Physiologic
Effects
- Thyroid nodules,
neoplasia, hypothyroidism, and autoimmune thyroiditis with or
without hypothyroidism are the main effects of internal exposure
to I-131.
- Benign neoplasms
are more common than malignant neoplasms.
Persons exposed to releases
of I-131 involving melted fuel at nuclear power plants, from production
of nuclear weapons, and from fallout from aboveground detonation
of atomic bombs have a higher risk for developing thyroid cancer
or thyroid disease, or both, than do unexposed populations. In particular,
persons exposed during childhood received higher doses, which in
many cases were repetitive over time. These persons were more vulnerable
than were those exposed as adults.
Radiation causes health
effects when either enough cells are killed quickly enough to disrupt
tissue function (acute health effects) or damaged cells are incompletely
repaired but still viable (carcinogenic, tumorigenic). I-131 radiation
might affect cells in the thyroid gland, leading to hypothyroidism
or thyroiditis, or might cause benign or malignant thyroid tumors
and nodules. The thyroid gland has one of the lowest cell-proliferation
rates of body tissues, and its regenerating ability is also low.
Thyroid
Tumors
Even in the absence
of exposure to I-131, thyroid tumors are the most common endocrine
neoplasms. Thyroid tumors are usually nodules localized to the thyroid
gland, and are often palpable on examination of the anterior neck.
I-131 exposure increases the risk of thyroid nodules and cancer.
Thyroid cancer is rare. The mean rate of spontaneous thyroid cancer
is one in 1 million for children (10 in 1 million for adults), with
a female-to-male ratio of 3 to 2. The increased risk for thyroid
cancer is especially important for exposures to I-131 during childhood.
The incidence of thyroid nodules increases with age. However, thyroid
cancer in children often presents at a more advanced stage than
in adults: more distant metastases, more lymph node involvement.
The risk of recurrence is higher in children, but the death rate
(at least over 20 years) is much lower in children than in adults.
- 5%-10% of palpable
nodules are thyroid cancer; the remainder are benign thyroid nodules.
When identifying those
nodules that are likely to be malignant, a careful history is crucial.
The history should include past medical history, occupational history,
environmental (exposure) history, family history including dietary
exposure, and social history. Of particular importance is a history
of external radiation exposure to the head, neck, or upper mediastinum
in infancy or childhood.
More information
on ionizing radiation, taking exposure histories, pediatric
environmental health, and reproductive and developmental hazards
is readily available in other ATSDR case studies (ATSDR 1993a,
1993b, 2001, 2002). |
Exposure of the thyroid
gland to moderate to high doses (from 6.5 to 2,000 centigray) of
I-131 linearly increases the risk for thyroid cancer. Nodular disease
occurs in about 20% of these patients, but it might not be apparent
until 30 or more years after the initial exposure. The risk for
thyroid neoplasm has been correlated directly with younger age at
radiation exposure, radiation dose, and sex. (A male patient with
a nodule should be regarded with greater suspicion because more
women have thyroid cancer-by a ratio of 2:1-and women have more
thyroid disease-by a ratio of about 8:1.) Thyroid nodules in children
and elderly patients are more likely to be malignant.
Excesses of thyroid
nodules and cancer (including 1,800 cases of thyroid cancer) reportedly
occurred from the Chernobyl nuclear power plant release. These results
might have reached higher levels than expected due to low dietary
intake of iodine in the region and high endemic rates of goiter.
Further research
related to iodine-131 would be useful in several areas including
the risk posed by low levels of exposure, possible differences in
radiation-related and naturally occurring thyroid cancers....
(Committee on Thyroid Screening 1999). The Hanford Thyroid Disease
Study found no dose-response relationship. An individual's increased
risk, however, cannot be ruled out. The Hanford thyroid dose estimates
were based on a dose reconstruction model using historical records
and assumptions that result in uncertainty of doses.
Thyroid
Cancer After X-Ray Exposure
Several cohorts of pediatric
patients irradiated in head and neck area for thymus hypertrophy,
tinea capitis, and chronic tonsillitis have been studied. These
studies suggested three findings. First, the thyroid of children
is more sensitive to carcinogenesis than is the thyroid of adults.
