Increasing incidence of thyroid cancer in the United States, 1973-2002.

W HILE THE INCIDENCE OF many head and neck cancers in the United States is decreasing, 1 a number of registries have reported that the incidence of thyroid cancer is increasing. 2-6 Some investigators have attributed the increase to environmental radiation, 5 while others have found no obvious source. 3 However, increasing cancer incidence can be either real or apparent because of increased diagnostic scrutiny. Supporting evidence for a real increase in incidence includes an increase in known or suspected risk factors for the cancer. If the increase were real, it would be expected across all stages, as opposed to a shift in the stage distribution toward earlier stages, as might be expected if there were changes in diagnostic criteria or increased diagnostic scrutiny. Moreover, the increase in incidence might be expected to be accompanied by increasing symptoms or associated mortality. Although some thyroid cancers can spread and cause death, for many people thyroid cancer has also long been recognized to exist in a subclini-cal form. More than 50 years ago, pathologists reported that thyroid cancer (particularly papillary histology) was a common autopsy finding, despite its never having caused symptoms during a person's life. 7 This finding has been replicated in several autopsy studies, 8-11 the most methodical of which was from Harach et al, 12 who systematically sectioned 101 thyroid glands in 2-to 3-mm slices. They found that 36% of people not known to have thyroid cancer during their lifetime nonetheless had 1 or more foci of thyroid cancer. However, because many of the cancers identified by Harach et al were small (far smaller than the 2-3 mm between the slices), they reasoned that many were missed by their technique. They went on to calculate that, if sectioned finely enough, virtually every person would be found to harbor a thyroid cancer. As diagnostic techniques for thyroid cancer have become more sensitive, particularly with the advent of ultrasound and fine-needle aspiration, it has become possible to detect this subclinical reservoir. 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 diagnos-For editorial comment see p 2179. Context Increasing cancer incidence is typically interpreted as an increase in the true occurrence of disease but may also reflect changing pathological criteria or increased diagnostic scrutiny. Changes in the diagnostic approach to thyroid nodules may …