 60 rad, 0.3 to 0.6 Gy). In an unexposed group (Rongelap residents who were
not on the island at the time of the BRAVO test), the prevalence was 0.3 to
0.4%. At about the same time, in 1964, cases of palpable thyroid gland nodules
began to be identified in health screening programs. The prevalence of thyroid
nodularity had an age/dose profile similar to that of thyroid hypofunction
(i.e., elevated serum TSH). In 1981, thyroid nodules were found in 77% Rongelap
residents exposed before the age of 10 years and in 13% of those exposed after
10 years. Prevalence in the Ailingnae populations was 29% in the population of
children exposed before age 10 years and 33% in the population exposed after age
10 years. In the Utrik population, the prevalence of thyroid nodules was 8% in
the population of children exposed before age 10 years and 12% in the population
exposed after age 10 years. The prevalence of thyroid gland carcinoma, mainly
papillary carcinomas, also appeared to be elevated in the exposed Rongelap
population (6%) compared to the unexposed group (1%). ... /Radioactive fallout
containing iodine/
/EPIDEMIOLOGY STUDIES/ /THYROID/ Subsequent to the release of radioactive
materials from the Chernobyl power plant in 1986, an increased prevalence of
thyroid nodules in children of the Belarus region was reported. An analysis of
the results of ultrasound screening of 20,785 people in Belarus conducted during
the period 1990-1995 revealed a prevalence of thyroid gland nodules that ranged
from 4 to 22 per 1,000. Prevalence was highest (16 to 22 per 1,000) among
residents from districts in which thyroid radiation doses were estimated to have
been above 1 Gy (1.3 to 1.6 Gy, 130 to 160 rad). Verified diagnoses from
patients who were referred for further examination as a result of ultrasound
results revealed a prevalence of thyroid cancer of 2.5 to 6.2 per 1,000, or
approximately 13 to 50% of nodule cases, among cases from districts where
thyroid radiation doses were estimated to have been above 1 Gy (1.3 to 1.6 Gy,
130 to160 rad). Adenoma was diagnosed in 7 to 12% of thyroid nodule cases,
nodular goiter was diagnosed in 5 to 22% of the thyroid nodule cases, and 7 to
64% of the nodule cases were diagnosed as benign cysts. ? The results of other
thyroid screening programs (e.g., the Chernobyl Sasakawa Health and Medical
Cooperation Project) also suggest a high prevalence of goiter among people born
in Belarus between the years 1976 and 1986, which would be consistent with a
high prevalence of iodine deficiency in the population. Therefore, iodine
deficiency may have contributed to the observed thyroid nodularity and also may
be a confounding variable in susceptibility to thyroid cancer. /Radioactive
iodine and thyroid deficiencies/
/EPIDEMIOLOGY STUDIES/ /Fertility/ Clinical cases of impaired testicular
function have been reported following oral exposures to iodine-13I for ablative
treatment of thyroid cancer. Effects observed included low sperm counts,
azospermia (absence of spermatozoa), and elevated serum concentrations of
follicle stimulating hormone (FSH), which persisted for more than 2 years of
follow-up. Exposures to radioiodine ranged from 50 to 540 mCi (1.8 to 20 GBq). A
study of 103 patients who received iodine-131 treatments for thyroid cancer
found low sperm counts and elevated serum FSH concentrations in some patients
when examined 10-243 months after treatment (mean, 94 months). Exposures to
radioiodine ranged from 30 to 1,335 mCi (1.1 to 49.4 GBq) with a mean exposure
of 167 mCi (6.2 GBq). /Iodine-131/
/EPIDEMIOLOGY STUDIES/ /THYROID CANCER/ The thyroid gland receives the highest
radiation dose of any organ or tissue following an internal exposure to
radioiodine and, therefore, cancer of the thyroid gland is the major health
concern associated with radioiodine exposures. Children, in particular, are
highly vulnerable to radioiodine toxicity. Cancer morbidity and mortality among
populations that received exposures to radioiodine have been examined in several
large-scale epidemiology studies. In general, these studies fall into several
