Increase In Thyroid Cancer Prevalence In Fukushima After The Nuclear Disaster In 2011 ñ A Potential Overdiagnosis?

KAMO K. ² , TSUGANE S. ¹

A thyroid ultrasound examination program has been conducted in Fukushima Prefecture, Japan after the nuclear disaster in 2011. Though high prevalence of thyroid cancer was observed, relevant quantitative evaluation was not conducted. We aimed to calculate the observed/expected (O/E) ratio of thyroid cancer prevalence for the residents aged ≤20 years. Observed prevalence was the number of thyroid cancer cases detected by the program through the end of April, 2015. Expected prevalence was calculated as cumulative incidence by a life-table method using the national estimates of thyroid cancer incidence rate in 2001-2010 (prior to the disaster) and the population of Fukushima Prefecture. The observed and estimated prevalence of thyroid cancer among residents aged ≤20 years was 160.1 and 5.2, respectively, giving an O/E ratio of 30.8 (95% confidence interval (CI): 26.2, 35.9). When the recent increasing trend in thyroid cancer was considered, the overall O/E ratio was 22.2 (95% CI: 18.9, 25.9). The cumulative number of thyroid cancer deaths in Fukushima Prefecture, estimated with the same method (annual average in 2009-2013), was 0.6 under age 40. Combined with the existing knowledge about radiation effect on thyroid cancer, our descriptive analysis suggests the possibility of overdiagnosis. Further evaluation including individual-level analysis is required to clarify the contribution of underlying factors.