Basic Epidemiological Problems in Thyroid Cancer

Two of the main features of thyroid malignancy are its rarity and its excellent prognosis in the overwhelming number of cases. On the other hand, these factors induce some specific difficulties in epidemiologic studies of thyroid cancer.

Apart from special aspects for thyroid malignomas, the validity of epidemiological data in general depends on a number of factors, which may cause pitfalls.

Each scientist who implements or analyzes studies in this field should bear in mind those fundamental difficulties, otherwise erroneous conclusions or fatal epidemiologic misinterpretations may be drawn. This chapter therefore opens with some basic methodological reflections.

Ideally, for epidemiological research, complete data on the entire population should be available. At least, it must be assured that the data sample is representative of the study population in every relevant aspect.

For this aim, well-organized structures for data acquisition and transmission to centralized agencies are necessary. Mandatory reporting of all diagnosed malignancies, cancer deaths, routine autopsies, and comparison of these data with information from the official register of births and deaths by a centralized cancer registry would be beneficial for oncological epidemiology. This ideal situation does not exist, even in many industrial nations with highly developed health care systems, due in part to ethical considerations as well as concerns about the protection of privacy. For example, in Germany, not until 1995 was legislation passed to allow physicians to report newly diagnosed malignancies to the registry (however, reporting is not mandatory). Meanwhile, most German federal states have established central cancer registries, but even at present the registration is not nationwide. In the USA, the National Cancer Data Base (NCDB) records approximately 60% of all cancer cases but, again, the majority of the information is provided on a voluntary basis.

Due to the limitations of centrally acquired data or even their complete absence in many countries, epidemiological studies are often based on clinical data, which are flawed by selection biases. The continual improvement of diagnostic modalities is another factor complicating data comparison, e.g., the improved sensitivity of ultrasonography since the 1980s makes the historical comparison of incidence data highly problematic. The reported increasing incidence of thyroid cancer, especially in the early stages, is at least in part due to improved diagnostic techniques, which lead to earlier detection _[69].

The slow growth rate of thyroid carcinomas causes another difficulty for epidemiological research. There are often decades between tumor induction and clinical manifestation, and in a substantial number of cases the disease remains undetected during life, and tumor mortality is generally low. In 1998 for example, in the United States 17,200 new cases of thyroid cancer from a total of 1,228,600 new cancer cases were estimated, with a male:female ratio of 1:3.7.