categories that can be distinguished by the sources of exposure and estimated
radiation doses to the thyroid gland and include: (1) exposure to high doses (10
to 20 mCi, 370 to 740 MBq; >10,000 rad, >100 Gy) achieved when iodine-131
is administered to treat hyperthyroidism ...; (2) exposures to moderately high
doses (40 to 70 ?Ci, 1.5-2.6 MBq; 80 to 130 rad, cGy) associated with clinical
administration of iodine-131 for diagnosis of thyroid gland disorders; (3) low
doses from exposures to fallout from nuclear bomb tests (BRAVO test, 300 to
2,000 rad, cGy; Nevada Test Site, 1 to 40 rad, cGy); (4) low to high doses from
exposures to releases from nuclear power plant accidents (Chernobyl, 10 to 500
rad, cGy); and (5) low to high environmental exposures from operational releases
from nuclear fuel processing plants (Hanford Nuclear Site, 0.0001 to 284 rad,
cGy). As a point of reference, the dose-response relationship for thyroid cancer
and external radiation appears to extend down to thyroid doses of 0.1 Gy (10 rad)
and predicts an excess relative risk (ERR) of 7/Gy for ages <15 years at
exposure. Studies of thyroid cancers and external radiation exposure have found
a strong dependence of thyroid cancer risk on age at exposure ? and this
increased risk persists, possibly for the lifetime. This same general trend in
age-dependence would be expected for internal exposures to radioiodine? . Breast
cancer is also a concern with exposures to high levels of radioiodine after
ablative therapy for hyperthyroidism because breast expresses NIS and can
transport and accumulate iodide. However, the epidemiological literature to date
has not implicated such exposures as a significant risk factor for breast
cancer. /Radioactive iodine from medical tests and fallout/
/EPIDEMIOLOGY STUDIES/ THYROID/ Within nine years after the thermonuclear
explosion at Bikini in 1954, thyroid nodules were noted in children on Rongelap
atoll, who had received the highest dose. Of those aged < 10 years, 67%
developed nodules. The doses to the thyroid were estimated to be 10-43 Gy. Five
children who were exposed when below the age of five years showed growth
retardation, which was most prominent among children aged 1-1.5 at the time of
exposure. The incidence of subclinical hypothyroidism was 31% among children who
were < 10 years old at the time of exposure to estimated doses of > 2 Gy
from iodine-131. /Radioiodine fallout/
/EPIDEMIOLOGY STUDIES/ The aim of the study was to assess whether radioactive
fallout from the Chernobyl accident in 1986 influenced thyroid cancer incidence
among children and adolescents in Finland. The population was divided into two:
those with thyroid doses less than 0.6 mSv and above 0.6 mSv. Cumulative
incidence of thyroid cancer was identified from the Finnish Cancer Registry from
a population aged 0 to 20 years in 1986 with a total of 1,356,801 persons. No
clear difference in underlying thyroid cancer incidences rates were found during
the pre-Chernobyl period (1970-1985) (rate ratio RR 0.95, 95% confidence
interval CI 0.81-1.10). During the post-Chernobyl period (1991-2003), thyroid
cancer incidence was lower in the more exposed population than in the less
exposed population (RR 0.76, 95% CI 0.59-0.98). /The authors concluded that
their/ results did not indicate any increase in thyroid cancer incidence related
to exposure to radiation from the Chernobyl accident. /Radioiodine from fallout/
/EPIDEMIOLOGY STUDIES/ A cohort of 32,385 individuals younger than 18 years of
age and resident in the most heavily contaminated areas in Ukraine at the time
of the /Chernobyl/ accident was invited to be screened for any thyroid pathology
by ultrasound and palpation between 1998 and 2000; 13 127 individuals (44%) were
actually screened. Individual estimates of radiation dose to the thyroid were
available for all screenees based on radioactivity measurements made shortly
after the accident and on interview data. The excess relative risk per gray (Gy)
was estimated using individual doses and a linear excess relative risk model.
