|
Answers
to Pretest and Challenge Questions
Pretest
(a) The thyroid gland
is the critical organ for I-131 exposure. Essentially all of the
iodine entering the body quickly becomes systemic (EPA 1988), with
approximately 30% depositing in the thyroid.
(b) The main route of
human internal I-131 exposure of humans is ingestion of contaminated
fresh dairy products, eggs, and leafy vegetables, depending on downwind
distance from the release; the major source of internal exposure
is milk consumption. Goat's and sheep's milk contain approximately
10 times the concentration of radioiodine found in cow's milk.
(c) I-131 in large amounts
can produce thyroiditis. Hypothyroidism and thyroid cancer can result
from smaller exposures of I-131 and its accumulation in the thyroid
gland.
(d) Infants and children
are the groups most sensitive to I-131 exposure. The dose to children
is much higher than for adults because the thyroid mass in children
is smaller than that for adults; the first week of life is an especially
vulnerable period. Populations in the former Soviet Union exposed
to much larger radiation doses (especially those affected by the
Chernobyl nuclear release) showed an increased incidence of thyroid
cancer for children younger than 15 years old at the time of the
release.
Challenge
1. The main sources
of I-131 in the environment have been from nuclear power plant releases
and nuclear weapons production and testing. The main sources in
the United States have been the Nevada test site and the Hanford
Nuclear Reservation.
2. Dietary intake of
iodine before exposure is important because a relative iodine deficiency
increases the thyroid uptake of I-131. After exposure, the most
critical dietary information needed is the amount and type of milk
and milk products consumed, their I-131 concentrations, and the
time they were consumed relative to the time of the release. The
concentration of I-131 in goat's and sheep's milk is 10 times that
of cow's milk. Fresh milk drunk directly on the farm has higher
amounts of I-131 than the amount in milk that has been sent from
the farm to the processing plant and then to a store. This variation
in amount of I-131 is related to the short half-life of I-131 and
the decay that occurs in the time the milk is processed. Although
cheese and other aged-milk products tend to have lower amounts of
I-131, it is also important to determine how much of these products
have been consumed and what their I 131 concentrations were when
they were consumed.
3. Infants and children
are the groups most sensitive to I-131 exposure. The dose to children
is much higher than the dose to adults because the thyroid mass
in children is smaller than that of adults. The first week of life
is an especially vulnerable period. Populations in the former Soviet
Union exposed to much larger radiation doses (especially those affected
by the Chernobyl nuclear release) showed an increased incidence
of thyroid cancer for children younger than 15 years old at the
time of the release.
4. I-131 has a strong
affinity for the thyroid gland, which is the critical target organ
for exposure. Essentially all of the iodine entering the body quickly
becomes systemic (EPA 1988), with approximately 30% distributing
to the thyroid.
5. This patient's exposure
history should include previous childhood head, neck, and upper
mediastinum radiation exposure; previous residences (proximity to
nuclear testing or release sites); dietary habits since childhood,
including milk consumption and source (fresh vs. processed milk;
whether milk was from a cow, sheep, or goat); source of drinking
water; occupational history; and hobbies. Patients who consumed
goat's milk contaminated with I-131 have a higher radiation dose
of exposure than if they drank contaminated cow's milk from the
same pasture. The patient also should be asked about symptoms consistent
with hypothyroidism, hyperthyroidism, and disorders of calcium metabolism.
6. Fine needle aspiration
biopsy (FNAB) is the procedure of choice for evaluating a palpable
nodule and determining whether or not it is malignant.
7. Serum TSH level is
a useful initial screening assay because it can identify patients
with either thyrotoxicosis or hypothyroidism. Chronic autoimmune
thyroiditis can present with an increased TSH level and a thyroid
nodule.
8. Serum FT4 and T3
should be measured if TSH is abnormal. A serum calcitonin level
should be obtained if either a medullary thyroid carcinoma or a
multiple endocrine neoplasia type II is suspected. Also, because
of the risk of hyperparathyroidism after exposure to I-131, serum
calcium, phosphorus, and parathyroid hormone should be assessed.
9. If the nodule is
found to be benign by FNAB, the patient could be treated with T4
in a dose sufficient to suppress serum TSH, which will limit glandular
growth. If the nodule decreases in size, the patient should be maintained
on T4 indefinitely and the nodule monitored with ultrasound. If
the nodule persists while on T4 therapy, the patient will need a
repeat FNAB. If the nodule grows during T4 therapy, a surgical resection
is indicated.
10. The patient and
her husband are at higher risk for developing thyroid disease or
thyroid cancer because they received higher levels of exposure to
I-131 than persons who did not live near the Hanford Reservation
during the time of the highest I-131 emissions. The daughter is
not at higher risk because she was born in 1963, and emissions decreased
after 1962. No higher risk for any other disease is known. Children
in the United States are screened at birth for thyroid function;
no further thyroid tests are needed if the child is growing normally
without other medical problems. Good prenatal care would be highly
advised for your patient's daughter. However, you could reasonably
reassure your patient that an abnormal pregnancy outcome as a result
of exposure to I-131 is unlikely because all of the potential exposures
for the patient and her daughter were in the past.
11. If you live within
50 miles of a nuclear facility that produces or is capable of releasing
I-131, you should work with your medical association, local or state
public health department, emergency response organizations, and
elected representatives to ensure that a stockpile of KI is available
and a distribution plan is in place to distribute it if required.
12. The perception of
risk by individuals or communities can be affected by several factors
other than how the risk is communicated. This includes culture,
social and economic level, geographic location, previous experiences,
and other variables inherent to the individual. In matters of high
concern and low trust, perception equals reality. Peter Sandman
(1993) developed the following framework for dealing with risk:
“Risks
that are
|
- natural
are more readily accepted than those that are man-made
- visible
and avoidable are more readily accepted than those that
are unseen and imposed by others
- voluntarily
assumed are more acceptable than those that are involuntarily
imposed
- familiar
are more acceptable than risks that are exotic or unfamiliar.”
|
13. The health
care provider can encourage trust and credibility by getting the
facts of the exposure straight, being forthcoming with information
that meets the needs of the individuals, coordinating efforts with
public health agencies, and avoiding giving mixed messages. It is
important to deal with the uncertainty; listen to and deal with
specific concerns; convey the same information to all segments of
the audience; and explain risk in language people understand, simplifying
language and presentation, but not content.
For a printer-friendly
version of the Radiation Exposure From Iodine 131 case study,
download the PDF version.
You need to have
Adobe Acrobat Reader to download a PDF version. Click here to
go to the Adobe Web site,
and follow the instructions to install this free software on your
computer.
Previous
Section
Next Section
|
