Rising Incidence and Incidence-Based Mortality of Thyroid Cancer in California, 2000-2017.

Context: The increased incidence of thyroid cancer globally over the past several decades is principally attributed to small, indolent papillary thyroid cancers. A possible concomitant increase in thyroid cancer-specific mortality remains debated.

Objective: The changes in thyroid cancer incidence and incidence-based mortality were assessed using a large population-based cohort over an 18-year period.

Design & Patients: A retrospective analysis of all thyroid cancers reported in the California Cancer Registry was performed (2000-2017). Age-adjusted incidence and incidence-based mortality rates were analyzed using a log-linear model to estimate annual percent change.

Results: We identified 69 684 individuals (76% female, median age 50 years) diagnosed with thyroid cancer. The incidence of thyroid cancer increased across all histological subtypes (papillary, follicular, medullary, and anaplastic) and all tumor sizes. The incidence increased from 6.43 to 11.13 per 100 000 person-years (average increase 4% per year; P < 0.001) over the study period. Thyroid cancer-specific mortality rates increased on average by 1.7% per year (P < 0.001). The increased mortality rates were greater in men (2.7% per year, P < 0.001) and patients with larger tumors (2-4 cm) (3.4% per year, P < 0.05).

Conclusions: Data from this statewide registry demonstrate that the incidence of thyroid cancer is increasing, and that this phenomenon is not restricted to small papillary thyroid cancers. Rising incidence in thyroid cancers of all sizes with concurrent increase of incidence-based mortality in men and those with larger tumors suggest a true increase in clinically significant disease.