The changing epidemiology of thyroid cancer

Abstract

Clinically recognized thyroid cancer accounts for less than 1% of human malignancies. In the mean, females are affected 1.5-3 times more often than males. The prevalence of occult thyroid malignancy is up to a factor of 104 higher than the incidence of clinical recognized thyroid cancer, and the cause-specific mortality rate is, for its part, about 3-10 times lower than the average incidence rate (about 1.5/100,000 per year in men and 3/100,000 per year in women). The rarity and favorable prognosis of clinically recognized thyroid cancer cause specific problems for epidemiological studies. There is a lack of long-term prospective, randomized clinical trials with large cohorts, which would be necessary to demonstrate changes in epidemiology and effects of environmental factors or improvements of medical methods on this data. Currently, the majority of available data on thyroid cancer derive from retrospective studies or from cancer registry databases. Additionally, the wider use and the improvement in sensitivity of diagnostic methods (especially ultrasound) influence epidemiologic data such as prevalence and incidence and complicate the comparison with historical data. Despite these problems in obtaining valid epidemiological data, the following trends are obvious in the majority of countries: The overall incidence of thyroid cancer has increased in both sexes during the past decades, but the latest available data do not indicate whether this trend will further continue. This increase is mainly due to papillary thyroid cancer, whereas anaplastic carcinoma has become rarer. Especially in countries where preexisting iodine deficiency has been supplemented, the above-mentioned current trend can be observed: not only the relative contribution, but also the incidence of papillary carcinoma has increased markedly. In contrast, the overall cause-specific mortality has remained constant or even declined, reflecting the growing proportion of less aggressive cancer types and the medical improvements leading to tumor detection at earlier stages. © Springer-Verlag Berlin Heidelberg 2001, 2005.