Report of the Research on Thyroid Gland Examinations, etc. of Workers at the TEPCO Fukushima Daiichi Nuclear Power Plant Has Been Released
FY2013 Health and Labour Science Research Grants Health and Labour Science Special Research Program
A report was compiled and released regarding the Research on Thyroid Gland Examinations, etc. of Workers at the TEPCO Fukushima Daiichi Nuclear Power Plant (chief researcher: Tomotaka Sobue (Professor, Environmental Medicine and Population Sciences, Graduate School of Medicine, Osaka University)).
This research, funded by the Health and Labour Science Research Grants, aims to epidemiologically analyze radiation effects on the thyroid gland by setting an exposed group (emergency workers exposed to radiation exceeding a thyroid equivalent dose* of 100 mSv) and a control group (thyroid equivalent dose of 100 mSv or less), performing an ultrasonic examination for both groups and comparing the results. The results of the analysis are to be evaluated from the viewpoint of clinical medicine in terms of radiation effects on the thyroid gland.
* Thyroid equivalent dose: Dose only focusing on thyroid exposure, which is calculated as the total of the internal exposure and the external exposure (including exposure prior to the accident); Thyroid equivalent dose is 1/20 in the case that it is converted to the whole-body exposure dose(effective dose)
Summary of the Research Results
- This is an interim report based only on the ultrasonic examination, prepared before definite diagnoses based on the detailed examination have become available. It would be dangerous to draw a conclusion based only on the results of this research due to bias in selection of study subjects and uncertainties resulting from estimation of their exposure doses.
- According to the research results, the percentage of workers who had the experience of receiving ultrasonic examinations was high for the exposed group (56.9% against 5.6% for the control group) while the percentage of workers who received this screening was low for the exposed group (31.8%; 62.0% for employees of TEPCO and 2.2% for employees of contractors). This suggests the possibility of a considerable bias in the incidence of cysts and nodules among workers with high doses.
- In other words, there is a possibility that workers for whom a secondary examination was judged unnecessary (level A2) in their previous ultrasonic examinations selectively participated in this research. Or, workers to whom a secondary examination was recommended (level B) or for whom a secondary examination was judged necessary (level C) in their previous ultrasonic examinations might have selectively dropped out of this research.
- With regard to workers whose internal exposure evaluation results are considered to be less reliable (those classified into reliability level C or D), quantitative evaluation of their internal exposure should be conducted carefully.
- Efforts should be made to collect and analyze the results of the detailed examinations for study subjects in whom an abnormality was detected in this screening, and also to collect and analyze the results of previous thyroid gland ultrasonic examinations for the exposed group.
- The results of the ultrasonic examinations (FY2012 on a voluntary basis) and the secondary examinations (detailed examinations; FY2012 and FY2013) have yet to be collected.
- A notice recommending a detailed examination was sent to study subjects to whom a secondary examination was recommended (level B) or for whom a secondary examination was judged necessary (level C), with a referral form to medical institutions where they can receive a detailed examination.
- Based on the above and the comprehensive judgment, it is found that there is no difference in the percentage of those to whom a secondary examination was recommended (level B) or for whom a secondary examination was judged necessary (level C) between the exposed group and the control group, without any correlation with thyroid equivalent doses. On the other hand, the percentage of those for whom a secondary examination was judged unnecessary (level A2) was relatively high for workers with high doses, and the same tendency was observed in the analysis using re-evaluated thyroid equivalent doses.
- For those workers for whom a secondary examination was judged unnecessary (level A2), a notice to that effect was sent.
- While no correlation was found between nodule size and thyroid equivalent dose, the research suggested that the incidence of relatively larger cysts* was high for workers with high doses.
* Cysts themselves do not need to be treated. However, as a large cyst may cause symptoms in the neck, any cyst of 20.1mm or larger was as level B (only one case).
- Outline of the Report PDF 373KB
- Summary Report (including a supplement) PDF 219KB
- Tables PDF 245KB