Background and Methods: After the Fukushima Nuclear Power Plant accident, thyroid ultrasound
screening was carried out on all residents of ages≤18 years at the accident. In order to find the cause for
thyroid cancer prevalence, area dose dependence of childhood thyroid cancer proportion in the first and
second round screening (E-I, E-II), and in 6 years as a whole (E-I+II) was analyzed for external dose
estimated by Fukushima Health Management Survey and effective dose estimated by UNSCEAR. The
results were analyzed by the regression analysis of Microsoft Excel.
Results: Thyroid cancer incidence within 4-6 years and prevalence in 6 years after the accident had tendency
to increase with doses. In OM-model according to Ohira’s map, thyroid cancer prevalence in E-I+II after 6
years showed linear prevalence-effective dose response with positive coefficient 5.6 (CI=3.4-7.9) (cancer
cases / 100,000 / mSv) in the 1.6–5 mSv range. In F-model according to official regional division, thyroid
cancer proportion in E-II and E-I+II showed linear response to UNSCEAR effective dose with R2>0.86 and
p<0.075. This correlation was higher than the correlation between external dose and effective dose with
R2=0.71.
Conclusion: The observed linear area dose relation in 6 years after exposure suggests highly probable
association between childhood thyroid cancer in Fukushima and radiation exposure. UNSCEAR effective
dose was found to give the best fit to linear regression line for E-II and E-I+II, irrespective of models. Linear
dose response for reasonable area division can be a direct evidence for radiation related thyroid cancer
because linear dose response cannot be observed in mass-screening for sporadic thyroid cancers.
Risk of thyroid cancer after the Fukushima nuclear power plant accident
