thyroid cancer is incomplete. In contrast to the case of screening for breast cancer, for example, there are no randomized, controlled trials of screening for thyroid cancer, either in the general population or in high-risk groups. Instead, the effect of screening must be inferred from observational studies regarding the prevalence of undiagnosed thyroid cancer and the potential benefit of early detection.

In this paper we address whether physicians should screen for thyroid cancer seen in asymptomatic patients thought to have been exposed to I-131. We focus on studies of screening in the general population and in high-risk groups. We examine the accuracy of the tests used for screening, the number of cancer patients detected with screening, and evidence that treatment of cancers found by screening improves outcomes.

Screening is "the application of a test to detect a potential disease or condition in a person who has no known signs or symptoms of that condition at the time the test is done" (Eddy, 1991). Studies of screening can be classified according to the setting in which the decision to screen takes place. In clinic-based screening, or casefinding , a screening test is performed in patients who visit a primary-care physician for an unrelated reason. Studies of casefinding programs provide the most realistic estimates of the effects and costs of screening in a clinic or office practice, but there have been very few studies of casefinding for thyroid cancer. Population-based studies contact, recruit, and follow patients in the context of an epidemiologic research effort. Such studies show the extent of unsuspected thyroid cancer in a population sample, but they do not reflect the yield or costs of screening in clinics or providers' offices. We used population-based studies as a benchmark against which the yield and benefits of clinic-based screening programs could be measured. Finally, studies of monitoring in high-risk groups describe efforts to monitor individuals with occupational exposure to radiation or with a history of head and neck irradiation for benign or malignant conditions. Such studies could be less relevant to screening in populations exposed to the much lower doses of radiation from the Nevada Test Site.

Before the consequences of screening can be estimated, it is necessary to formulate the screening problem. To formulate a strategy researchers must specify the intended population for screening; the screening tests, follow-up tests, and treatments that will be used; and the type of outcomes influenced by the tests. In this section, we enumerate the information gaps, and we suggest a strategy for a "baseline" screening program.