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Institute of Medicine (US) Committee on Thyroid Screening Related to I-131 Exposure; National Research Council (US) Committee on Exposure of the American People to I-131 from the Nevada Atomic Bomb Tests. Exposure of the American People to Iodine-131 from Nevada Nuclear-Bomb Tests: Review of the National Cancer Institute Report and Public Health Implications. Washington (DC): National Academies Press (US); 1999.

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Exposure of the American People to Iodine-131 from Nevada Nuclear-Bomb Tests: Review of the National Cancer Institute Report and Public Health Implications.

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Executive Summary

For nearly 50 years, public concern and scientific debate have surrounded the program of above-ground nuclear weapons testing that the United States conducted in Nevada during the period 1951 to 1962. In August 1997, as the National Cancer Institute (NCI) was preparing to a release a report on exposure to iodine-131 from test fallout, the Secretary of the Department of Health and Human Services (DHHS) asked the Institute of Medicine (IOM) and the National Research Council (NRC) to undertake an independent assessment of the public health and medical implications of the estimated iodine-131 doses received by the American people from this testing and to advise the Department on steps that might be taken in response.

This report presents the conclusions of two committees appointed by the IOM and NRC, which are referred to here collectively as the committee. In developing these conclusions, the committee

  • Assessed the soundness of the NCI analyses, which were presented in October 1997 in the two-volume report Estimated Exposures and Thyroid Doses Received by the American People from Iodine-131 in Fallout Following Nevada Atmospheric Nuclear Bomb Tests.
  • Analyzed evidence linking I-131 exposure to thyroid cancer and other thyroid conditions.
  • Reviewed estimates of thyroid cancer cases in the United States that may have resulted from the weapons tests.
  • Examined possible clinical and public health responses to I-131 exposure that would be consistent with scientific evidence about the possible benefits and harms of routine screening for thyroid cancer and with research on communicating risk information to the public and the medical community.
  • Considered directions for further research.

The committee reached the following general conclusions in three subject areas.

Estimates of national, county, and individual exposure to iodine-131

  1. In attempting to fulfill its Congressionally mandated tasks, the NCI undertook a very difficult task that depended on limited data of uncertain reliability and validity. The NCI report represents a careful, detailed, and responsible effort to estimate iodine-131 exposure from the Nevada atomic weapons tests. The NCI's methods were generally reasonable, although specific elements can be questioned (Chapter 2).
  2. The estimate of the American people's collective dose from I-131 is consistent with the committee's analysis and is unlikely to greatly over- or understate the actual levels (Chapter 2 on Validation and Uncertainty in Collective or Average Dose Estimates).
  3. The levels of detail presented in the report, specifically, county-specific estimates of iodine-131 thyroid doses, are probably too uncertain to be used in estimating individual exposure. For the most part, direct measures of fallout for any particular weapons test were made for only about 100 places nationwide (except near the Nevada Test Site itself). Estimates of county-specific exposures may also have little relevance to specific individuals for whom exposure depends on such critical factors as varying individual consumption of milk and other foods and variations in the source of those foods (Chapter 2).
  4. Individual-specific estimates of past exposure to iodine-131 from the Nevada tests are possible but uncertain, often highly so, because critical data are often not available or of questionable reliability. A small minority of the population—those who were young children at the time of testing and who routinely drank milk from backyard cows or, especially, goats—had a significant exposure to I-131 (Chapter 2 on From Fresh Milk to Human Intake and Chapter 3 on Normal Thyroid Physiology).

Estimates of cancer risk


Exposure to I-131 as a by-product of nuclear reactions can cause thyroid cancer as shown conclusively by the 1986 nuclear accident in Chernobyl, which resulted in high level exposure for many people. The NCI dose reconstruction model indicates that the level of exposure to I-131 was sufficient to cause and continue to cause excess cases of thyroid cancer. Because of uncertainty about the doses and the estimates of cancer risk, the number of excess cases of thyroid cancer is impossible to predict except within a wide range (Chapter 3 on Thyroid Cancer Risk Based on NCI Estimates of I-131 Doses).


