The precise (anonymized) individual data on where the participants lived, when the screenings were carried out, and when the diagnoses were established, have not yet been released. We requested this information from one of the directors of the Fukushima Medical University to carry out independent investigations and to publish epidemiological studies. Unfortunately, this data was not provided or made public. Note, the concealment of these data is partly due to restrictions outlined in the informed consent by the FHMS participants. Therefore, we estimated the corresponding proxy information from the published screening schedule tables, the number of the municipality-specific cumulative examination participants, and the thyroid cancer cases in each municipality. This data was announced approximately once every three months, which is why we assumed that the date when an individual had been examined can be estimated up to a precision of 90 days. For more accurate analyses, it is required to provide open access to the (anonymized) information as to where any participating person lived and when he or she was examined and diagnosed. With this more precise data, our analyses can easily be replicated. Since our approach inevitably entails spatial-temporal non-differential misclassification, it is likely that ‘true’ effects based on individual location and individual exposed person-time observed will turn out to be stronger and more precise compared to the results reported in the present paper. From this point of view, our quantitative estimates may be considered conservative.
There are few data, if any, which allow estimating validly the internal thyroid exposure after the FDNPP accidents for larger numbers of residents. It is a disadvantage of our study that information on the external dose resulting from the initial plume, the inhalation, and the diet contamination could not be assessed directly. However, such information cannot be obtained by simple physical measurement; exposure measures are rather dependent on various conversion coefficients and many other factors that introduce considerable uncertainty.
It was not possible to analyze the data by comparing the radiation dose-rate between the individuals that developed thyroid cancer and the individuals that remained healthy. In other words, we were not able to carry out a case-control study, which is a powerful epidemiological instrument. The reason is that the geographical locations where the thyroid cancer cases and the healthy participants lived as well as the date when the participants were examined or diagnosed have not yet been made available. The confounding factors age and sex at the municipality level have not been disclosed by the Fukushima Health Management Survey as yet. So, it is not possible to adjust our analyses for age and sex. We are also not aware of representative BMI data and iodine intake statistics at the municipality level that could be used for our study. Nevertheless, we think that our approach and our analyses are valid and important and that our work may motivate more refined and better adjusted analyses in the future.
We suggest an innovative statistical technique to determine the municipality-specific average exposed person-time of the participants in the ’Fukushima Health Management Survey’. The knowledge of the exposed person-time enables the assessment of the association between the radiation dose rate and the thyroid cancer detection rate more precisely than in previous studies. The thyroid cancer detection rate and the radiation dose-rate in the 59 municipalities in the Fukushima prefecture show statistically significant dose-response relationships. The detection rate ratio per μSv/h was 1.065 (1.013, 1.119) based on all data in both examination rounds combined. In the 53 municipalities subjected to less than 2 μSv/h, the detection rate ratio was considerably higher: 1.555 (1.096, 2.206). Therefore, it became evident that the radiation contamination due to the Fukushima nuclear power plant accidents is positively associated with the thyroid cancer detection rate in children and adolescents. This corroborates previous studies providing evidence for a causal relation between nuclear accidents and the subsequent occurrence of thyroid cancer.
We are most grateful to five reviewers for many constructive comments on earlier drafts and for references to lesser known, however important literature.
Hagen Scherb orcid: 0000-0002-2730-5619.
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