Critical data is missing for two-thirds of children tested for thyroid problems in Fukushima Prefecture, because their families have failed to declare the children's hour-by-hour whereabouts during the immediate post-disaster period.
Investigators need residence records for the four months following March 11, 2011, when radioactive fallout was high from a nuclear disaster and ingested iodine isotopes could have damaged the thyroid glands of growing children.
The records are arduous to complete, but are needed in order to estimate a possible dose. Absence of a dose estimate could make people ineligible for compensation if they later develop serious problems such as thyroid cancer, because of an inability to establish a cause.
By late September, about 100,000 children had their thyroid glands tested because of their relative proximity to the melted-down reactors at the Fukushima No. 1 nuclear power plant, following the Great East Japan Earthquake and tsunami.
The tests targeted those living in districts such as the 13 municipalities near the crippled nuclear plant, and Fukushima city, the prefectural capital. Results for 80,000 tests are now available.
The thyroid gland testing program is run by the Fukushima Medical University. It plans to petition parents and guardians for the missing data, at briefing sessions from October.
Of the 38 test subjects who took more detailed follow-up tests, one was diagnosed with cancer and 27 were found to have benign tumors. But prefectural government officials have said those cases are unlikely to have anything to do with the nuclear disaster because it occurred only 18 months ago.
As it investigates the possible impact of radiation on health, the prefectural government is evaluating likely doses received during the first four months of the nuclear disaster. All residents of Fukushima Prefecture are eligible for that survey.
But Fukushima Medical University officials say only slightly more than 30 percent of the 80,000 children who had thyroid gland tests have received estimates of their external radiation dose.
External doses are calculated on the basis of the individuals' whereabouts in the immediate aftermath of the disaster. The factors include records of airborne radiation levels and the time the individuals spent outdoors.
Examinees are required to fill out detailed 24-hour records for the first two weeks, and the number of hours they spent outdoors each day for the rest of the four-month period.
If Fukushima Prefecture residents develop cancer of the thyroid gland in the future, authorities will determine the likelihood of the disease being linked to the nuclear disaster. They will base the decision on comprehensive assessments of individual doses and any increase in the cancer rate in the wider community.
Radiation damage to the thyroid gland depends largely on the body's internal dose, but knowing external doses helps to inform the general trend in internal conditions, experts said.
When dose data is unavailable, there is no way to tell whether an abnormality in the thyroid gland has anything to do with the nuclear accident.
"Dose estimates are essential for evaluating a causal relationship between disease and radiation in those cases where people unfortunately fall ill and consider applying for compensation," said Saburo Murata, deputy director of the Hannan Chuo Hospital in Osaka Prefecture.
In the past, Murata has helped atomic bomb survivors and nuclear plant workers apply for health compensation.
"I advise people to keep records of any changes in their health conditions and their whereabouts, including from now on," he said.
(This article was written by Yuri Oiwa and Teruhiko Nose.)
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