Epidemiological analyses of past thyroid cancer incidence and mortality rates provide little evidence of widespread increases in thyroid cancer risk related to the pattern of exposure to I-131 described in the NCI report (Chapter 3 on Epidemiologic Analyses Using Cancer Registries). They suggest that any increase in the number of thyroid cancer cases is likely to be in the lower part of the ranges estimated by NCI. The epidemiologic analyses are, however, subject to many limitations and uncertainties.


Given the uncertainties in both the dose reconstruction model and the epidemiological analyses, further epidemiologic studies will be necessary to clarify the extent to which Nevada tests increased the incidence of thyroid cancer (Chapter 2). Pending these studies, it is prudent for DHHS to plan its responses as if excess cases of thyroid cancer have occurred.


Individual-specific estimates of the probability of developing thyroid cancer from exposure to fallout from the Nevada testing program are uncertain to a greater degree than the dose estimates because of the additional uncertainty, in particular, about the cancer-causing effect of low doses of I-131 (Chapter 3 on Thyroid Cancer Risk Based on NCI Estimates of I-131 Doses).


The type of thyroid cancer, papillary carcinoma, usually linked to radiation exposure is uncommon and rarely life threatening. Even among those with exposure to iodine-131, few will develop thyroid problems (Chapter 3 on Incidence of Clinically Manifest and Occult Disease).

Thyroid cancer screening, public education, and research


There is no direct evidence that early detection of thyroid cancer through systematic screening (rather than through routine clinical care) improves survival or other health outcomes (Chapter 4 on Benefits and Harms of Screening for Thyroid Cancer).


A program of systematic screening for thyroid cancer is not recommended either in the American population generally or among regional populations believed to have been exposed to iodine-131 from the Nevada tests (Chapter 4 on Committee Findings and Recommendations). For concerned patients who consult their physicians about screening for thyroid cancer, the decision about screening should be jointly made following a discussion of thyroid cancer risks following exposure to iodine-131, and possible benefits and harms of screening.


Rather than promoting systematic thyroid cancer screening, the Department of Health and Human Services should focus on a program of public information and education about the consequences of the Nevada weapons tests. It should involve members of the public and health professionals in developing and testing information strategies and materials. This program should explain basic facts about the Nevada tests, the link between I-131 and thyroid cancer (including symptoms), the problems with estimating individual exposure to iodine-131 and risk of thyroid cancer, and the pros and cons of screening (Chapter 4 on Information and Decisionmaking).


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, and people's perceptions and understanding of the benefits and harms of screening (Chapter 6).


In July 1945, the United States conducted the world's first atomic bomb test in Alamogordo, New Mexico. Soon thereafter, U.S. bombers dropped atomic bombs on Hiroshima and Nagasaki as part of a strategy to end World War II. After that war's end, the Cold War led to new national security anxieties and prompted the government to initiate a program of nuclear-weapons testing in Nevada.

During 1951-1962, the United States conducted nearly 100 above-ground nuclear weapons tests in Nevada; another dozen tests were conducted at depths below ground where some atmospheric release of radioactive material was possible. In addition to U.S. tests, other nations including the United Kingdom, the former Soviet Union, France, China, India, and, recently, Pakistan have conducted nuclear tests (the latter two nations only tested underground). Other nations' tests in the 1950s as well as U.S. testing in the Pacific have complicated efforts to estimate the fallout of radioactive materials from the Nevada tests and the health consequences of that fallout. Some systematic studies of fallout patterns were initiated with the first tests, but studies of health effects did not begin until much later.

Public concern about radioactive fallout from the Nevada testing began to emerge by the mid-1950s. Congressional hearings were held in the late 1950s and in 1963 to consider possible effects of the fallout on Americans. The limited test-ban treaty of 1963 was one reflection of public concern about nuclear weapons tests. Litigation was another consequence. By the late 1970s, hundreds of damage claims had been filed against the U.S. government alleging that illnesses, primarily cancers, resulted from the nuclear tests.

