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 Table of Contents  
ORIGINAL ARTICLE
Year : 2017  |  Volume : 8  |  Issue : 4  |  Page : 174-179

Thyroid nodule prevalence among young residents in the evacuation area after fukushima daiichi nuclear accident: Results of preliminary analysis using the official data


1 Department of Epidemiology and Preventive Medicine, Kagoshima University Graduate School of Medical and Dental Sciences, Kagoshima, Japan
2 Department of Radiological Sciences, Japan Health Care College, Sapporo, Japan
3 Department of Environment Health, University of Occupational and Environmental Health, Kitakyushu, Japan

Date of Web Publication8-Jan-2018

Correspondence Address:
Dr. Suminori Akiba
Department of Epidemiology and Preventive Medicine, Kagoshima University Graduate School of Medical and Dental Sciences, Kagoshima
Japan
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jrcr.jrcr_42_17

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  Abstract 

Introduction: The nuclear accidents at Fukushima Daiichi Nuclear Power Plant released more than 10 EBq (exabecquerel) of the radionuclides into the atmosphere. A primary health concern after the nuclear accident is the internal exposure of children to radioactive iodines, which are known to accumulate in the thyroid, and to cause neoplasm. Fortunately, studies conducted so far have shown that the thyroid doses from internal exposure to 131I were low, and therefore, any excess risk of thyroid cancer among residents is considered unlikely to be detected in the future. Data and Analysis: Approximately half a year after the accident, the Fukushima Health Management Survey was started. It includes the thyroid screening survey using ultrasonography and a program to estimate the individual radiation dose of residents and evacuees. Results and Discussions: The first-round thyroid survey, which was conducted during the period 2011–2013, covered 300,476 young residents, approximately 82% of residents eligible for the survey, and found thyroid nodules in 3990 examinees. The prevalence of nodules in the evacuation zone was similar to that in the nonevacuation zone. The second-round survey, which was conducted during the period 3–6 years after the accident, detected 3788 participants with thyroid nodules among 270,511 examinees (approximately 71% of eligible residents). The prevalence of thyroid nodules in the evacuation zone was significantly higher than that in the rest of area (relative risk = 1.32; 95% confidence interval = 1.19, 1.45). Conclusion: Further studies are necessary to evaluate the scientific significance of present findings.

Keywords: Accident, Fukushima, nodules, nuclear, thyroid


How to cite this article:
Akiba S, Nandakumar A, Higuchi K, Tsuji M, Uwatoko F. Thyroid nodule prevalence among young residents in the evacuation area after fukushima daiichi nuclear accident: Results of preliminary analysis using the official data. J Radiat Cancer Res 2017;8:174-9

How to cite this URL:
Akiba S, Nandakumar A, Higuchi K, Tsuji M, Uwatoko F. Thyroid nodule prevalence among young residents in the evacuation area after fukushima daiichi nuclear accident: Results of preliminary analysis using the official data. J Radiat Cancer Res [serial online] 2017 [cited 2018 Apr 26];8:174-9. Available from: http://www.journalrcr.org/text.asp?2017/8/4/174/222444


  Introduction Top


On 11 March 2011, earthquake and tsunami hit the Fukushima Daiichi Nuclear Power Plant (FDNPP) of Tokyo Electric Power Company and crippled its reactor units 1, 2, and 3, which were under operation at that time. At 20:50 on that day, residents within a 2-km radius of the FDNPP were told to evacuate. On March 12 morning, the evacuation zone was extended to 10-km radius and was extended further to 20-km radius on that afternoon. The municipalities included in the 20-km radius were the entire area of Futaba, Okuma, and Tomioka, and a part of Tamura, Minamisoma, Naraha, Kawauchi, Namie, and Katsurao. Those municipalities constitute the main parts of the evacuation zone designated by the government. On March 15 morning, a nonoperating reactor at FDNPP, the reactor unit 4 with 1532 fuel rods in its spent fuel pool was also damaged by an explosion. At 11:00 on that day, residents in the 20–30 km radius were instructed to shelter indoors.[1]

The Fukushima nuclear accident released more than 10 EBq (exabecquerel) of the radionuclides into the atmosphere.[2] The most massive radiation leak from the crippled reactors took place on March 15. In the afternoon, the plume went west and then northwest, and contaminated the areas in this direction, including Koriyama, Iitate, Namie, and Kawamata.[3],[4],[5] Until the morning of March 15, a large number of people, including evacuees from the coastal districts of Namie and neighboring municipalities, such as Minamisoma, stayed in the Northwestern part of Namie. In that afternoon, most of them moved to areas further north and/or west. However, such a village-wide evacuation was not conducted in Iitate on that day.

