Office of Cancer Communications

Building 31,Room 10A24

Bethesda, MD 20892

National Cancer Institute

September 23, 1997

NCI Press Office
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Questions and Answers on the NCI Fallout Report

1. What is the NCI Fallout Report?

The National Cancer Institute (NCI) report contains an assessment of radioactive Iodine-131 (I-131) fallout exposure from the nuclear bomb tests that were carried out at the Nevada Test Site in the 1950s and 1960s. In 1982, Congress passed legislation calling for the Department of Health and Human Services to develop methods to estimate I-131 exposure to the American people, to assess thyroid doses from I-131 received by individuals across the country from the Nevada tests, and to assess risks for thyroid cancer from these exposures. The fallout report fulfills the first two of these three requirements.

The report includes county-by-county estimates of average I-131 doses to the thyroid for persons living or born in the contiguous 48 states during the period when the bomb tests were carried out, mostly in the 1950s. Estimates of thyroid dose have been made for people by age category, sex, and amount and source of milk consumed (see Question 11). Doses have been estimated for each of the 90 tests that together released nearly 99 percent of the I-131 produced during the bomb testing program, and summed over test series and for all the tests combined.

2. What does the report show?

The report shows that everyone living in the contiguous 48 states was exposed to I-131 at some level. The average cumulative thyroid dose to the approximately 160 million people in the country at the time was about 2 rads. (By comparison, a routine I-131 diagnostic thyroid scan of a child in the 1950s gave approximately 200 to 300 rads to the thyroid. Today, a thyroid scan would give about 0.4 to 4 rads to the thyroid, depending on the radionuclide used. A rad is a unit of measurement that stands for radiation absorbed dose, the amount of radiation that is absorbed by the tissues in the body. Scientists now measure radiation dosage in grays: One gray equals 100 rads.)

Children and persons who drank large quantities of milk, or who drank milk from goats or family-owned cows, are estimated to have received higher doses. (Goat's milk concentrates I-131 more than cow's milk. Estimates of I-131 doses in milk consumed soon after it was taken from a family-owned cow are usually higher than for store-bought milk because store-bought milk takes time to process and ship, allowing more time for the radioactivity in I-131 to diminish.) Most children aged 3 months to 5 years probably received about three to seven times the average dose for each test, because in general they drank more milk than adults did, and because their thyroids were smaller. Most adults, by contrast, probably received doses two to four times lower than the average estimated dose.

Doses varied widely according to geographic area. In general, persons living in Western states to the north and east of the test site had higher doses than those living in West Coast, Southern, and Eastern states. In the 24 counties with the highest exposures, estimated average cumulative doses ranged from 9 to 16 rads.

3. Does radiation cause thyroid cancer? Does I-131 cause cancer?

The risk of thyroid cancer from childhood exposure to external radiation such as X-rays has been known for some time. Increased risk of thyroid cancer has continued for many years after the atomic bombings in Hiroshima and Nagasaki, and after childhood X-ray exposures to treat conditions of the head and neck, such as acne and tonsillitis. A combined analysis of seven studies of persons exposed to a wide variety of external radiation sources -- including atomic bombs and treatment for cancer, tinea capitis (ringworm), and enlarged tonsils or thymus glands -- found that on average, persons exposed as young children to 100 rads (1 gray) had a 7.7 times higher risk of thyroid cancer compared with persons not exposed to radiation. But the overwhelming majority of thyroid cancers are not related to X-ray treatments.

The limited data on persons exposed as children to I-131 from the nuclear test fallout have provided suggestive but not conclusive evidence that it is linked to thyroid cancer. I-131 is thought to be no more likely to cause thyroid cancer than the same dose of external radiation, and it may be as little as one-fifth as hazardous. However, the radiation doses received by young children, especially those who lived in areas with high fallout levels, and particularly those who drank a great deal of milk, may well have increased the risk of thyroid cancer. The level of increased risk can be estimated but it is highly uncertain.

4. What has NCI done to assess the risk of thyroid cancer from the fallout? What further studies are planned?

In 1993, researchers at the University of Utah, Salt Lake City, published results of an NCI-sponsored study of children living in parts of Utah and Nevada that had high I-131 fallout levels, and a comparable number living in parts of Arizona that had relatively little fallout. The average dose for children in the study was estimated at about 10 rads. The researchers found a statistically significant association between I-131 exposure and all thyroid neoplasms combined (including cancer and benign tumors). They found some evidence of an association between estimated dose and thyroid cancer, but this was not statistically significant. The level of uncertainty was high because the number of cancer cases involved was very small. Reflecting this uncertainty, the researchers estimated that between zero and six of the eight observed thyroid cancers might have been caused by the fallout. The risks observed in the study were consistent with reported risks from external radiation (X-rays).

