Cari M. Kitahara, PhD, MHS

THE REPORTED INCIDENCE of thyroid cancer has markedly increased in the United States in recent decades, largely reflecting an increase in reported rates of papillary thyroid cancer. It has been unclear whether the increase in incidence is reflected in mortality rates over time. In a study of Surveillance, Epidemiology, and End Results 9 (SEER 9) data reported in JAMA, Hyeyeun Lim, PhD, of the National Cancer Institute, and colleagues found that papillary thyroid cancer mortality has increased along with disease incidence, suggesting that the reported increase in overall incidence is a true increase in addition to overdiagnosis involving small indolent tumors.¹ 

Cari M. Kitahara, PhD, MHS, of the Division of Cancer Epidemiology and Genetics, National Cancer Institute, is the corresponding author for the JAMA article. 

Study Details 

IN THE STUDY, data from the SEER 9 cancer registry program were used to analyze trends in thyroid cancer incidence from 1974 to 2013 and incidence-based mortality from 1994 to 2013. (The SEER 9 registries are Atlanta, Connecticut, Detroit, Hawaii, Iowa, New Mexico, San Francisco–Oakland, Seattle–Puget Sound, and Utah.) 

Analysis of incidence-based mortality used information from the SEER 9 cancer registry incidence file to link characteristics of the cancer at diagnosis with death certificate information. Rates were calculated as the number of cases or deaths per 100,000 person-years and age-adjusted to the 2,000 U.S. standard population. Annual percentage changes in rates were calculated using log-linear regression. 

Incidence Trends 

AMONG THE TOTAL of 77,276 patients in the registry diagnosed with thyroid cancer, mean age was 48 years, 75% were women, and 82% were white. The most common histologic types were papillary thyroid cancer (84%) and follicular thyroid cancer (11%). 

The incidence of thyroid cancer increased from 4.56 per 100,000 person-years in 1974 to 1977 to 14.42/100,000 person-years in 2010 to 2013, representing an average annual increase of 3.6% (95% confidence interval [CI] = 3.2%–3.9%, P < .001). Incidence rates increased by 6.7% per year (95% CI = 6.1%–7.2%, P < .001) during 1997 to 2009, with no significant increase observed during 2009 to 2013 (annual percentage change = 1.8%, 95% CI = −0.7% to 4.4%, P = .20). From 1997 to 2013, incidence rates increased for all sex, race, and age groups. 

Annual increases were 4.4% (95% CI = 4.0%–4.7%, P < .001) for papillary thyroid cancer (with increases by stage of 4.6% for localized, 4.3% for regional, 2.4% for distant, and 1.8% for unknown), 0.6% (95% CI = 0.2%–0.8%, P < .001) for follicular thyroid cancer, and 0.7% (95% CI = 0.2%–1.1%, P = .005) for medullary thyroid cancer. For 2009 to 2013, incidence rates did not increase for overall, localized, stage I, or small (≤ 2 cm) papillary thyroid cancers, but there was no evidence of reduction in the increase in regional, distant, or large papillary thyroid cancers. 

“Among patients in the United States diagnosed with thyroid cancer from 1974 to 2013, the overall incidence of thyroid cancer increased 3% annually, with increases in the incidence rate and thyroid cancer mortality rate for advanced-stage papillary thyroid cancer.”

— Hyeyeun Lim, PhD, and colleagues

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Incidence-Based Mortality 

INCIDENCE-BASED MORTALITY rates were underestimated in the earlier years included in the study but were consistent with observed SEER 9 mortality rates during 1994 to 2013. A total of 2,371 deaths from thyroid cancer occurred, with 1,063 from papillary thyroid cancer. Incidence-based mortality increased from 0.40/100,000 person-years in 1994 to 1997 to 0.46/100,000 person-years in 2010 to 2013, yielding an average annual percentage increase of 1.1% (95% CI = 0.6%–1.6%, P < .001). Positive annual percentage changes were observed for most demographic subgroups and were statistically significant among female (1.2%, P = .01), white (0.9%, P = .01), and black patients (3.8%, P = .04) as well as patients diagnosed after age 79 years (1.3%, P = .002). 

Among subtypes, the annual increase was limited to patients with papillary thyroid cancer (1.7%, P = .01). Among papillary thyroid cancer cases, positive annual percentage changes were observed for all stages at diagnosis and were statistically significant for patients with distant disease (2.9%, P = .003), stage IV disease (12.9%, P < .001), or both, and among those with tumor size ≤ 2 cm (6.8%, P = .004) and > 2 to ≤ 4 cm at diagnosis (4.3%, P = .01). 

The investigators concluded: “Among patients in the United States diagnosed with thyroid cancer from 1974 to 2013, the overall incidence of thyroid cancer increased 3% annually, with increases in the incidence rate and thyroid cancer mortality rate for advanced-stage papillary thyroid cancer. These findings are consistent with a true increase in the occurrence of thyroid cancer in the United States.” ■

DISCLOSURE: The study was supported by the Intramural Research Program of the National Cancer Institute. For full disclosures of the study authors, visit jama. jamanetwork.com. 

REFERENCE 

1. Lim H, Devesa SS, Sosa JA, et al: Trends in thyroid cancer incidence and mortality in the United States, 1974-2013. JAMA 317:1338-1348, 2017.