A primary health concern after the Fukushima accident has been the internal exposure of children to radioactive iodines, which are known to accumulate in the thyroid and to cause thyroid neoplasm.[6] In July 2011, the Fukushima Prefecture Government started the Fukushima Health Management Survey. This survey includes the thyroid screening survey using ultrasonography and a program to estimate individual radiation doses of residents. Note that thyroid dose measurements could not be conducted extensively. Only 1200 or so residents underwent direct thyroid dosimetry by the end of April 2011, when 131I decreased to almost undetectable levels due to its short half-life (=8 days). Various nonofficial efforts have also been made to estimate thyroid dose. One of them is the study reported by Hosoda et al.[7] They reported thyroid equivalent dose estimates in Namie, one of the most heavily contaminated district in the evacuation zone, using the results of whole-body counting examinations, which were carried out by the Japan Atomic Energy Agency a few months after the nuclear accident, and the 131I/134Cs activity ratio which were estimated from their own survey. The maximum internal exposure of the thyroid to 131I was estimated to be 18 mSv.[7] Their estimates are similar to those reported by the National Institute of Radiological Sciences (NIRS).[8] Those dose estimates are not large enough to cause a detectable increase of thyroid cancer risk among the exposed population.

In 2013, Professor Yutaka Hamaoka of Keio University analyzed the open-to-public thyroid screening data obtained from the Fukushima Health Management Survey in 30 municipalities in Fukushima Prefecture (Fukushima Prefecture Government. November 12, 2013). He found a statistically significant increase in the frequency of small thyroid nodules (<5 mm) in relation to thyroid dose,[9] including the doses estimated by the NIRS and WHO.[8],[10] The excess prevalence of small thyroid nodule is evident in Namie Town and Iitate Village, and their neighboring areas, which were hit by heavily radioactive plume on March 15 afternoon, 2011. Further studies are necessary to understand the implication of his findings.

In the present study, we conducted a preliminary analysis of thyroid survey data obtained from the first- and second-round surveys to compare the prevalence rates of thyroid nodules in the evacuation zone and the other areas.

Data and analytical methods

Data

The thyroid screening program was conducted to examine all the Fukushima residents aged 18 years or younger as of March 11, 2011. If considered necessary, aspiration biopsy was also conducted. The first-round thyroid survey started in October 2011 and ended at the end of fiscal year (FY) 2013, which was March 2014. In this survey, all the municipalities in Fukushima Prefecture were divided into three groups of municipalities on the basis of geographical location and radionuclide contamination. The surveys were planned to cover one of the three groups in each FY. The survey in the 1st year covered areas with relatively heavy radionuclide contamination. The survey in the 3rd year covered Aizu, Ishikawa, and Higashishirakawa Counties, which are considered least contaminated with radionuclides. The second-round survey was started in April 2014 (the beginning of the FY 2014) and ended at the end of the FY 2016, which ends in March 2017. Currently, the third round survey is going on. In the present study, the data obtained from the first and second round surveys are analyzed.

Expected numbers

Expected prevalence rates: Age-specific expected prevalence rates were calculated using the data obtained by the study conducted in three “control” areas, that is, Nagasaki, Yamanashi, and Aomori Prefectures.[11] This study examined residents aged 3–18 years, and the numbers of examinees for age groups of 3–4, 5–9, 10–14, and 15–18 years were 71, 1092, 1863, and 1339, respectively. Small nodules (with the large axis length 5 mm or shorter) detected among those four age groups numbered 0, 4, 12, and 12, respectively. Larger nodules (with the large axis length longer than 5 mm) were found in 0, 3, 16, and 24 examinees of those age groups, respectively. From statistical analysis using those data, the following statistical models with maximum likelihood estimates were obtained: the expected prevalence for small thyroid nodules = exp (−7.805) x age1.091; and the expected prevalence for larger thyroid nodules = exp (−10.61) x age2.322.