In the 1980s, continued concerns prompted a number of further studies to reevaluate radiation exposures of the population following weapons tests in Nevada. In 1983, Public Law 97-414, section 7(a) directed the Secretary of Health and Human Services to conduct research and develop estimates of the thyroid doses received by the American people from iodine-131 in fallout from the Nevada atmospheric tests. In 1983, NCI established a task group to assist it in a program of technical and scientific work that extended for more than a decade. The NCI report, Estimated Exposures and Thyroid Doses Received by the American People from Iodine-131 in Fallout Following Nevada Atmospheric Nuclear Bomb Tests, was published in October 1997. Publication of the report followed newspaper stories that presented information from the report's summary and noted that the analyses were still not available to the public some six years after the NCI had completed most of its mandated work. The NCI report did not include estimates of health risks of exposures. These were provided in separate analyses.


In fulfilling its Congressional mandate, the NCI faced a very formidable task of methodology development, information collection and generation, and data analysis. Challenges included a paucity of data directly relevant to dose reconstruction for events occurring nearly 50 years ago and disparities between the availability of data originally collected for other purposes and the requirements for current use in dose reconstruction. A contributing factor was that the potential health effects of I-131 were not entirely recognized in the 1950s.

Deposition of iodine-131 was estimated based on measurements taken at fewer than 100 sites in the entire continental United States. Even for these sites, amounts had to be estimated because technology at that time did not allow for a direct measurement of I-131 alone; only gross (total) beta activity was measured.

Overall, the committee concluded that the NCI report reflects an intensive effort to collect or generate the data needed for a complicated series of analyses, although documentation of methods, analyses, or results was insufficient in a few places. The committee concluded that the NCI was unlikely to have grossly over- or underestimated the collective I-131 dose, but it was less confident that the NCI had realistically determined the uncertainty associated with the estimate.

For the U.S. population overall, the NCI report presents a comprehensive rationale for assuming that the Nevada atomic weapons tests during the 1950s resulted in an average cumulative thyroid dose from I-131 fallout of 0.02 Gy (2 rad) for the American population collectively (approximately 160 million people) and an average cumulative dose of 0.1 Gy (10 rad) for a large part of the population that was under age 20 at time of exposure.

At the individual level, estimates of exposures to iodine-131 from the Nevada test are uncertain to varying degrees. Critical data on events that occurred almost 50 years ago are often unavailable or of questionable reliability. No matter how painstaking the analysis, the NCI report cannot compensate for the gap between the information available and the information needed to develop valid and reliable dose estimates for individuals. In particular, even with sophisticated methods for geographic extrapolation beyond specific measurement sites, the direct measurements of fallout are far too sparse to provide precise dose estimates at the county or state level for the entire continental United States. Although the committee understood the motivations and initial analytic thinking that led the NCI to calculate and to present county level estimates of exposure for every test, it concluded that the level of dosimetry detail provided for individual counties is inappropriate given the high level of uncertainty associated with the estimated doses. Rather than distinguish between average dose received in many of the counties, it is more appropriate to limit distinctions to much larger geographic areas and, even then, to distinguish only broad exposure levels.


The NCI's two-volume dose reconstruction report did not include estimates of thyroid cancer risk related to its estimates of iodine-131 exposure from the Nevada weapons tests. Instead, a separate, later memorandum provided estimates of lifetime thyroid cancer risk and excess cancer cases associated with that exposure. In addition to reviewing the NCI estimates, assumptions, and methods, the committee also reviewed the literature on cancer associated with iodine-131 exposure and examined epidemiological data on the prevalence of thyroid cancer in the United States. The most important conclusion from the literature review is that a link between thyroid cancer and exposure to iodine-131 from fallout—long considered uncertain—is now considered to be established based on studies following the nuclear reactor accident at Chernobyl in 1986. Because these studies have focused on children and are necessarily limited in followup time, the implications for adults now in middle age are unclear.

