10 Septembre 2016
September 10, 2016
By TERU OKUMURA/ Staff Writer
Researchers have found no correlation between radiation exposure and the incidence rate of thyroid cancer among 300,000 children living in Fukushima Prefecture at the time of the 2011 nuclear disaster.
But the team at Fukushima Medical University, which carried out the study, cautioned that the health of local children should continue to be monitored to be more definitive.
“At the present stage, we have found no evidence pointing to any relationship between doses of external radiation resulting from the nuclear accident and the thyroid cancer rate,” said Tetsuya Ohira, a professor of epidemiology at the university. “But we need to continue to look into the situation.”
The study involves 300,476 children in Fukushima Prefecture who were aged 18 or younger when the Fukushima No. 1 nuclear power plant went into a triple meltdown in March 2011 after the Great East Japan Earthquake and tsunami.
The children underwent the first round of health checks between October 2011 and June 2015.
Of the total, 112 were tentatively diagnosed as having thyroid cancer.
There are two types of radiation exposure: external exposure in which a person is exposed to radiation in the atmosphere, and internal exposure in which a person is exposed through the intake of contaminated food, water and air.
For the study, municipalities in the prefecture were classified into three groups based on the estimate for residents' external exposure. That data was obtained during a prefecture-wide health survey carried out after the disaster occurred.
The first group is a zone where people with an accumulative dose of 5 millisieverts or more represented 1 percent or more of the population there. The second group is a zone where people with an accumulative dose of up to 1 millisievert account for 99.9 percent or more of the population. The third group is a zone that falls into neither of the other two groups.
The scientists looked at the incidence rate for thyroid cancer in each group and concluded there is almost no difference among the groups.
The number of subjects diagnosed with thyroid cancer was 48 per 100,000 people in the first group, 41 in the second group and 36 in the third group.
The finding was similar to a separate survey in which researchers looked into the possible association among 130,000 or so children whose radiation exposure had been estimated.
Hokuto Hoshi, head of a health survey panel set up at the prefectural government after the nuclear disaster, said he will closely follow the results of future studies to offer a more conclusive finding.
“The outcome of the recent study provides one indication in making any overall judgment,” said Hoshi, who also serves as vice chairman of the Fukushima Medical Association. “The study is substantial and we are going to pay attention to the findings of further studies.”