Ages at the time of examination

The average age at the time of accident for each municipality was calculated using the proportions of four age groups (ages 0–5, 6–10, 11–15, and 16–18). Average ages at examination for the first-round survey were calculated by adding 0.75, 1, and 2.5 year to average ages at the time of accident for the municipalities where examinations were conducted in the FYs 2011, 2012, and 2013, respectively. For the second survey, we calculated average ages at examination, using four age groups at the time of examination presented in the official report.

Relative risk

Area-specific relative risks (RRs) were estimated using observed and expected numbers of nodule cases described above. RRs and 95% confidence intervals were obtained from Poisson regression analysis, using AMFIT. In the analysis using AMFIT, person-years were replaced by expected numbers of cases.[12] The evacuation zone includes the following municipalities: Iitate, Namie, Minamisoma, Kawamata, Katsurao, Tamura, Hirono, Okuma, Naraha, Tomioka, Futaba, and Kawauchi.


  Results Top


In the first-round survey, 300,476 young residents (151,683 males and 148,793 females) underwent the thyroid survey. This is approximately 82% of residents eligible for the survey. In the second-round survey, examinees numbered 270,511 (approximately 71% of eligible subjects). In this survey, thyroid nodules were detected in 3990 examinees. In addition, 113 participants were found to have carcinomas or suspected to have carcinomas. In the present study, thyroid cancer risk was not analyzed. [Table 1] presents the ratios of prevalence proportions between the evacuation and nonevacuation zones. The prevalence of small nodules in the evacuation zone was significantly increased when compared to that in the nonevacuation zone. On the other hand, the prevalence of large nodules in the evacuation zone was significantly decreased than that in nonevacuation zone. When small and larger nodules were combined, the risk ratio was close to the unity.
Table 1: Thyroid nodule prevalence in the first-round survey

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In the second-round survey, which was conducted during the period 3–6 years after the accident, thyroid nodules were detected in 3788 of examinees. In addition, 71 participants were found to have carcinomas or suspected to have carcinomas. [Table 2] shows that risk ratios for small and larger nodules were significantly larger than the unity.
Table 2:Thyroid nodule prevalence in the second-round survey

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When municipality-specific prevalence in the evacuation zone was examined, larger nodule prevalence in Iitate was higher than the other areas, whereas the small nodule prevalence showed no evident area difference. Note, however, a lack of statistical power in municipality specific analysis made it difficult to evaluate the area distribution of nodule prevalence in the evacuation zone.


  Discussion Top


The second-round survey, which was conducted during the period 3–6 years after the accident, found a significantly higher prevalence of thyroid nodules among examinees in the evacuation zone while the first-round survey did not show such an increase. It should be pointed out that the involvement of factors other than radiation exposure may be an explanation for the elevated prevalence in the evacuation zone. For example, dietary habits and other lifestyles are suspected to be involved in the development of thyroid disorders, including nodules.[13],[14] Note, however, that iodine deficiency, which is a well-known risk factor of thyroid nodules,[15] is unlikely an explanation since Japan is known for its high iodine intake. Anyway, it is difficult to tell whether nonradiation factors can explain the development of the area difference 3 years after the accident.

In the first-round survey, the evacuation zone appeared to have an elevated prevalence of small nodules as pointed out by Hamaoka.[11] Note, however, that the first-round survey was conducted within 1 year after the accident in most of the municipalities in the evacuation zone. Thyroid nodule development in such an early period after exposure is not known in the literature. For example, a survey conducted 4 years after the Chernobyl accident found no increase of thyroid nodule in the fallout areas.[16]

It should be of note that the sensitivity of thyroid screening tests using ultrasonography has increased markedly since the early 1990s, when Chernobyl surveys were conducted, thanks to the availability of high-resolution thyroid ultrasonography. A scanning device with a transducer producing an ultrasound frequency as high as 10 MHz is known to be capable of detecting nodules as small as 2 mm.[17] A neoplasm with a diameter of 2 mm is estimated to have approximately one million cells.[18] If a thyroid nodule starts as a single cell and becomes a tumor consisting of one million cells in a year, its doubling time needs to be less than 18 days. It is difficult to tell whether benign neoplasm cells have such a short doubling time or not.

In the first-round survey, the evacuation zone had an excess of small nodules but no increase of all-sized nodules (small and larger nodules together). This finding suggests that long-axis lengths in the evacuation zone were measured to be shorter than those in the nonevacuation zone. Radiation exposure is unlikely to be the cause of such findings.