According to the NCI's revised estimates, which are not broken down by state or county, exposure to I-131 from the Nevada atmospheric tests will produce between 11,300 and 212,000 excess lifetime cases of thyroid cancer with a point or central estimate of 49,000 cases. The committee considered the NCI approach to developing estimates of excess cancer cases due to iodine-131 exposure generally reasonable, but the committee did raise questions about certain assumptions. In particular, it noted that there is disagreement within the scientific community about the assumption of dose-response linearity, that is, the assumption that even the smallest dose of iodine-131 to the thyroid results in some excess risk of cancer. Most exposure to iodine-131 following the Nevada tests was low-level exposure for which evidence of cancer risk is very limited.

Several epidemiological analyses provide little support for the higher estimates of excess cancers related to iodine-131, although these analyses also have limitations that restrict their ability to detect the effects of I-131 fallout on the population. Given the limitations, the analyses suggest that the excess of cancer cases is far below the highest value in the estimated range provided by NCI and is probably in the lower part of the range. Additional analysis (based on the assumption that the risk of developing cancer is constant over a person's lifetime) suggests that about 45 percent of iodine-131 related thyroid cancers have already appeared. Although the committee cautioned against using the NCI county-level dose estimates as a basis for assessing individual risk of thyroid cancer, it suggested that the chance of a significant exposure is highest for those who were young children at the time and who routinely drank milk from backyard cows and in particular, goats.


In developing advice for DHHS about thyroid cancer screening and other clinical and public health issues, including communication with health care practitioners and organizations, the committee relied on the evidence-based approaches to clinical practice that have been developing over more than two decades. These approaches stress systematic processes for (1) analyzing scientific evidence about the benefits and harms of clinical practices, (2) developing guidelines for clinician and patient decisions about specific health problems, and (3) clearly presenting the assumptions, evidence and rationale for practice recommendations.

From the perspective of clinical practice and public health policy, a critical problem with the NCI estimates of I-131 exposure and cancer risk discussed above is that they provide little help in identifying those individuals with a significantly elevated risk of developing thyroid cancer. In addition, thyroid cancer caused by radiation cannot be distinguished from naturally occurring thyroid cancer.

More precise estimates would not, in any case, significantly change the conclusions reached by the committee against routine screening of the population generally or of subgroups possibly exposed to I-131. Fundamental to this recommendation is the lack of evidence that early detection of thyroid cancer through screening (rather than through routine clinical care) improves health outcomes. The committee noted that papillary thyroid cancer, the most common form of naturally occurring thyroid cancer and the form linked to radiation exposures, has a high survival rate, regardless of cause, when detected in routine clinical practice without screening. Ninety percent of those diagnosed with papillary thyroid cancer are alive 30 years later.

Screening has the potential for harm as well as benefit. Routine screening for thyroid cancer by palpation and, especially, by ultrasound will identify many nodules, most of which will not be malignant. This will prompt further testing by FNA biopsy. FNA biopsies will discover a few cancerous nodules, will not find cancer in most nodules, and will yield a significant proportion of indeterminate or unsatisfactory samples (20 to 30 percent or more) that may lead to unnecessary thyroid surgery for many people who do not have thyroid cancer or who have very small cancers that would never progress to cause health problems.

Given popular fears of cancer and concern about radiation, the often modest reach of public information programs, and conflicting recommendations from other groups, clinicians will likely see some patients who express concern about possible exposure to radioactive fallout and who request screening for thyroid cancer. Although the committee recommends against policies that encourage or promote routine screening, it is essential that clinicians respond sensitively and constructively to concerned patients who come to them seeking advice. Such a response will involve listening to the patients concerns; discussing their possible exposure to iodine-131 and other risk factors for thyroid cancer; explaining that thyroid cancer is uncommon even in people with some exposure to I-131 and that the thyroid cancer linked to I-131 exposure is rarely life threatening; describing the process, benefits, and harms of screening and the lack of evidence showing that people are better off with it than without it; checking patient understanding of the information presented; and, then, jointly deciding how to proceed. Although the committee believed the decision about a physical examination including thyroid palpation should be a joint one, it recommended against screening by ultrasound for patients without symptoms.