... Forty-five pathologically confirmed cases of thyroid cancer were found
during the 1998-2000 screening. Thyroid cancer showed a strong, monotonic, and
approximately linear relationship with individual thyroid dose estimate
(P<.001), yielding an estimated excess relative risk of 5.25 per Gy (95%
confidence interval (CI) = 1.70 to 27.5). Greater age at exposure was associated
with decreased risk of radiation-related thyroid cancer, although this
interaction effect was not statistically significant. ... Exposure to
radioactive iodine was strongly associated with increased risk of thyroid cancer
among those exposed as children and adolescents. In the absence of Chornobyl
radiation, 11.2 thyroid cancer cases would have been expected compared with the
45 observed, i.e., a reduction of 75% (95% CI = 50% to 93%). The study also
provides quantitative risk estimates minimally confounded by any screening
effects. Caution should be exercised in generalizing these results to any future
similar accidents because of the potential differences in the nature of the
radioactive iodines involved, the duration and temporal patterns of exposures,
and the susceptibility of the exposed population. /Radioiodine from fallout/
/BIOMONITORING/ /GENOTOXICITY/In this study, the sister chromatid exchange (SCE)
method was employed to investigate acute and late chromosomal damage (CD) in the
peripheral lymphocytes of 15 patients who received various doses of iodine-131
(259-3,700 MBq), either for thyrotoxicosis (TTX) or for ablation treatment in
differentiated thyroid cancer (DTC). The SCE frequencies in cultured peripheral
lymphocytes were determined before treatment (to assess basal SCE frequencies),
on the 3rd day (to assess acute SCE frequencies) and 6 months later (to assess
late SCE frequencies). The basal, acute and late SCE frequencies (mean+/-SD)
were 3.19+/-0.93, 10.83+/-1.72 and 5.75+/-2.06, respectively, in the whole
group, and these values differed significantly from each other ( P<0.001).
/Iodine-131/
/BIOMONITORING/ /GENOTOXICITY/After the Chernobyl accident a statistically
significant increase in the number of children with thyroid tumours was
observed. In this study 166 children with and 75 without thyroid tumours were
analysed for micronucleus formation in peripheral blood lymphocytes using the
cytochalasin B approach. The following factors did not significantly affect
micronucleus formation: gender, age at the time of the first iodine-131
treatment, tumour stage, tumor type, or metastases; a statistically significant
increase in the number of micronuclei, however, was observed for the residents
of Gomel compared to other locations, such as Brest, Grodno, and Minsk. The
children with tumors received iodine-131 treatment after surgical resection of
the tumors. This gave /the authors/ the opportunity to systematically follow the
effect of iodine-131 on micronucleus formation. A marked increase was observed 5
days after the iodine-131 treatment followed by a decrease within a 4-7 months
interval up to the next application, but the pre-treatment levels were not
achieved. Up to 10 therapy cycles were followed each including an analysis of
micronucleus formation before and 5 days after iodine-131 application. The
response of the children was characterized by clear individual differences and
the increase/decrease pattern of micronucleus frequencies induced by iodine-131
was correlated with a decrease/increase pattern in the number of
lymphocytes./Iodine-131/
/BIOMONITORING/ /GENOTOXICITY/ The presence of chromosomal aberrations in human
peripheral blood lymphocytes is a recognized indicator of exposure to radiation
in vivo, an increase in the frequency of chromosomal aberrations above the
background level reflecting direct exposure of circulating lymphocytes and/or
hematopoietic precursor cells in the bone marrow. ... Chromosomal aberrations
and gene mutations were ... observed in many studies in cells of people exposed
internally to specific radionuclides, including the beta-particle emitter ...
iodine-131 ... /Iodine-131/
/OTHER TOXICITY INFORMATION/ The Chernobyl reactor accident resulted in massive
releases of iodine-131 and other radioiodines. Beginning approximately 4 years
after the accident, a sharp increase in the incidence of thyroid cancer among
children and adolescents in Belarus and Ukraine (areas covered by the
radioactive plume) was observed. In some regions, for the first 4 years of this
striking increase, observed cases of thyroid cancer among children aged 0
through 4 years at the time of the accident exceeded expected number of cases by
30- to 60-fold. During the ensuing years, in the most heavily affected areas,
incidence is as much as 100-fold compared to pre-Chernobyl rates ... . The
majority of cases occurred in children who apparently received less than 30
centaGy to the thyroid ... .. A few cases occurred in children exposed to
estimated doses of < 1 cGy; however, the uncertainty of these estimates
confounded by medical radiation exposures leaves doubt as to the causal role of
these doses of radioiodine ... .. The evidence, though indirect, that the
increased incidence of thyroid cancer observed among persons exposed during
childhood in the most heavily contaminated regions in Belarus, Ukraine, and the
Russian Federation is related to exposure to iodine isotopes is, nevertheless,
very strong... .. /The FDA/ concluded that the best dose-response information
from Chernobyl shows a marked increase in risk of thyroid cancer in children
with exposures of 5 cGy or greater ... .. Among children born more than nine
months after the accident in areas traversed by the radioactive plume, the
incidence of thyroid cancer has not exceeded preaccident rates, consistent with
the short half-life of iodine-131 /Iodine-131 from fallout/.
