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Determine Who’s At Risk | Clinical Evaluation |
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Treatment and Management
 Determine Who’s At Risk
Incidence of thyroid cancer depends on
many factors,
including thyroid dose and age at the time of exposure.
Epidemiologic studies on thyroid cancer and I-131 at
Chernobyl confirmed that the risk for thyroid cancer is
dependent on the absorbed dose, the age and location of
persons at time of exposure, and the absence of immediate
iodine prophylaxes for I-131 exposure.
A strong relationship exists among the incidences of
thyroid neoplasia, hypothyroidism, and autoimmune
thyroiditis and the received dose. In the most contaminated
area after the nuclear release at Chernobyl, thyroid cancer
incidence was significantly higher compared with other
regions. There are no earlier studies comparable to those
for Chernobyl because no studies were conducted around U.S.
weapon production facilities when I-131 was released. The
Hanford Thyroid Disease Study, released in June 2002, found
no statistically significant association between I-131 dose
to the thyroid from radiation released at the Hanford
Nuclear Reservation and any of twelve different thyroid
disease outcomes.
Children are the most sensitive group for
exposure to I-131.
Age is a factor for exposure to I-131 because of the
differences between thyroid doses for children and adults.
The dose to children is much higher than that to adults
because the thyroid mass in children is smaller, and because
milk, as the main route of contamination, is consumed in
higher quantities during childhood. For an equivalent uptake
of I-131, a child’s thyroid receives a higher radiation dose
because the same amount of energy is deposited in a smaller
tissue mass (more energy per gram = higher dose). For
newborns, the thyroid dose is about 16 times higher than
that for adults for the same ingested radioactivity;
similarly, the absorbed dose is about 8 times higher for
children under 1 year old and 4 times higher for children 5
years old.
Special considerations exist for pregnant
and nursing mothers.
During pregnancy, the maternal thyroid has an increased
ratio of I-131 uptake, especially during the first
trimester. I-131 crosses the placental barrier. During the
second and third trimesters, the fetal thyroid takes up and
stores iodine in increasing amounts. During the first
postpartum week, thyroid activity increases up to fourfold.
This critical period lasts for a couple of days. Infants and
children are at high risk from radioiodine exposure because
their thyroids are small. This risk decreases as children
age, although it continues until they are about 20 years
old. About one-quarter of the iodine ingested by the mother
is secreted in breast milk, which adds an additional risk
factor for the breast-feeding infant.
Drinking fresh versus pasteurized milk
leads to a higher dose of I-131.
The geographic distribution of persons exposed to I-131
is important for three reasons. First, the risk is higher
for those in rural areas because fresh milk is often
consumed. This is important when the milk is produced from a
contaminated pasture. The delay between the production and
consumption of milk contributes to decreasing radioactivity
for urban populations. Second, the risk is higher for
populations with endemic deficiency of iodine. I-131
absorption is higher for these populations. Third, different
types of milk and dairy products are consumed in some rural
areas. In goat’s milk and sheep’s milk, I-131 concentrations
are up to 10 times higher than in cow’s milk for the same
concentration of I-131 in the pasture.
Thyroid Evaluation of
Patients Who are Concerned About Hanford Iodine-131
Releases: A Guide for Physicians
Individual Dose and
Risk Calculator for Nevada Test Site Fallout (National
Cancer Institute)
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