The historical and political contexts surrounding the Nevada testing program and its aftermath combined with the technical nature of the analyses of iodine- 131 exposure, health risks, and screening strategies will make it difficult to communicate information in ways that will be perceived as equally believable and understandable by all those concerned about the consequences of the testing program. The above-ground nuclear tests were purposive, man-made phenomena that left behind a toxic residue, albeit most of it short-lived. From almost the beginning of the testing program, governments and residents of areas adjacent to the test sites have engaged in an intermittent, often acrimonious debate about the release of accurate information and the possible health effects of fallout. Evidence of contamination in more distant areas and from other sources (e.g., the Hanford Nuclear Reservation) have prompted concerns beyond areas near the test site.

Two legacies of the testing program and its aftermath are the considerable, and sometimes intense, distrust of the government as a source of information and advice, and the fact that a segment of the public is convinced the health impacts of exposure are significant and severe. These circumstances create significant challenges for DHHS in constructing a credible and effective public communication program.

The potential audiences for a DHHS communication and information effort will include individuals and groups who have, over the course of decades, developed strong beliefs about the risk and about the trustworthiness of their government, people with little past interest or concern who will want more information when they learn of the exposure issue from news media or other sources; elected officials with varying degrees of concern about the issue; and clinicians and public health officials with varying levels of knowledge about thyroid conditions and screening options. It will be important for DHHS to (1) recognize the existence of these different audiences, (2) decide the audiences they most need to reach, and then (3) use a growing body of empirical and experiential knowledge about communicating risks to develop information strategies that are sensitive to the concerns and perceptions of these groups. For example, if DHHS wishes to communicate effectively with people living in the regions where the tests were conducted, it will need to bring representatives from these areas into the information planning process at the very beginning.

The building blocks for the information and communication program should include several kinds of materials. Brochures or the equivalent should be developed for clinicians, public health departments, and others to give to concerned patients. In addition, although many people lack access to computers, an updated Internet site for both the lay public and clinicians can be a convenient, in-depth source of information for many others. Electronic and written sources of information should explain basic facts about the Nevada tests, I-131, and thyroid cancer and should provide basic education about screening tests, including explanations of possible benefits and harms.

The committee recognized the particular challenge of communicating its conclusions about screening in ways that respond to understandable public concerns about the likely (albeit small) risk of thyroid cancer linked to exposure to iodine-131 from the Nevada atomic weapons tests. Screening programs—whether or not they are supported by scientific evidence—can be a popular response to risk. Notwithstanding this appeal, the committee recommends that DHHS concentrate on developing a program of information and education for the concerned public and clinicians that builds on the analyses, conclusions, and information approaches described in the committee report and on the experience the Department and others have gained in developing similar informational materials.


The committee suggests that DHHS consider additional research in several areas. These areas include (a) the relative effectiveness of external radiation versus internal radiation in producing thyroid cancer; (b) the relative malignancy of radiation-related versus spontaneous thyroid neoplasms; (c) the role of genetic events in the development of thyroid cancer, in particular, the role of ret/PTC oncogene as it may affect the nature of the dose-response relationship for thyroid cancer; (d) people's perceptions of the benefits and risks of screening for thyroid and other cancers and the factors affecting such perceptions including the way quantitative information is presented; and (e) the effectiveness of existing programs to communicate radiation risks. The committee considered public comments calling for further research on the total radiation exposure resulting from all radionuclides deposited in radioactive fallout from nuclear tests in both the United States and other countries. It concluded that such research was unlikely to benefit public health and would divert resources from other uses of greater probable benefit and this as well as the cost of such research should be clearly understood before a decision is made to undertake it due to public concern.

Copyright © 1999, National Academy of Sciences.
Bookshelf ID: NBK100833


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