/OTHER TOXICITY INFORMATION/ THYROID/ The thyroid gland in adults is considered
to be radioresistant in terms of cell death and failure of function. It has the
capacity to actively concentrate iodine? . Radioiodine can, therefore, deliver
considerable doses to the gland. ? A dose of at least 300 Gy is required to
cause total ablation of the thyroid within a period of two weeks. This can be
achieved with a single oral dose of 1,850-3,700 MBq of iodine-131, resulting in
an uptake of about 37 MBq by the thyroid. /Iodine-131/
/OTHER TOXICITY INFORMATION/ THYROID/ Also, thyroid doses were estimated for the
208 emergency workers admitted to Hospital 6 in Moscow within 3 to 4 weeks after
the /Chernobyl/ accident; most of the thyroid doses were less than 1 Gy, but
three exceed 20 Gy. It is interesting to note that the measurements of
iodine-131 and iodine-133 among the five emergency workers with the highest
thyroid doses showed that iodine-133 contributed less than 20% to the thyroid
doses. The specific values of the contributions from iodine-133 were 18% (with
74% from iodine-131), 11% (81% from iodine-131), 6% (86% from iodine-131), 10%
(82% from iodine-131) and 14% (78% from iodine-131) for the five workers.
/Iodine-131 and -133/
/OTHER TOXICITY INFORMATION/ THYROID/ Radioactive fall-out from a thermonuclear
explosion at Bikini in the Pacific Ocean was deposited on the Marshall Islands
in 1954. Inhalation and ingestion of radioactive iodine (mainly iodine-131,
iodine-132, iodine-133, iodine-134) by the population resulted in significant
internal exposure. Twenty-five years later, the population on the nearby atoll
of Rongelap still showed a significantly impaired thyroid reserve, while at
least four of 43 persons suffered from thyroid malfunction. Three of these were
estimated to have received doses < 3.5 Gy. /Iodine radioisotopes/
/OTHER TOXICITY INFORMATION/ /Thyroid Cancer/ Radioiodine not only locally
irradiates the thyroid gland but also becomes associated with thyroid hormones,
thus influencing other organs of the body. Thyroid cancers can be differentiated
(papillary, follicular and medullary) or undifferentiated (anaplastic
carcinoma). The thyroid cancer known to be caused by ionizing radiation is
papillary carcinoma, as shown among the atomic bomb survivors and recently in
the Chernobyl area. In a study of 577 Ukrainian patients less than 19 years of
age in whom thyroid cancer was diagnosed, 290 cases were evaluated
histopathologically and > 90% were found to be papillary carcinomas. Similar
frequencies were seen in a study in the USA of 4,296 patients previously
irradiated for benign disorders: thyroid cancers were found in 41 children (mean
age at diagnosis, 16 years), of which 95% were papillary carcinomas. Thyroid
nodules have also been related to exposure to radioiodine. /Radioiodine/
/OTHER TOXICITY INFORMATION/ /Thyroid Cancer/ Among survivors of the atomic
bombings, the most pronounced risk for thyroid cancer was found among those with
a dose to the thyroid of > 1 Sv before the age of 10 years, and the highest
risk was seen 15-29 years after exposure; the risk subsequently began to
decline, but it was still elevated 40 years after exposure. ? a pooled analyses
of seven cohorts of individuals exposed to ionizing radiation ? found an excess
relative risk (ERR) at 1 Gy of 7.7 (95% CI, 2.1-28.7) for persons exposed in
childhood. They also reported that the ERR decreased by a factor of about 2 for
each successive five-year interval of age at exposure over the range 0-14 years
of age. /Radioactive fallout/
/OTHER TOXICITY INFORMATION/ /Thyroid Cancer/ Thyroid cancers can be classified
into differentiated thyroid cancers (papillary, follicular and medullary) and
non-differentiated tumors (anaplastic carcinoma). Papillary carcinoma is the
thyroid cancer known to be caused by ionizing radiation, as shown among the
atomic bomb survivors and recently in the Chernobyl area. ? The carcinogenic
effect of iodine-131 is less well understood than that of external photon
radiation. Before the Chernobyl accident, the effects of radioiodine in children
had not been studied to any extent, since children are rarely examined medically
or treated. The childhood thyroid gland, red bone marrow and premenopausal
female breast are the most radiosensitive organs in the body. Although thyroid
carcinomas are known to be more aggressive in children, their prognosis is
better than that of adults. /Iodine-131and ionizing radiation/
/OTHER TOXICITY INFORMATION/ /Parathyroid Gland/ Cases of hypo- and
hyperparathyroidism are rare outcomes in patients who receive iodine-131
treatments for ablative therapy of thyroid cancer or hyperthyroidism. The
parathyroid gland is in close proximity to the thyroid gland. Although in most
people, the parathyroid and thyroid glands are separated by more than 1 cm, in
approximately 20% of people, the parathyroid gland is located within the thyroid
gland capsule. The latter configuration would result in irradiation of the
parathyroid gland with beta emission from iodine-131 ? . Cases of parathyroid
dysfunction have been reported after exposures to iodine-131 ranging from 4 to
30 mCi (0.15 to 1.1 GBq). /Iodine-131/
Populations at Special Risk:
Results from studies of atomic bomb survivors and persons exposed to external
irradiation have shown that exposure at the youngest ages is associated with the
greatest risk of thyroid cancer. The available data on exposure from the
Chernobyl accident are largely in agreement with this observation. For example,
a recent paper found the highest incidence of thyroid cancer among those exposed
at ages 0-4, who also had the highest doses.
