Trends in Thyroid Cancer Incidence and Incidence-Based Mortality in California

papillary carcinoma of thyroid
Thyroid cancer incidence is increasing, with data showing a consistent increase not only in the diagnosis of small papillary thyroid carcinoma but also larger and more aggressive tumors.

Thyroid cancer incidence increased in California between 2000 and 2017, with data showing a consistent increase not only in the diagnosis of small papillary thyroid carcinoma but also larger and more aggressive tumors, according to study results published in The Journal of Clinical Endocrinology & Metabolism.

Although previous studies have reported that the observed worldwide increase in thyroid cancer incidence is mainly due to incidental diagnosis of small thyroid cancers, data show that large tumors are also diagnosed more frequently today. The data on thyroid cancer-specific mortality rates in the United States have been inconsistent.

In the study, the researchers explored trends in thyroid cancer incidence and incidence-based mortality in California from 2000 to 2017.

The researchers identified 68,684 patients (76.3% women; median age, 50 years, range 38 to 61 years) from the California Cancer Registry. The most common histologic type was papillary thyroid carcinoma (87.8%) followed by follicular thyroid carcinoma (7.2%) and medullary thyroid cancer (1.8%).

The overall thyroid cancer incidence ranged from 6.43 to 11.13 per 100,000 person-years, with an increase of 10.06 diagnoses per 100,000 person-years (95% CI, 9.99-10.14) observed during the study period. The average annual percent change in incidence was 4% (P <.001) from 2000 to 2017, with a peak incidence rate of 12.73 per 100,000 person-years (95% CI, 12.38-13.07) in 2015, which declined slightly thereafter. The incidence rate increase was evident in both sexes, across all racial and ethnic groups, and in all histologic subtypes except for follicular carcinoma.

The increase in thyroid cancer incidence was documented across all papillary tumor sizes (all P <.001), including papillary microcarcinoma (≤1 cm; average annual percent change, 5.8%) and tumors measuring 1 to 2 cm (average annual percent change, 5.4%), 2 to 4 cm (average annual percent change, 3.9%), and >4 cm (average annual percent change, 4.6%).

In total, 2562 thyroid cancer-specific deaths occurred during the study period. The majority of deaths (75.2%) occurred in patients older than 60 years. The average annual mortality rate was 0.34 deaths per 100,000 person-years (95% CI, 0.33-0.36) and the overall thyroid cancer incidence-based mortality increased by 1.7% per year (P <.001).

While mortality rates remained stable for follicular thyroid carcinoma, there was an increase in mortality rates for papillary (average annual percent change, 2.3%) and anaplastic cancers (average annual percent change, 3.5%) and a decrease in mortality rates for medullary cancer (average annual percent change, -3.8%).

Increasing mortality rates were evident in men and patients with regional stage papillary thyroid carcinoma, but not in women or patients with localized or distant stages.

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In addition, the mortality rate for papillary microcarcinoma remained stable, but increased mortality rates were observed in papillary thyroid carcinoma measuring 1 to 2 cm (average annual percent change, 5.1%), 2 to 4 cm (average annual percent change, 3.4%), and >4 cm (average annual percent change, 2.6%).

The researchers acknowledged several study limitations, including the relatively short follow-up period and a limited number of cases in some tumor subgroups.

“The increased incidence of thyroid cancer in California is due in part to more sensitive detection and diagnosis of what is usually an indolent disease. However, this data also suggests a true biological rise as evidenced by the concurrent rise in incidence and incidence-based mortality amongst larger [papillary thyroid carcinomas],” wrote the researchers, adding that future research should investigate potential contributors and risk factors associated with this trend.

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Yan KL, Li S, Tseng CH, et al. Rising incidence and incidence-based mortality of thyroid cancer in California, 2000-2017 [published online March 13, 2020]. J Clin Endocrinol Metab. doi:10.1210/clinem/dgaa121