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Increase in thyroid cancer in US attributed to improved early detection

May 10, 2006

Although the incidence of thyroid cancer has more than doubled in the past 30 years, the rise is being attributed to improved diagnostic techniques of previously undetected disease, rather than a true increase in the occurrence of thyroid cancer, according to a study in the May 10 issue of JAMA.

While the incidence of many head and neck cancers in the United States is decreasing, a number of registries have reported that the incidence of thyroid cancer is increasing. Some investigators have attributed the increase to environmental radiation, while others believe it could be from increased diagnostic scrutiny, according to background information in the article.

Although some thyroid cancers can spread and cause death, thyroid cancer has also been recognized to exist in a subclinical (before symptoms) form. Autopsy studies have revealed that many individuals not known to have thyroid cancer during their lifetime had thyroid cancer, particularly papillary cancers. As diagnostic techniques for thyroid cancer have become more sensitive, such as with the advent of ultrasound and fine-needle aspiration, it has become possible to detect this subclinical disease. Thus, while increasing incidence of thyroid cancer might reflect an increase in the true occurrence of disease, it might also reflect increased diagnostic scrutiny or changes in diagnostic criteria. Examination of the reasons underlying an increase in the incidence of thyroid cancer is important, because if there is an increase in the true occurrence of disease, efforts should be made to address its cause and aid those at greatest risk of developing the disease.

Louise Davies, M.D., M.S., and H. Gilbert Welch, M.D., M.P.H., of the Department of Veterans Affairs Medical Center, White River Junction, Vt., examined the trends in thyroid cancer incidence, histology, size distribution, and death in the U.S. to determine whether the patterns suggest a true change in thyroid cancer incidence or an apparent change based on increased diagnostic scrutiny. The researchers analyzed data, from 1973-2002, from the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) program and data on deaths attributed to thyroid cancer from the National Vital Statistics System.

The researchers found that the incidence of thyroid cancer rose from 3.6 per 100,000 in 1973 to 8.7 per 100,000 in 2002-a 2.4-fold increase. This 5.1 per 100,000 growth in the incidence of thyroid cancer was virtually entirely due to an increase in papillary thyroid cancer, which rose by 5 per 100,000, a 2.9-fold increase. Most of this increase is the result of increased detection of small cancers. Forty-nine percent of the increase consisted of cancers measuring 1 centimeter (cm) or less; 87 percent consisted of cancers measuring 2 cm or less. There was no significant change in the incidence of other types of thyroid cancer. Despite increasing incidence, the death rate from thyroid cancer has remained stable.

"Given the known prevalence of small, asymptomatic papillary thyroid cancers at autopsy, we believe this suggests that increased diagnostic scrutiny has caused an apparent increase in incidence of cancer rather than a real increase," the researchers write. "Because many of these cancers would likely never have caused symptoms during life, epidemiologists have labeled the phenomenon 'overdiagnosis'-a term perhaps most familiar in the setting of prostate cancer," the authors write.

"Further studies will be needed to determine if a more cautious diagnostic approach-perhaps simply providing follow-up for symptomatic thyroid nodules- is worthwhile. In addition, papillary cancers smaller than 1 cm could be classified as a normal finding," the researchers conclude.

JAMA and Archives Journals