It has also been postulated that the risk of thyroid cancer may be especially
high among persons exposed in utero, as developing fetal thyroid tissue may be
highly susceptible to thyroid cancer induction by iodine-131 exposure.
/Iodine-131/
Iodine deficiency may also be an important modifier of the risk of
radiation-induced thyroid cancer. Some regions contaminated by the Chernobyl
accident are areas of mild to moderate iodine deficiency. To date, only two
published studies have investigated the relationship between iodine deficiency,
radiation dose and the risk of thyroid cancer in young people. In a study
carried out in the Bryansk region of Russia, ...a significantly increased risk
of thyroid cancer with increasing radiation dose from Chernobyl that was
inversely associated with urinary iodine excretion levels /was reported/. At one
Gy, the excess relative risk in territories with severe iodine deficiency was
approximately two times that in areas of normal iodine intake, thereby
suggesting that iodine deficiency may enhance the risk of thyroid cancer
following radiation exposure. .../Researchers/ also investigated the effects of
iodine deficiency and its interaction with radiation exposure in the risk of
thyroid cancer. Subjects who resided in the areas of lowest soil iodine content
had a 3.1 times (95% confidence interval 1.7, 5.4) higher risk at one Gy than
subjects residing in areas of higher soil iodine content. It is noted that
administration of potassium iodide as a dietary supplement significantly reduced
the risk of radiation induced thyroid cancer. /Iodine deficiency/
Probable Routes of Human Exposure:
Following the Chenobyl accident in April, 1986, radioiodines were one of the
main sources of fallout exposure beyond the 30-km zone however the 131- and 133-
nuclides are short-lived and had disappeared before measurements could be
made(1). Iodine-129, while produced in copious amounts, delivers only a minor
dose to the thyroid owing to its 17X10+06 year half-life. Iodine deposited in
fallout does remain in the top few centimeters of soil for decades which could
be used in future in reconstructing doses received(1). The principal route of
human absorption to iodine-131 is via fresh milk by the grass, cow, milk food
chain, with rapid distribution milk being a primary consideration(1). Direct
exposure to iodine-131 occurs to those being treated for hyperthyroidism(2).
Medical personnel that are exposed to patients receiving iodine-131 therapy for
thyroid oblation or to treat thyroid carcinoma or thyrotoxicosis receive
radiation doses of 1.43X10-4 rem/hour per MBq of iodine-131 in the patient at a
distance of 0.1 meters from the patient. At increasing distances from the
patient the dose decreases to 0.18X10-4 and 0.07X10-4 rem/hour per MBq at 0.5
meters and 1.0 meters, respectively. Typical thyroid burdens of iodine-131 in
medical personnel range from 35-18,131 pCi with an average of 2,400 pCi.
Radiochemists and their support staff can have activities of 0.03-200 nCi per
year from exposures to iodine-131. Workers at a nuclear fuel reprocessing
facility were found to receive doses, via inhalation, as high as 47.4-68.1 rem
to the thyroid and 0.024-0.047 rem whole body. Laboratory personnel that work
directly with iodine-125 were found to have thyroid burdens of iodine-125 at
levels ranging between 9-560 nCi(1).
