International and subnational variation thyroid cancer incidence and mortality over 2008–2012

Abstract

Introduction Thyroid cancer incidence varies widely by country, but has been reported to increase dramatically since the 1980s in several countries. Meanwhile, thyroid cancer mortality has remained low and stable or even declined. We present the most up to date comprehensive international report on thyroid cancer incidence and mortality. Methods Thyroid cancer (ICD-10 C73) cases for 2008–2012 were extracted from population-based cancer registries in Cancer Incidence in Five Continents (CI5) volume XI ( http://www.ci5.iarc.fr/CI5-XI ) from the International Agency for Research on Cancer. Data from regional registries were combined to obtain a proxy of the national incidence. In total, 56 national or combined regional registries covering at least 0.5 million inhabitants aged 15–74 were selected. National thyroid cancer deaths for 45 of the selected countries were available and were extracted from the WHO mortality database ( http://www.who.int/healthinfo/mortality_data/en ), for the corresponding period. Age-standardized incidence and mortality rates (ASR) were computed by sex, for ages 15 to 74, using the World Standard population. Incidence ASRs were compared between countries and between regional registries within the same country; mortality ASRs could only be compared between countries. Results Incidence ASRs varied more than 100-fold among the selected countries and were most elevated in some of the highest resource counties worldwide including eastern and western Asia, Northern America, Oceania, and Europe. The Republic of Korea reported, by far, the largest incidence ASRs in both men and women (26.8 new cases and 130.6 new cases per 100,000, respectively). Meanwhile, the lowest incidence ASRs were observed in Africa: 0.2 new cases per 100,000 men in Zimbabwe, Harare and 0.9 new cases per 100,000 women in South Africa, Eastern Cape. In all countries, incidence ASRs were always higher in women than in men. In contrast, mortality ASRs were very low and varied very little between countries: from 0.1 deaths per 100,000 in Saudi Arabia to 0.9 deaths per 100,000 in Philippines among men and from 0.2 deaths per 100,000 in Cyprus to 1.6 deaths per 100,000 in Philippines among women. Within countries differences in incidence ASRs were substantial, with 10-fold differences in some countries. For instance, among women, incidence ASRs of 9.3 in Brazil, Pocos de Calda vs. 97.8 in Florianopolis were observed; 1.2 in both India, Puna and Tripura (a mountainous state with 60% of its land covered by forest) vs. 12.8 in the urban district of Kamrup; 3.7 in Canada, Yukon vs. 34.7 in Ontario. Conclusions The previous observation of very high rates in some high-resource countries is still valid, but now extends to other countries, or some regions of countries with lower resources. Notwithstanding the progress in thyroid cancer treatment over time, the stable and extremely low mortality rates together with the stark variations in incidence within the same country (particularly higher in more urban regions) suggest that the prevalence of thyroid cancer risk factors is not the only driver for the very high observed incidence rates. Rather, the inflated number of new cases could be due to increased detection of tumors that would otherwise cause no symptom or death if left undetected and untreated–also known as overdiagnosis. Notably, incidence ASRs seem to now also be high in some regions of countries in socioeconomic transition (e.g. Brazil and India). This calls for revised guidelines for diagnostic procedures to avoid unnecessary harm in a growing number of populations.