Geographic Distribution and Evolution of Thyroid Cancer Epidemic in South Korea

Dear Editor:

The incidence of thyroid cancer has increased worldwide, with the greatest increase in incidence being in South Korea (1). Despite the rapid increase in the number of thyroid cases in this country, little is known about the geographic distribution and temporal trends of thyroid cancer diagnosis.

Using the Korean Cancer registry data, which covers the entire population (the completeness of incidence data for 2010 was 97.1%) and is maintained by the Korea National Cancer Center (2), we computed annual age- and sex-standardized thyroid cancer incidence for 16 geographic regions between 2002 and 2012, and mapped the spatial/time distribution of the incidence rates using ArcGIS^® software v10.2, with threshold level in each cell determined by setting the classification methods as manual without smoothing tools.

Thyroid cancer incidence increased from 10.6 cases per 100,000 people in 2002 to 73.6 cases per 100,000 people per year in 2012. In 2002, the geographic variation in thyroid cancer incidence ranged from 6.6 cases per 100,000 people in Busan to 17.9 cases per 100,000 people in Gwangju, with a rate ratio of 2.7. The areas with higher rates were clustered in the southern region of the country (Fig. 1). From 2002 to 2012, all geographic regions demonstrated an increased incidence of thyroid cancer diagnosis, with an average annual percentage change (APC) of 21.4%. This increase, however, was accentuated in the central and northwest regions of the country (Supplementary Table S1 and Fig. S1; Supplementary Data are available online at www.liebertpub.com/thy). For instance, the APC for the southern regions Gwangju and Chonnam was 17% and 18.2%, respectively, whereas for central-northern regions such Dajon and Seoul, the APC was 24.6% and 20.2%, respectively. By 2012, the geographic variation in thyroid cancer incidence decreased, ranging from 38.9 cases per 100,000 people in Kangwon to 93.6 cases per 100,000 people in Daegu, with a rate ratio of 2.4. Consequently, the spatial distribution of thyroid cancer in 2012 showed a more homogeneous distribution, where all geographic regions show high rates of thyroid cancer. Finally, urbanization seems to affect incidence, with higher incidence occurring in metropolitan areas (82 cases per 100,000 people in 2012), lower in those areas bordering metropolitan areas (65 cases per 100,000 people in 2012), and lowest in areas not bordering metropolitan areas (57 cases per 100,000 people in 2012).

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FIG. 1.

Geographic distribution and temporal trends of thyroid cancer in South Korea in rates per 100,000 people.

Rarely, a chronic disease becomes an epidemic with the speed and dispersion comparable to that of an infectious disease. Remarkably, for cancer, this phenomenon has never been observed before. The factors contributing to this epidemic and its distribution pattern are not fully understood. Recent evidence, however, points to thyroid cancer screening as an important driver; thyroid cancer screening is strongly associated with the incidence of thyroid cancer in Korea (r ² = 0.75, p = 0.001) (1). The role of other driving factors such as sociodemographic and age-related characteristics, as well as access to healthcare, may need to be elucidated. For other countries, such as the United States (3) that are experiencing their own epidemic of thyroid cancer but with a less intensity, the Korean experience should guide further research and help to alter the course of this epidemic.