The increased incidence of thyroid cancer in the past 30 years may be due to improved diagnostic techniques rather than an increase in the occurrence of the disease, researchers write in the Journal of the American Medical Association.
"While an 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," the authors write.
To address this issue, Louise Davies and Gilbert Welch, from the White River Junction Veterans Affairs Medical Center in Vermont, USA, examined trends in thyroid cancer incidence, histology, size distribution, and mortality using data from 1973-2002 contained in the National Cancer Institute's Surveillance, Epidemiology, and End Results program.
Results revealed that there was a 2.4-fold increase in the incidence of thyroid cancer from 1973 to 2002, rising from 3.6 to 8.7 per 100,000 individuals.
Moreover, the increased incidence of thyroid cancer was almost entirely due to an almost 3-fold increase in the prevalence of papillary thyroid cancer, which rose from 2.7 to 7.7 cases per 100,000 individuals from 1973 to 2002.
Forty-nine percent of the increase consisted of cancers measuring 1 cm or less, and 87% consisted of cancers measuring 2 cm or less. There was no significant change in the incidence of other types of thyroid cancer over the study period.
Interestingly, although the prevalence of thyroid cancer increased, there was no change in the mortality rates from thyroid cancer between 1973 and 2002.
"Given the 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," Davies and Welch note.
"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."
In an accompanying editorial, Ernest Mazzaferri, from the University of Florida in Gainesville, USA, refers to the recommendations made by the American Thyroid Association for patients with thyroid nodules and differentiated thyroid cancer for the appropriate diagnosis of thyroid cancer.
He wrote: "First, thyroid sonography should be performed in all patients with one or more suspected thyroid nodules; second, fine-needle aspiration biopsy (FNAB) is the procedure of choice for evaluating thyroid nodules; and third, when several nodules larger than 1 to 1.5 cm are present, those with a suspicious sonographic appearance should be biopsied preferentially.
"Patients with nodules that are 8 to 9 mm in size and that have suspicious ultrasonographic findings, suspicious cervical lymph nodes, or a history of radiation exposure or familial thyroid cancer should be considered for ultrasound-guided FNAB. Smaller nodules most likely can be followed up over several years without FNAB if they are not increasing in size."