Introduction

On March 11, 2011, the Fukushima Dai-ichi Nuclear Power Plant in Fukushima Prefecture was damaged by the Great East Japan Earthquake and a radioactive plume was dispersed into the atmosphere. After the accident, the Fukushima prefectural government started conducting the Fukushima Health Management Survey to evaluate the initial external dose of radiation exposure and to monitor the health conditions of local residents who were likely to have been heavily influenced by the accident [1]. One of the detailed surveys conducted in Fukushima included thyroid ultrasound examinations targeting all prefectural inhabitants aged between 0 and 18 years on March 11, 2011 (approximately 360,000 inhabitants) [1]. The findings of thyroid ultrasonography performed in approximately 38,000 inhabitants until March 2012 showed that about 40 % of the children had small cysts with a diameter of ≤2.0 mm [1].

Clinically, small cysts without solid components do not require further examination and/or treatment. However, affected inhabitants generally worry because it is well known that the rate of childhood thyroid cancer increased for 4–5 years after the Chernobyl Nuclear Power Plant accident in 1986 [2–4].

No large-scale ultrasound examinations of the thyroid in the general population of children in particular have been carried out until recently. Therefore, the frequency of thyroid nodular lesions, such as small cysts, on high-quality ultrasonography has remained unknown. We conducted ultrasound examinations of the thyroid among Japanese children in the general population of three Japanese prefectures (Aomori, Yamanashi and Nagasaki Prefectures) in order to investigate the frequency of thyroid nodular lesions using the same ultrasound procedures as those used in the Fukushima Health Management Survey.

Subjects and methods

Study areas

The study was conducted in Aomori Prefecture, Yamanashi Prefecture and Nagasaki Prefecture by Hirosaki University, Yamanashi University and Nagasaki University, respectively. These areas were chosen because they are geographically dispersed throughout the Eastern, Middle and Western regions of Japan and are thought to have been unaffected by radioactive material from the Fukushima Nuclear Power Plant accident. Additional reasons for choosing these areas were that these prefectures had thyroid ultrasound specialists and medical facilities equipped for further examinations.

Participants

One kindergarten, one elementary school, one junior high school and one high school in each prefecture were contracted for this study, and all of the Japanese children aged between 3 and 18 years at each kindergarten and school were invited to participate in the study. Children for whom their parents refused the examinations were excluded from the study.

Ethics statements

The study was approved by the ethical committees of Hirosaki University, Yamanashi University and Nagasaki University, respectively. It was conducted in accordance with guidelines expressed in the Declaration of Helsinki. Written informed consent was obtained from the parents of all children examined.

Ultrasound examinations

The ultrasound examinations were performed between November 2012 and January 2013. For all examinations, we used the 7.75-MHz probe [12L-RS (GE Healthcare, Japan) and LOGIQ e Expert (GE Healthcare, Japan)], the same ultrasonography equipment used in the Fukushima Health Management Survey [5]. The examination protocol was also the same as that used in the Fukushima Health Management Survey.

Ultrasound findings

We identified nodules and cysts in the thyroid. Cysts with solid components were defined as nodules. We classified the thyroid findings of all participants into three categories—‘A’ (‘A1’ and ‘A2’), ‘B’ or ‘C’, according to the same criteria used in the Fukushima Survey [1, 5] based on the guidelines of the Japan Association of Breast and Thyroid Sonology. ‘A’ indicated that no further examinations were required, and ‘B’ indicated the presence of nodules with a diameter of ≥5.1 mm or cysts with a diameter of ≥20.1 mm. ‘C’ indicated the presence of thyroid findings that required immediate further examinations in a hospital. We further classified the category ‘A’ into sub-categories ‘A1’ and ‘A2’. ‘A1’ was defined as the absence of nodules or cysts, and ‘A2’ was defined as the presence of nodules with a diameter of ≤5.0 mm or cysts with a diameter of ≤20.0 mm. Unclear findings were evaluated by an expert working group who made the final diagnosis.