Second, the delay between the external irradiation and the appearance
of the cancer is at least 10 years (average delay 20 years). Third,
the dose-response curve is linear for persons exposed before 15
years of age (even down to 0.1 Gy [10 rad]). The excess relative
risk per unit of exposure (ERR/Gy) for childhood exposure is 7.7
(95% confidence interval, 2.1-28.7).
Reproductive
and Developmental Effects From Therapeutic Uses of I-131
I-131 has become a standard
treatment for thyroid ablation in persons with hyperthyroidism or
thyroid cancer. At least seven case reports and small case series
on adverse reproductive outcomes of medical I-131 use have been
published; however, sufficient information was not provided to determine
whether the fetus was exposed to I-131 radiation and, if so, to
what extent. The authors concluded that although the abnormalities
found could not be directly attributed to the therapy, it is prudent
to avoid pregnancy for 1 year after radiation treatment.
Other
Effects
- Increased risk for
thyroid neoplasm remains elevated for at least
40 years after exposure.
Thyroid exposure to
either internal or external radiation might trigger an immune response.
Changes in thyroid autoimmunity after I-131 therapy have been attributed
to the production and release of autoantigens as a result of radiation
damage.
Acute/Recent
Exposure
Acute radiation thyroiditis
occurs within 2 weeks after high exposure to I-131 and is characterized
by local pain and tenderness over the gland. Occasionally, significant
systemic symptoms have been associated with a massive release of
stored thyroid hormone. This syndrome can require treatment with
anti-inflammatory agents and beta-adrenergic antagonist agents.
Clinically significant acute radiation thyroiditis is unlikely to
occur at thyroid I-131 doses below 20,000 rad. Radioactive iodine
can accumulate during pregnancy in the fetal thyroid and cause its
permanent ablation. Because of the risk for fetal exposure to I-131,
women of childbearing age must take a pregnancy test before undergoing
medical radioiodine treatment.
Psychosocial
Effects
- Elevated levels of
psychologic stress, which can lead to increased risk of depression,
anxiety, and posttraumatic stress disorders in some people, can
occur after incidents involving nuclear releases.
- Most people want
to receive information as quickly as possible after notification
of a nuclear or chemical release. They want public health officials
or their primary care providers to give them advice about potential
health risks and what actions to take to prevent serious consequences
of their exposure. Timely and correct information is key to preventing
stress and relieving its psychosocial effects.
A nuclear release or
known past exposure from any type of radiation can lead to increased
psychologic stress because of the invisible nature of the event
and concern for serious health-related effects from a radiologic
contaminant. Results from studies in communities affected by previous
nuclear releases, such as Three Mile Island (TMI), showed that area
residents experienced long-term elevations of stress with increases
in community rates of subclinical depression, anxiety, demoralization,
and a heightened perception of risk. Indeed, the high levels of
psychosocial stress in communities affected by the TMI release remained
elevated for 6 years after the release and did not return to normal
until 10 years after the incident (Baum et al. 1983).
- Mothers are especially
susceptible to psychological effects because of their concern
about the effects of radiation on their children's health. Pregnant
women have added worry about risks to their unborn children.
Health care providers
may hear concerns from people who might have been exposed to radioactive
releases from nuclear tests or facilities. It is common for these
people to talk about uncertainties about their health concerning
previous or present exposure and the effect on their health. Patients'
health concerns may center around whether they will get ill and-if
so-when, what caused the illness, whether the illness can be diagnosed
properly, and what its prognosis, treatment, and financial impact
will be. Because of fear of cancer and the uncertainty of when or
whether it might occur, patients can have emotional stress, risk
for developing anxiety, and depression (Vyner 1988).
Social consequences
to exposure to radiation can also occur. Many of the men and women
exposed in Nagasaki and Hiroshima during World War II were perceived
as "damaged" and shunned for marriage because of the potential
for "damaged genes."
Challenge
questions
5. What would you
include in your patient's exposure history?
6. What is the procedure of choice to study a palpable nodule
of the thyroid gland?
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