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HTDS Guide > Findings and
Interpretations > How the HTDS Findings Compare to Other Radiation
Studies
HTDS Guide
| Findings and Interpretations |
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How the HTDS Findings Compare to Other Radiation Studies
No other radiation studies are directly comparable to the Hanford
situation, and drawing comparisons with other studies is difficult
because many factors affect thyroid health. Factors include the type of
radiation (internal or external), the doses received, the rate at which
the doses were received, how much non-radioactive iodine a person gets
in his or her diet, a person's age at the time of exposure, how much
time has passed since the exposure and the methods researchers used to
search for the disease.
Nevertheless, much can be learned from other radiation studies. For
example, scientists agree that high doses of external radiation, as well
as high doses of internal radiation received in short periods of time,
increase the risk of certain thyroid diseases. There is also evidence
that the risk from radiation is greater for people exposed at younger
ages.
Much less is known about situations such as Hanford, where
populations are exposed to lower levels of internal radiation from
iodine-131 over prolonged periods of time.
The following is a review of other important radiation studies (refer
to the chart below).
External Exposures
Studies of survivors of the atomic bombings of Hiroshima and
Nagasaki during World War II show that people exposed to high
levels of external gamma radiation face an increased risk of thyroid
cancer. People whose thyroid glands were exposed in childhood to high
levels of external radiation from x-ray treatments of the head
and neck are also at increased risk. (Medical uses of x-ray treatments
of children have changed since the 1960s.)
What distinguishes these situations from Hanford is that the
exposures were external and not from iodine-131, the radiation doses
were much higher and the doses were delivered in very short periods of
time.
Mix of Internal and External Exposures
Studies show that children and adults exposed to internal and
external radiation from nuclear testing in the Marshall Islands
(largest test in 1954) have an increased risk of thyroid nodules. Other
studies show a large increase in thyroid cancer among children exposed
to iodine-131 and other radiation from Chernobyl in 1986.
Neither situation is directly comparable to Hanford because the
radiation received in both instances was a mixture of internal and
external exposures, and the doses were higher and delivered over shorter
periods of time. In the case of Chernobyl, iodine deficiency in the
population may also be a factor in the rate of thyroid disease.
A study of people exposed as children to fallout from nuclear testing
in the 1950s at the Nevada Test Site is called the Utah Study.
This study found an increased risk of thyroid neoplasia, which is a
category of disease that the researchers defined to include both thyroid
cancer and certain kinds of benign nodules. However, there was no clear
evidence that the risk of thyroid cancer alone was increased.
The study is considered the most comparable to the Hanford situation,
but there are important differences. While exposure was largely from
iodine-131, it also likely included short-lived radioactive iodines as
well as external radiation. In addition, the Utah exposures were
concentrated over shorter time periods.
Internal Exposures
There have been epidemiological studies of the effect of iodine-131
alone. However, these have all been based on people who received
iodine-131 to treat thyroid disease, or to diagnose suspected
thyroid disease. People exposed to radioactive iodine for medical
reasons (primarily for the treatment of Graves disease) generally
received very high doses. While several studies found an increased risk
of thyroid cancer, the researchers concluded this was likely due to the
patients' underlying thyroid diseases, not their iodine-131 exposures.
People exposed to radioactive iodine for diagnostic purposes received
much lower doses. Studies have not found an increased risk of thyroid
cancer associated with diagnostic exposures.
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1. X-ray Treatment of Children |
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Exposure to x-ray treatment. |
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Exposure lasted seconds to minutes. |
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Studied exposure in childhood. |
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Exposure resulted in increased risk of thyroid
cancer. |
2. Hiroshima and Nagasaki |
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Exposure primarily to gamma rays; some neutron
exposure. |
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Exposure lasted seconds. |
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Studied exposure at all ages. |
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Exposure resulted in increased risk of thyroid
cancer, with greatest risk in children; limited evidence of an
increased risk of autoimmune thyroiditis. |
3. Marshall Islands |
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Exposure to iodine-131 plus other internal and
external radionuclides. |
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Exposure lasted weeks. |
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Studied exposure at all ages. |
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Exposure resulted in increased risk of thyroid
nodules and hypothyroidism, primarily in children. |
4. Chernobyl |
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Exposure to iodine-131 plus other internal and
external radionuclides. |
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Exposure lasted days to weeks. |
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Studied exposure in childhood. |
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Exposure resulted in large increased risk of
thyroid cancer in children. Iodine deficiency in the diet may also
be a contributing factor. |
5. Nevada Test Site |
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Exposure to iodine-131 plus other internal and
external radionuclides. |
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Exposure lasted weeks to months. |
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Studied exposure in children. |
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Exposure resulted in increased risk of thyroid
nodules; some indication of an increased risk of thyroid cancer. |
6. Medical Uses of Iodine-131 for Treatment |
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Exposure to iodine-131. |
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Exposure lasted days to weeks. |
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Studied exposure at all ages, primarily adults. |
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Exposure resulted in increased risk of
hypothyroidism (intent of the treatment); no clear evidence of an
increased risk of thyroid cancer from iodine-131 exposure. |
7. Medical Uses of Iodine-131 for Diagnosis |
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Exposure to iodine-131. |
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Exposure lasted days to weeks. |
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Studied exposure at all ages, primarily adults. |
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Exposure not associated with an increased risk
of thyroid cancer. |
8. Hanford |
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Exposure almost entirely to iodine-131. |
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Exposure lasted months to years. |
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Studied exposure in children. |
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Exposure not associated with an increased risk
of thyroid disease. |
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