When one reviews the comments of the Director General report of IAEA in Aug 2015 to the Tsuda et.al's study, there are valid issues noted which will need to be addressed in the future by Tsuda et.al regarding children living near the Fukushima nuclear meltdown affected by thyroid cancers at a rate 20 to 50 times when compared to children elsewhere .
Highly sensitive equipment is being used, which has detected asymptomatic thyroid abnormalities among a significant number of surveyed children (which would not have been detectable by clinical means). The abnormalities identified in the survey are unlikely to be associated with radiation exposure from the accident and most probably denote the natural occurrence of thyroid abnormalities in children of this age.
The incidence of thyroid cancer in children is the most likely health effect after an accident involving significant releases of radioiodine. Because the reported thyroid doses attributable to the accident were generally low, an increase in childhood thyroid cancer attributable to the accident is unlikely.
The conclusions of the study by Tsuda Toshihide et.al. in Oct 2015 mentions that "An excess of thyroid cancer has been detected by ultrasound among children and adolescents in Fukushima Prefecture within 4 years of the release, and is unlikely to be explained by a screening surge." The locations of nuclear reactors in Japan can be found here.
In conclusion, among those ages 18 years and younger in 2011 in Fukushima Prefecture, approximately 30-fold excesses in external comparisons and variability in internal comparisons on thyroid cancer detection were observed in Fukushima Prefecture within as few as 4 years after the Fukushima power
plant accident. The result was unlikely to be fully explained by the screening effect. In Chernobyl, excesses of thyroid cancer became more remarkable 4 or 5 years after the accident in Belarus and Ukraine, so the observed excess alerts us to prepare for more potential cases within a few years.
Exposure doses for residents were higher than the official report or the dose estimation by the World Health Organization, because the number of thyroid cancer cases grew faster than predicted in the World Health
Organization’s health assessment report.
Survey Methodology: The Fukushima Prefectural Government had examined 298,577 children under age 18 in 2011 out of a population of 367,687 and surveyed them again in 2014. The children's thyroid gland was examined through the use of ultrasound and the results were that 110 have been diagnosed with or are suspected of having thyroid cancer.
Several limitations for the study mentioned in the question are recorded in the paper by the authors themselves.
Limitation #1: Coverage of the screening program and secondary examination
did not include all eligible residents of Fukushima at the
time of the 2011 accident. This may induce overestimation in internal comparisons when prevalence of the 2013 fiscal year districts was used as a reference.
Limitation #2: The effect of the length of time elapsed between the accident and timing of screening should be considered further.
Limitation #3: Before the accident, no evidence existed that natural radiation was higher in Fukushima Prefecture than in the rest of Japan.
Experts listed below state several reasons which are skeptical of the conclusions of Tsuda et.al. study.
- Per IAEA in August 2015, "The screening has detected asymptomatic thyroid abnormalities — nodules, cysts and cancers — that would have gone undetected if asymptomatic children had been screened using standard equipment. Similar results were obtained when the same screening was carried out on children living far away from the areas affected by the accident."
These factors suggest that the thyroid abnormalities detected in the survey are unlikely to be associated with radiation exposure due to the accident. On the basis of the data made available on indirect measurements of external dose equivalent due to activity in the thyroid, thyroid equivalent doses in children appear to have been low. For the levels of doses reported, increases in thyroid cancer in children would not be attributable to
- Geraldine Thomas a specialist in the molecular pathology of cancer at Imperial College London who also helped establish the Chernobyl Tissue Bank, which analyses samples from people exposed to radiation after the nuclear disaster in 1986 disputes the Tsuda study's findings.
The analysis incorrectly compares the screening in Fukushima to clinical cases of thyroid cancer in which patients are already sick. The comparison falsely suggests thyroid cancer in Fukushima is elevated by as much as 50 times compared with the general population. "This is not a very good paper to be basing opinions on," she says. More accurate comparisons between residents within different parts of Fukushima prefecture show no statistically significant variations in cancer rates, she says.
- David Brenner director of the Columbia University's Center for Radiological Research director also disputes the study's findings saying that the study makes no effort to trace the exposure of patients.
"It's simply relating geographic regions to cancer risks and not looking at individual radiation doses," he says, adding that without that information, it's virtually impossible to connect the screenings to the accident. "It really doesn't tell us the whole story," he says.
Scott Davis, professor at the Department of Epidemiology in the Seattle-based School of Public Health said that the key limitation of Tsuda's study is the lack of individual-level data to estimate actual radiation doses.
Another study by Hajime Watanobe et.al. in Dec 2014 found no confirmation of any discernible deleterious effects of the emitted radioactivity on the thyroid of young Fukushima residents at 20-30 months after the accident. However, the sample size of 1137 subjects was relatively small when compared to Tsuda's study.
The results obtained revealed no discernible deleterious influences of the emitted radioactivity on the young thyroid. In addition, we did not find any significant relationship between the thyroid ultrasonographic findings and thyroid-relevant biochemical markers.
- Referring to John D Boice Jr in 2012, "estimates to date of population doses suggest very low uptakes of radioactive iodine which was a major determinant of the epidemic of thyroid cancer following childhood exposures around Chernobyl."