Abstract
Background: The incidence of papillary thyroid cancer, including advanced-stage and larger tumors, increased markedly in the United States, and obesity is now recognized as an important risk factor. We evaluated the extent to which trends in papillary thyroid cancer incidence rates in the United States have been impacted by changes in the prevalence of overweight and obesity. Methods: We used data from the NIH-AARP Diet and Health Study (participants aged ≥50 years) to estimate hazard ratios (HRs) for papillary thyroid cancer (overall and by stage and size of the tumor at diagnosis) by categories of body mass index (BMI), adjusted for demographic (e.g., age, sex, race/ethnicity, education) and other lifestyle-related (alcohol intake, smoking) factors with Cox regression models. Population attributable fractions (PAFs) for overweight/obesity (assuming a 10-year latency period) were calculated using the estimated HRs, overweight/obesity prevalence estimates derived from annual National Health Interview Surveys, and cancer incidence data from the Surveillance, Epidemiology, and End Results (SEER)-13 Program. Results: During follow-up of the NIH-AARP Study, 604 papillary thyroid cancers were identified. Compared to individuals who were normal-weight (18.5-24.9 kg/m(2)), those who were overweight (25.0-29.0 kg/m(2)) or obese (30+ kg/m(2)) experienced a 26% (HR=1.26, 95% CI 1.05-1.53) and 30% (HR=1.30, 95% CI 1.04-1.61) increased risk of papillary thyroid cancer and a nearly 3-fold (HR=2.93, 95% CI 1.25-6.87) and >5-fold (HR=5.42, 95% CI 2.24-13.1) increased risk of large (4+ cm) papillary thyroid cancers. Between 1995 and 2015, the PAF for overweight/obesity increased from 12 to 16% for all papillary thyroid cancers and 52 to 63% for large papillary thyroid cancers. Overweight/obesity accounted for 6% and 37% of the increase in incidence rates for total and large papillary thyroid cancers, respectively. In the absence of overweight/obesity, we would have observed an estimated 0.84, 2.1, and 3.4 fewer papillary thyroid cancers per 100,000 individuals in 1995, 2005, and 2015, respectively. Conclusions: The rising prevalence of overweight and obesity in the United States contributed to the increase in papillary thyroid cancer incidence rates from 1995 to 2015. By 2015, one of every 6 papillary thyroid cancers (and nearly two-thirds of large papillary thyroid cancers) was attributed to overweight or obesity.