Administration of stable iodine tablets, after the accidents, is considered effective in reducing internal exposure of the thyroid to radioactive iodine. Note, however, whether stable iodine administration can prevent thyroid cancer development is yet unclear. For example, a Danish study did not show any solid evidence for a protective effect of iodine supplementation on thyroid cancer development.[19]

On March 16, 2011, the Nuclear Emergency Response Local Headquarters sent a notice to the municipality offices in a 20-km radius zone to give stable iodine tablets at the time of evacuation. Before this notice was issued, in several municipalities, stable iodine tablets were distributed to residents: in Tomioka Town on March 12 and in Naraha Town on March 15, to residents <40 years old; in Futaba Town on March 12, to children at evacuation sites. However, its administration was not indicated by the health authority there. Miharu Town was the only municipality where an official decision was made to administer stable iodine to its residents; there, approximately 7000 residents aged <40 years took the stable iodine pill on March 15.[20] In the other areas, some children might have been given stable iodine tablets by local physicians.

After the Fukushima accident, the Japanese government changed its policy regarding stable iodine distribution. Now, it instructs local governments to “predistribute” stable iodine tablets to residents living within a 5-km radius of nuclear facilities.[21]

It is difficult to evaluate the contribution of radioiodines other than 131I. The thyroid dose given by 132I is considered negligible since its half-life of is only 2.3 h. However, when 132Te (half-life: 3.2 days) is present in the lung,132I, the only decay product of 132Te, is provided to the bloodstream continuously; as a result, the de facto half-life of 132I can become long enough to give significant radiation exposure to the thyroid.[22] After the Fukushima accident, 88PBq of 132Te was released into the atmosphere.[23] This amount is only one-tenth of that released into the environment after Chernobyl accident.[24] However, the plume with highly concentrated 132Te might have affected relatively densely populated areas on March 15, 2011.

The findings made in the present study are at variance with what was found in high-background radiation areas in Yangjiang, China and Kerala, India. In those studies, the prevalence of thyroid nodules was not elevated among middle-aged women in the area with high natural background radiation.[25] Relatively low-dose rates in those areas may be an explanation for the difference.

Since thyroid nodule prevalence increases with age and is higher among women,[15] age and sex are potential confounding factors when analyzing thyroid nodule prevalence. In the present study, age adjustment was based on four age categories (ages 0–5, 6–10, 11–15, and 16–18). Although this approach was far from ideal, data using finer age categories were not open to public. Sex adjustment could not be made in this study. It was because area-specific data were not stratified by sex. However, the proportions of men/boys among participants are unlikely to show evident area differences; therefore, it is unlikely for sex to confound the results seriously.

The detection rates of thyroid nodules particularly small nodules, are affected by the skills of examiners and the resolution of ultrasonography devices. The detection of smaller nodules is more likely to be affected by the diagnostic skills of examiners than larger nodules. Using a 7.5-MHz transducer, the Japanese study reported by Hayashida et al.[11] detected small and larger nodules among 1.6% of examinees. Regarding the Fukushima Thyroid Survey, the frequencies of transducers are not reported in the de facto official report made by Suzuki et al.[26] This survey detected thyroid nodules among 1.4% of examinees. Although this figure is slightly smaller than that in the study of Hayashida et al.,[11] there is no reason to doubt that the two surveys used ultrasound devices which has evidently different capability to detect tumors. Note, however, the Fukushima survey does not have a system to assure the sensitivity of nodule detection.

The clinical significance of detecting thyroid nodules using ultrasonography is doubtful since most of nodules do not pose any health risk. Russ et al.[27] recommended that nodules <10 mm among those younger than 35 years of age do not require further evaluation. In the official report of the Fukushima Thyroid Survey, the number of nodules with their long axis longer than 10 mm is not described.

In the first-round and second-round surveys, 113 and 71 participants were found to have carcinomas (or suspected to have carcinomas), respectively. In the present study, thyroid cancer risk was not analyzed. Suzuki et al.[26] reported that the prevalence of thyroid carcinoma found in the first-round survey did not show any significant area differences.


  Conclusion Top


In summary, the thyroid nodule prevalence observed in the evacuation zone was significantly higher than that in the nonevacuation zone in the second-round survey while such an increase was not observed in the first-round survey. Further studies are necessary to evaluate those findings in relation to radiation exposure.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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