To provide more accurate information on the risk of thyroid cancer from radioactive fallout, NCI investigators are collaborating with other U.S. government agencies, international organizations, and governments and scientists in Belarus and Ukraine to study thyroid cancer among persons in those countries who were exposed during childhood to fallout from the Chernobyl nuclear accident in 1986. The tens of thousands of children exposed to the fallout received radiation doses to the thyroid that ranged from comparatively small doses to doses ten times higher than U.S. residents received from the Nevada tests, and a clear increase in thyroid cancer has been seen in this population. The dose to each person in the study group will be reconstructed based on thyroid measurements made within about two months of the accident, residential and dietary histories, and other environmental measurements. Because of the wide range of exposures, the large numbers of persons exposed, and the large number of thyroid cancer cases observed, information from the Chernobyl studies is expected to be relevant to the assessment of the impact of the U.S. exposures.

Researchers sponsored by the Centers for Disease Control and Prevention are studying the health effects of the radioactive iodine released from the Hanford, Wash., nuclear weapons plant in the 1940s and 1950s. Results are expected in 1998.

The new fallout data from NCI could be used to correlate fallout levels with thyroid cancer rates in specific geographic areas; however, such studies will encounter difficulties. Thyroid cancer is an uncommon disease, particularly among children. The small number of cases in any one geographic area -- particularly in sparsely populated areas -- makes it difficult to determine with confidence whether rates are significantly elevated. And while some states have registries with historical data on all cancer cases in the state, most do not. In addition, many children in the 1950s were exposed to medical X-rays at levels known to increase risk for thyroid cancer, and it is difficult to disentangle these effects from those of fallout exposure.

5. What is the U.S. government doing in light of this report?

The Department of Health and Human Services has requested that the National Academy of Sciences' Institute of Medicine (IOM) review the data to assess the risks to individuals, and to recommend to physicians how to treat persons who might be at risk of disease because of their exposure to radioactive iodine.

This review will be comprehensive and public as are all IOM studies. IOM is expected to establish a diverse expert panel including epidemiologists, public health authorities, risk communication experts, specialists in thyroid disease and its treatment, family physicians, public interest groups, and citizen representatives. In the meantime, persons concerned about fallout exposure should consult a health professional.

The administration will establish an interdepartmental group to look at broader policy issues.

6. Is there danger of exposure to I-131 fallout now?

No. The I-131 that was released from the tests is no longer present in the environment. I-131 has a radioactive half-life of about eight days, meaning that its radioactivity decreases by half every eight days. This means that nearly all the exposures took place primarily within two months following each test.

7. Why did the study take 14 years to complete?

The study involved the identification, collection, synthesis, and analysis of an enormous amount of data from many sources, including the Department of Energy, the National Oceanic and Atmospheric Administration, the Department of Agriculture, the Department of Commerce, agencies in each state, county agencies, the American Dairy Herd Improvement Association, the dairy industry, and numerous private individuals knowledgeable in some aspect of the study.

Early in the study an advisory committee was established to guide and assist in the study's conduct. From 1985 until 1993, it provided guidance regarding what data existed and where, which data might be useful and how, the retrieval and compilation of data (much of which was not in computerized form), the analysis of data, the methodologies to be used in reconstructing fallout deposition levels, milk concentration of I-131, and in the calculation of dose estimates by test, county, age, and dietary assumptions. During 1994 and 1995, the study report of approximately 1,000 pages was redrafted several times. Since that time, efforts have been made to review, organize, and summarize the results (about 100,000 pages) in an understandable format, and to revise and improve the clarity of the text.

8. When will the report be released?

The study's executive and technical summaries, along with a list of the 24 counties that were most highly exposed to the fallout, were released Aug. 1, along with a list of all U.S. counties with their estimated average thyroid dose levels. The complete report, about 1,000 pages long, is scheduled to be published by Oct. 1, as well as the full data set with annexes and subannexes.

The complete report will include a description of the study methods, the most important measurement results, and summary information on estimated I-131 doses to individuals. The full data set will include tabulated results of all the dose information by county, by age, and by milk consumption. It will include the amount of I-131 fallout in each county after each test, and the amount of I-131 that got into the food supply. Users of the data set will be able to group and present data by county, by test, by test series, and for all tests. Maps displaying the tabulated data will also be available.

9. What is radioactive fallout? What is iodine-131?

Radioactive fallout refers to a variety of airborne radioactive particles that fall to the ground during and following nuclear weapons tests. People and animals may ingest these particles in their food or inhale them from the air. Iodine-131 is a radioactive isotope of iodine -- an altered form of the element that is chemically the same as the naturally occurring element but is radioactive. I-131 is found in fallout from atmospheric nuclear bomb explosions along with a variety of other radionuclides such as strontium-90. I-131 has been the focus of most concern because it concentrates in the thyroid, particularly in children, and may increase the risk for thyroid cancer. There appears to be little risk of thyroid cancer for persons exposed to radiation as adults. Because iodine concentrates in the thyroid, high doses of I-131 are used to treat some types of benign thyroid disease and thyroid cancer.

