Increasing thyroid cancer incidence in Queensland, Australia 1982-2008 - true increase or overdiagnosis?

Background Thyroid cancer incidence has been increasing worldwide. Some suggest greater ascertainment of indolent tumours is the only driver, but others suggest there has been a true increase. Increases in Australia appear to have been among the largest in the world, so we investigated incidence trends in the Australian state of Queensland to help understand reasons for the rise. Methods Thyroid cancers diagnoses in Queensland 1982-2008 were ascertained from the Queensland Cancer Registry. We calculated age-standardized incidence rates (ASR) and used Poisson regression to estimate annual percentage change (APC) in thyroid cancer incidence by socio-demographic and tumour-related factors. Results Thyroid cancer ASR in Queensland increased from 2.2 to 10.6/100 000 between 1982 and 2008 equating to an APC of 5.5% [95% confidence interval (CI) 4.7-6.4] in men and 6.1% (95% CI 5.5-6.6) in women. The rise was evident, and did not significantly differ, across socio-economic and remoteness-of-residence categories. The largest increase seen was in the papillary subtype in women (APC 7.9%, 95% CI 7.3-8.5). Incidence of localized and more advanced-stage cancers rose over time although the increase was greater for early-stage cancers. Conclusion There has been a marked increase in thyroid cancer incidence in Queensland. The increase is evident in men and women across all adult age groups, socio-economic strata and remoteness-of-residence categories as well as in localized and more advanced-stage cancers. Our results suggest 'overdiagnosis' may not entirely explain rising incidence. Contemporary aetiological data and individual-level information about diagnostic circumstances are required to further understand reasons for rising thyroid cancer incidence.