23 Mars 2015
March 23, 2015
By YURI OIWA/ Staff Writer
A team of researchers has developed a method to more precisely estimate the doses of radiation to the thyroid glands of people who received medical examinations shortly after the Fukushima nuclear crisis unfurled.
The method came after researchers looked at the radiation exposure records for about 500 people within about a week after the accident in March 2011 and their daily whereabouts.
The team, led by researchers at International University of Health and Welfare in Otawara, Tochigi Prefecture, hopes that the new calculation method will also help estimate doses of radiation to thyroid glands of another 42,000 people who received radiation checkups at evacuation centers.
Gen Suzuki, the director of International University of Health and Welfare Clinic, who heads the team, said it may be used to determine the relationship between thyroid cancer among Fukushima residents and their exposure to radioactive iodine.
“The lack of a reliable estimate on the level of internal exposure in their thyroids, together with the lack of knowledge about the causal relationship between thyroid cancer and exposure to radioactive iodine, is a main cause of anxiety among Fukushima residents,” Suzuki said.
Internal exposure comes from mainly absorbing radioactive materials into the body through consuming contaminated food and other means, whereas external exposure comes from outside sources of radiation.
Radioactive iodine accumulates in the thyroid and causes internal exposure. It is suspected to be a cause of thyroid cancer. But there is almost no data on the amount of such radiation exposure among residents, with the material having a short half-life of around eight days.
To assess the amount of exposure of each resident to radioactive iodine, the team used records of external radiation exposure checkups conducted on a total of 42,500 people at Fukushima Medical University and evacuation centers within about a week from the accident.
The records have not been used for the purpose because the simple test did not specify the radioactive materials they were exposed to.
However, the team discovered that a portion of about 500 people who took an additional test at the medical university have results that specify each radioactive material, including radioactive iodine, and its rate of external exposure.
By adding up the total amount of radiation exposure and the rate of each radioactive material, the researchers say that it is possible to estimate the total amount of radioactive iodine in the environment around each subject.
Thus, the researchers say they can assess the amount of radioactive iodine taken in by each person and the level of internal exposure caused by the material.
The 500 examinees also have detailed records of what they did and where they went in the wake of the accident at the Fukushima No. 1 nuclear power plant.
The researchers said they can estimate the amount of radioactive iodine taken into the body of residents who do not have a record on the percentage of radioactive material for their external exposure by closely examining their daily accounts following the accident.
They said they will also investigate if the remaining 42,000 people who took the test at evacuation centers have similar daily accounts to assess the amount of their internal exposure through radioactive iodine.
Of medical tests on about 385,000 people in Fukushima Prefecture who were 18 years or younger when the nuclear disaster started, 87 have been found to be suffering from thyroid cancer by the end of 2014.