Systematic Review of Trends in the Incidence Rates of Thyroid Cancer

BACKGROUND: A large proportion of global increase in thyroid cancer (TC) incidence has been attributed to increased detection of papillary thyroid cancer (PTC). Nonetheless, some reports support a real increase in incidence. We aimed to perform a systematic review to evaluate the changing trends in thyroid cancer incidence and summarise potential risk factors predisposing to this trend. METHODS: English language literature published between 1980 and August 2014 was searched via PubMed (MEDLINE) and OvidSP (EMBASE). Original studies on changes in thyroid cancer incidence in defined geographic areas that described clear methods of case selection and population estimates were included. Data on incidence rates and risk factors were collected. RESULTS: Of 4719 manuscripts, 60 studies were included; 31 from Europe, 13 from North America and the rest from Asia (9), Oceania (4) and South America (3). 53 articles reported a significant increase in incidence (highest was a 10 fold increase in South Korea), 6 reported stable rates and 1 noted a decrease. PTC was the commonest type reported to have increased in incidence (in 10 studies with relevant data). Follicular thyroid cancer (FTC) increased in incidence (in 4 studies); albeit at a lower rate compared to PTC. Data on risk factors was sparse; factors discussed included ionising radiation, iodine deficiency and supplementation. CONCLUSION: This systematic review strongly supports a widespread and persistent increase in thyroid cancer incidence. Evidence for over-detection of PTC as the predominant influence includes increased numbers of smaller size tumours and improved or unchanged survival.