Toxicity Summary:
There are 36 isotopes of iodine having masses between 108 and 143. Only one
isotope is stable (iodine-127); the remaining are radioactive. Most of these
have radioactive half-lives of minutes or less. Twelve have half-lives that
exceed 1 hour, and six have half-lives that exceed 12 hours (iodine-123,
iodine-124, iodine-125, iodine-126, iodine-129, and iodine-131). Four isotopes
(iodine-123, iodine-125, iodine-129, and iodine-131) are of particular interest
with respect to human exposures because iodine-123 and iodine-131 are used
medically and all four are sufficiently long-lived to be transported to human
receptors after their release into the environment. The U.S. population has been
exposed to radioiodine in the general environment as a result of atmospheric
fallout of radioiodine released from uncontained and/or uncontrolled nuclear
reactions. Historically, this has resulted from surface or atmospheric
detonation of nuclear bombs, from routine and accidental releases from nuclear
power plants and nuclear fuel reprocessing facilities, and from hospitals and
medical research facilities. Estimates have been made of radiation doses to the
U.S. population attributable to nuclear bomb tests conducted during the 1950s
and 1960s at the Nevada Test Site; however, dose estimates for global fallout
have not been completed. Geographic-specific geometric mean lifetime doses are
estimated to have ranged from 0.19 to 43 cGy (rad) for a hypothetical individual
born on January 1, 1952 who consumed milk only from commercial retail sources,
0.7-55 cGy (rad) for people who consumed milk only from home-reared cows, and
6.4-330 cGy (rad) for people who consumed milk only from home-reared goats.
Additional information is available on global doses from nuclear bomb tests and
doses from nuclear fuel processing and medical uses can be found in United
Nations Scientific Committee on the Effects of Atomic Radiations. Individuals in
the United States can also be exposed to radioiodine, primarily iodine-123 and
iodine-131, as a result of clinical procedures in which radioiodine compounds
are administered to detect abnormalities of the thyroid gland or to destroy the
thyroid gland to treat thyrotoxicosis or thyroid gland tumors. Diagnostic uses
of radioiodine typically involve administration, by the oral or intravenous
routes, of 0.1-0.4 mCi (4-15 MBq) of iodine-123 or 0.005-0.01 mCi (0.2-0.4 MBq)
of iodine-131. These correspond to approximate thyroid radiation doses of 1-5
rad (cGy) and 6-13 rad (cGy) for iodine-123 and iodine-131, respectively.
Cytotoxic doses of iodine-131 are delivered for ablative treatment of
hyperthyroidism or thyrotoxicosis; administered activities typically range from
10 to 30 mCi (370-1,110 MBq). Higher activities are administered if complete
ablation of the thyroid is the objective; this usually requires 100-250 mCi
(3,700-9,250 MBq). Thyroid gland doses of approximately 10,000-30,000 rad (300
Gy) can completely ablate the thyroid gland. An administered activity of 5-15
mCi (185-555 MBq) yields a radiation dose to the thyroid gland of approximately
5,000-10,000 rad (50-100 Gy). The health effects of exposure to radioiodine
derive from the emission of beta and gamma radiation. Radioiodine that is
absorbed into the body quickly distributes to the thyroid gland and, as a
result, the tissues that receive the highest radiation doses are the thyroid
gland and surrounding tissues (e.g., parathyroid gland). Tissues other than the
thyroid gland can accumulate radioiodine, including salivary glands, gastric
mucosa, choroid plexus, mammary glands, placenta, and sweat glands. Although
these tissues may also receive a radiation dose from internal radioiodine, the
thyroid gland receives a far higher radiation dose. The radiation dose to the
thyroid gland from absorbed radioiodine varies with the radiation emission
properties of the isotope (e.g., type of radiation, energy of emission,
effective radioactive half-life). The highest total doses are achieved with
iodine-131 which has an effective half-life in the thyroid gland of 177 hours.
Radioiodine is cytotoxic to the thyroid gland at high radiation doses and
produces hypothyroidism when doses to the thyroid gland exceed 25 Gy (2,500 rad).
Thyroid gland doses of approximately 300 Gy (30,000 rad) can completely destroy
the thyroid gland. This dose can be achieved with an acute exposure to
approximately 25-250 mCi (0.9-9 GBq) of iodine-131. Although, a rare outcome,
cytotoxic doses of iodine-131 can also produce dysfunction of the parathyroid
gland, which can receive a radiation dose from emission of iodine-