Do US thyroid cancer incidence rates increase with socioeconomic status among people with health insurance? An observational study using SEER population-based data

OBJECTIVES The US thyroid cancer incidence rates are rising while mortality remains stable. Trends are driven by papillary thyroid cancer (PTC), the predominant cancer subtype which has a very good prognosis. We hypothesised that health insurance and high census tract socioeconomic status (SES) are associated with PTC risk. DESIGN Relationships between thyroid cancer incidence, insurance and census tract SES during 2007-2010 were examined in population-based cancer registries. Cases were stratified by tumour histology, size and demography. SETTING Surveillance, Epidemiology, and End Results (SEER) registries covering 30% of the US population. RESULTS PTCs accounted for 88% of incident thyroid cancer cases. Small PTCs (≤ 2 cm) accounted for 60% of cases. Unlike non-PTC cases, the majority of those diagnosed with PTC were <50 years of age and had ≤ 2 cm tumours. Rate ratios (RR) of PTC diagnoses increased monotonically with SES among fully insured cases. The effect was strongest for small PTCs, high-SES versus low-SES quintile RR=2.7, 95% CI 2.6 to 2.9, two-sided trend test p<0.0001. For small PTC cases with insurance, the monotonic increase in incidence rates with rising SES persisted among cases younger than 50 years of age (RR=3.3, 95% CI 3.0 to 3.5), women (RR=2.6, 95% CI 2.5 to 2.8) and Caucasians (RR=2.5, 95% CI 2.4 to 2.7). Among the less than fully insured, rates generally decreased with increasing SES. CONCLUSIONS The >2.5-fold increase in risk of PTC diagnosis among insured individuals associated with high SES may be informative with respect to the contemporary issue of PTC overdiagnosis.