The catastrophic great east Japan earthquake on March 11, 2011 triggered the explosion of Fukushima Daiichi Nuclear Power Plants. As a result, high volatility fission products including ^129mTe, ¹³¹I, ¹³⁴Cs, ¹³⁶Cs, and ¹³⁷Cs were released into the air and deposited on the ground. Moreover, highly radioactive contaminated water continued to escape from the nuclear plant after 1 year after the accident. Although the impacts of radiation exposure on rates of non-thyroid solid cancers and leukemia in the exposed population is still unclear, strong evidence of an increased risk of developing secondary thyroid cancer was found among children who were exposed to radioactive iodine after the Chernobyl Nuclear Power Plant accident on 26 April 1986 in Ukraine. Ideally, potassium iodide should be prophylactically administered before or within several hours after ¹³¹I exposure. Administration of potassium iodide more than 1 day after exposure has a limited effect and is not recommended unless further exposure is expected [1]. In the case of Fukushima, children were not given prophylactic potassium iodide immediately. Thyroid screening for children who are among the evacuees was started in October 2011 and has gradually increased, and screening for the all citizens who are younger than 18 years will be started in Fukushima prefecture after 2014.