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FEDERAL DRUG ADMINISTRATION REPORT
Thyroid Cancers in the Aftermath
of Chernobyl
The Chernobyl reactor accident resulted in massive releases of 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 cGy 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.
We have 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 131 I.
Safety
The use of KI in Poland after the Chernobyl accident provides us
with useful information regarding its safety and tolerability in
the general population. Approximately 10.5 million children under
age 16 and 7 million adults received at least one dose of KI. Of
note, among newborns receiving single doses of 15 mg KI, 0.37 percent
(12 of 3214) showed transient increases in TSH (thyroid stimulating
hormone) and decreases in FT4 (free thyroxine). The side effects
among adults and children were generally mild and not clinically
significant. Side effects included gastrointestinal distress, which
was reported more frequently in children (up to 2 percent, felt
to be due to bad taste of SSKI solution) and rash (~1 percent in
children and adults). Two allergic reactions were observed in adults
with known iodine sensitivity.
Use of KI in Radiation Emergencies:
Rationale, Effectiveness, Safety
For the reasons discussed above,
the Chernobyl data provide the most reliable information available to date on the relationship
between internal thyroid radioactive dose and cancer risk. They suggest that the risk of
thyroid cancer is inversely related to age, and that, especially in young children, it may
accrue at very low levels of radioiodine exposure. We have relied on the Chernobyl data
to formulate our specific recommendations below.
The effectiveness
of KI as a specific blocker of thyroid radioiodine uptake is well
established as are the doses necessary for blocking uptake. As such,
it is reasonable to conclude that KI will likewise be effective
in reducing the risk of thyroid cancer in individuals or populations
at risk for inhalation or ingestion of radioiodines.
Short-term administration
of KI at thyroid blocking doses is safe and, in general, more so
in children than adults. The risks of stable iodine administration
include sialadenitis (an inflammation of the salivary gland, of
which no cases were reported in Poland among users after the Chernobyl
accident), gastrointestinal disturbances, allergic reactions and
minor rashes. In addition, persons with known iodine sensitivity
should avoid KI, as should individuals with dermatitis herpetiformis
and hypocomplementemic vasculitis, extremely rare conditions associated
with an increased risk of iodine hypersensitivity.
Thyroidal side
effects of stable iodine include iodine-induced thyrotoxicosis,
which is more common in older people and in iodine deficient areas
but usually requires repeated doses of stable iodine. In addition,
iodide goiter and hypothyroidism are potential side effects more
common in iodine sufficient areas, but they require chronic high
doses of stable iodine. In light of the preceding, individuals with
multinodular goiter, Graves' disease, and autoimmune thyroiditis
should be treated with caution, especially if dosing extends beyond
a few days. The vast majority of such individuals will be adults.
The transient
hypothyroidism observed in 0.37 percent (12 of 3214) of neonates
treated with KI in Poland after Chernobyl has been without reported
sequelae to date. There is no question that the benefits of KI treatment
to reduce the risk of thyroid cancer outweigh the risks of such
treatment in neonates. Nevertheless, in light of the potential consequences
of even transient hypothyroidism for intellectual development, we
recommend that neonates (within the first month of life) treated
with KI be monitored for this effect by measurement of TSH (and
FT4, if indicated) and that thyroid hormone therapy be instituted
in cases in which hypothyroidism develops.
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FDA: Says You Need Potassium Iodide
FDA: Nuclear Radiation Bombs
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