Incidence and mortality trends of thyroid cancer from 1980 to 2016

Thyroid cancer incidence rates have been increasing globally over past decades. However, no study examining those trends in the canton of Zurich, Switzerland exists. In this study, we describe the incidence and mortality trends of thyroid cancer in the canton of Zurich during a 37-year period (1980-2016) including factors such as sex, histological subtypes and age at diagnosis.

We analysed population-based cancer registry data from 1980-2016 for the canton of Zurich, Switzerland. We estimated the age-standardised incidence and mortality rates using the European standard population. Joinpoint regression was used to detect average annual percentage changes (AAPCs) and their corresponding 95% confidence intervals (CIs).

We included 2972 primary cases of thyroid cancer (72.3% in women). The papillary cases accounted for the majority of incident cases (65.8%). In 2016, women had a higher age-standardised incidence rate than men for both papillary (10.4 and 3.3, respectively, per 100,000) and non-papillary (1.6 and 0.7, respectively, per 100,000) thyroid cancer. In both men and women, the incidence rates of thyroid cancer increased significantly over the study period with AAPCs of 1.4% (95% CI 0.6-2.2%) and 2.6% (95% CI 2-3.1%), respectively. These increasing incidence trends are mainly driven by papillary thyroid cancer with AAPCs of 3.4% in men (95% CI 2.3% to 4.4%) and 4.3% in women (95% CI 3.7% to 5%). Mortality rates significantly decreased in both sexes (men AAPC -3.6%, 95% CI -4.7% to -2.4%; women AAPC -3.7%, 95% CI -4.8% to -2.6%).

Our results show significantly increasing age-standardised incidence rates of thyroid cancer over time in both sexes, mainly due to papillary thyroid cancer, the most frequent histological subtype, and the only subtype for which a significant increase was observed. It is possible that many indolent thyroid cancers, and more specifically papillary microcarcinomas, are increasingly diagnosed, which may not lead to symptoms if undetected. Therefore, targeted diagnostic strategies are necessary to avoid overdiagnosis of thyroid cancer. Nevertheless, we cannot completely exclude a partly true increase.