10. How were people exposed to I-131?

For most people, the greatest I-131 exposure came from drinking contaminated milk. I-131 fell on pasture lands, and the vegetation was eaten by cows, contaminating their milk. Smaller amounts came from breathing contaminated air and eating other contaminated dairy products, eggs, and leafy vegetables.

11. How was the study conducted?

To assess the thyroid doses of I-131 received by persons residing in different areas of the country, researchers had to estimate the amount of I-131 deposited on soil and vegetation after each bomb test, the amount of contaminated vegetation consumed by dairy cows, the amount and source of milk (and to a lesser extent, other foods) that people consumed, and the proportion of I-131 accumulating in the thyroid.

For most of the bomb tests, passive fallout collection devices were placed in up to 100 locations across the United States in an attempt to measure the fallout. But these devices measured only overall radioactivity, not specific radionuclides such as I-131. To estimate the amount of I-131 that fell in each county following each test, the NCI researchers used a reanalysis of the original data collected at the time of the tests, along with meteorological records on wind and rainfall patterns at the time, plus mathematical models. (In Western states most I-131 fell to the ground in dry form, while in Eastern states most was associated with rainfall.)

To determine the amount of I-131 consumed by cows, the researchers used other mathematical models to estimate what proportion of I-131 fell on vegetation, and combined these estimates with data on whether cows were on pasture in numerous regions of the country at the time of the tests, how much fresh pasture grass was eaten by cows in each state, and how much of the I-131 ingested by cows gets into their milk. The researchers then used records of milk production and distribution within each state and in the entire United States during the time of the tests to estimate how much I-131 was in the milk consumed by the populations of each county. Finally, data on average milk consumption levels for persons of different ages and in different regions of the country were combined with data from the analyses described above to derive estimates of average I-131 thyroid doses following each test for persons residing in each U.S. county. The data will be reported for 13 age categories (including four fetal age groups, four age groups for infants, four age groups for children, plus adults as a single group, with men and women reported separately). Within each of these age categories, four scenarios for milk consumption are being calculated: persons who drank average quantities of milk with average I-131 contamination levels for the county, persons who drank higher-than-average levels of the most highly contaminated milk available in the county, persons who drank milk from family-owned "backyard" cows (see Question 2), and persons who drank no milk. Much less detailed analyses are being done for exposures from foods other than milk, and for exposure through inhalation, because these exposure pathways are much less important than drinking milk.

12. How reliable are the study's estimates of I-131 exposure?

There are large uncertainties in the estimated thyroid doses that will be given in the report because it is impossible to know all the information needed to determine exact doses. To evaluate the validity of the mathematical models used in the study, results obtained from the model were compared with the limited data collected at the time of the tests. The comparisons also provide an estimate of the uncertainty attached to the calculated doses.

These comparisons show a relatively good agreement between actual data and predictions made by the mathematical models. For example, independent analysis of urine samples volunteered by soldiers at Army bases throughout the United States following one of the test series showed iodine-131 dose levels consistent with or lower than the doses predicted. However, it should be noted that the comparison between measured and predicted values required the use of several assumptions, and there is no assurance that the samples measured were representative of county averages.

13. Can an individual person calculate his or her own thyroid radiation dose?

Because of the large number of variables involved in calculating individual dose estimates, it would likely be difficult for an individual to estimate his or her own thyroid dose. However, detailed information will be provided in the full report so that individual cumulative doses can be estimated. This information is expected to be most useful to state or county health departments and epidemiologists who wish to assess the impact of the fallout on specific populations.

In general, the uncertainty of the thyroid dose from iodine-131 fallout for a representative individual included in the study is about a factor of 3. For example, if the thyroid dose estimate for an individual is 3 rads -- based on county of residence, age and assumed milk consumption at the time of the tests -- the true dose will likely lie between 1 and 9 rads.

However, the uncertainty attached to the thyroid dose for an actual, specific person is larger, because of uncertainties about the dietary habits and the metabolism of any individual.

14. How common is thyroid cancer? How treatable is it?

Thyroid cancer is relatively uncommon compared with other forms of cancer, accounting for about 1 percent of all cancers diagnosed in the United States. An estimated 16,100 U.S. cases will be diagnosed in 1997. The number of new cases is about 10 times higher than the number of deaths, estimated at 1,230 in the United States in 1997. Thyroid cancer occurs more than twice as often in women as in men. Incidence rates for 1994 -- the most recent year available -- were 7.6 per 100,000 U.S. women of all ages combined, and 2.8 per 100,000 U.S. men. Thyroid cancer mortality rates for that year were 0.3 per 100,000 for both men and women. Lifetime risk for thyroid cancer is estimated at 0.66 percent for women (about 1 in 152) and 0.27 percent for men (about 1 in 370). Thyroid cancer is highly curable: The 5-year survival rate is about 95 percent.

15. What should people do who are concerned about cancer risk from fallout exposure?

Anyone who is concerned about cancer risk from fallout should request a thyroid examination as part of their next visit to a physician.

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