Incidence of Clinically Relevant Thyroid Cancers Remains Stable for Almost a Century: A Population-Based Study

Objective: To examine the trends in incidence of clinically relevant thyroid cancers within the overall rising incidence of thyroid cancers. Patients and Methods: This is a population-based cohort study conducted using the Rochester Epidemiology Project database to identify all new cases of thyroid cancer in Olmsted County, Minnesota, between January 1, 1935, and December 31, 2018. We extracted information about demographics and tumor pathologic type, size, and invasiveness. Clinically relevant cancers included aggressive histology or presence of metastatic disease, size larger than 4 cm, and gross extrathyroidal tumor invasion. Results: Between 1935 and 2018, 596 thyroid cancer cases were diagnosed (mean age, 46.4 years; 72% female; 87% papillary cancers; and median tumor size, 1.5 cm). The sex- and age-adjusted incidence of thyroid cancer increased from 1.3 per 100,000 person-years (p-y) from 1935–1949 to 12.0 per 100,000 p-y in 2010–2018, corresponding to an absolute change per decade of 1.4 (95% CI, 0.7 to 2.2). There was a nonsignificant period absolute change for patients with tumor greater than 4 cm (0.03; 95% CI, -0.2 to 0.3), with evidence of tumor invasion (0.1; 95% CI, -0.1 to 0.4), and with aggressive histology or presence of metastatic disease (0.2; 95% CI, -0.1 to 0.6). Thyroid cancer mortality was unchanged over the observation period. Conclusion: Incidence rates of clinically relevant thyroid cancers, as defined by histology, size, and invasiveness, have not changed significantly in 80 years. The rising thyroid cancer incidence is driven by indolent thyroid cancers.