Correlations between Thyroid Radiation Dose and Thyroid Cancer Risk.

Abstract

After the atomic bombings in 1945, the incidence of thyroid cancer increased among residents of Hiroshima and Nagasaki, Japan. Similarly, the incidence of thyroid cancer substantially increased among the exposed children or adolescents in Belarus, Ukraine, and the four other affected regions of the Russian Federation after the Chernobyl disaster. This trend has been maintained even 20 years after exposure, with 5127 cases (among those aged < 14 years in 1986) and 6848 cases (among those aged < 18 years in 1986) of thyroid cancer have been reported between 1991 and 2005 in Belarus, the Ukraine, and the affected regions in the Russian Federation.

The average thyroid radiation dose, mainly due to consumption of milk contaminated with I-131 during the first few weeks following the accident, was estimated be approximately 490 mGy. There is no doubt that this increased exposure to radioiodine due to the Chernobyl disaster has substantially contributed to the increase in the incidence of thyroid cancer. Therefore, the possible health risks in the evacuees of the Fukushima disaster is of concern, particularly those of infant thyroid cancer due to the release of radioactive iodine 131 _{I into the environment. Local health authorities measured thyroid activity of 1149 children} aged <15 years in Iwaki City, Kawamata Town, and Iitate Village, Japan, from March 24–30 in 2011. The maximum dose was considered to be equivalent to a thyroid dose of 35 mSv. 131 _{I activity measurements in the thyroids of residents and evacuees in Namie Town during} the period from April 12–16 were also conducted. The Tsushima District of Namie Town is located within a 30-km radius of the reactor. ¹³¹I activity in the thyroids of 46 out of the 62 residents and evacuees were measured. The median thyroid equivalent dose was estimated to be 4.2 mSv and 3.5 mSv in children and adults, respectively. The maximum thyroid doses for children and adults were 23 mSv and 33 mSv, respectively. Considering the relatively low levels the present estimated thyroid gland equivalent radiation dose, an increased in the incidence of thyroid cancer among Fukushima disaster survivors is less likely.