Results

The overall participation rate was 85.0 %. In total, 4,365 children (1,630 children in Aomori, 1,366 in Yamanashi and 1,369 in Nagasaki) underwent ultrasound examinations. The characteristics of the participants are shown in Table 1. In total, 2,075 participants (47.5 %) were male and 2,290 (52.5 %) were female. One hundred and eighty-nine (4.3 %), 1,275 (29.2 %), 1,995 (45.7 %) and 906 (20.8 %) of the participants were aged 3–5, 6–10, 11–15 and 16–18 years, respectively.

Table 1 Characteristics of the participants in the three examination areas
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Table 2 shows the numbers of participants classified according to the thyroid findings. Overall, 4,321 participants were classified with a status of ‘A’ (99.0 %), consisting of 1,853 participants with a status of ‘A1’ (42.5 %) and 2,468 participants with a status of ‘A2’ (56.5 %). Forty-four participants were classified with a status of ‘B’ (1.0 %). No participants were classified with a status of ‘C’.

Table 2 Number of participants in the three examination areas classified according to the thyroid findings
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Table 3 shows the numbers of participants with thyroid nodules and cysts. Thyroid nodules were identified in 72 participants (1.6 %; size range 1.9–23.5 mm, median size 5.9 mm) and thyroid cysts were identified in 2,483 participants (56.9 %; size range 0.8–12.1 mm, median size 3.1 mm). Thyroid nodules with a maximum diameter of ≥5.1 mm were identified in 44 participants (1.0 %) and those with a maximum diameter of ≤5.0 mm were identified in 28 participants (0.6 %). No thyroid cysts with a maximum diameter of ≥20.1 mm were identified (0.0 %), while thyroid cysts with a maximum diameter of ≤20.0 mm were identified in 2,483 participants (56.9 %).

Table 3 Number of participants with thyroid nodules and cysts in the three examination areas
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Table 4 shows the numbers of participants by age and gender classified according to the thyroid findings. The figure shows the percentage of participants with each thyroid finding. Among the males (Fig. 1a) aged 3–5 years, 69.8, 29.2 and 1.0 % were classified with a status of ‘A1’, ‘A2’ and ‘B’, respectively. Among the males aged 6–10 years, 45.7, 54.1 and 0.2 % were classified with a status of ‘A1’, ‘A2’ and ‘B’, respectively. Among the males aged 11–15 years, 44.6, 54.7 and 0.7 % were classified with a status of ‘A1’, ‘A2’ and ‘B’, respectively. Among the males aged 16–18 years, 44.5, 54.7 and 0.8 % were classified with a status of ‘A1’, ‘A2’ and ‘B’, respectively. Among the females (Fig. 1b) aged 3–5 years, 71.0, 29.0 and 0.0 % were classified with a status of ‘A1’, ‘A2’ and ‘B’, respectively. Among the females aged 6–10 years, 43.3, 56.4 and 0.3 % were classified with a status of ‘A1’, ‘A2’ and ‘B’, respectively. Among the females aged 11–15 years, 34.3, 64.1 and 1.6 % were classified with a status of ‘A1’, ‘A2’ and ‘B’, respectively. Among the females aged 16–18 years, 37.8, 59.7 and 2.5 % were classified with a status of ‘A1’, ‘A2’ and ‘B’, respectively.

Table 4 Number of participants by age and gender classified according to the thyroid findings
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Fig. 1
figure 1

Percentage of participants by age and gender classified according to the thyroid findings. a and b show the percentages of males and females, respectively. ‘A1’, no nodules or cysts; ‘A2’, the presence of nodules with a diameter of ≤5.0 mm or cysts with a diameter of ≤20.0 mm; ‘B’, the presence of nodules with a diameter of ≥5.1 mm or cysts with a diameter of ≥20.1 mm

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Discussion

Before the popular usage of thyroid ultrasonography, most thyroid tumors were detected using thyroid palpation. A large-scale report of Japanese adults who underwent general health check-ups showed that the frequency of thyroid nodules detected on ultrasound examinations (18.55 %) was much higher than that detected on palpation (1.46 %) and that the frequency of thyroid cysts with a diameter of ≥3 mm was 27.6 % [6]. However, no information regarding ultrasonographic thyroid nodular findings in a general population of children is available.

The current study conducted ultrasound examinations of the thyroid among general Japanese children aged between 3 and 18 years in order to investigate the frequency of thyroid nodules and cysts. We chose three examination areas distributed geographically throughout Japan, that are thought to have been unaffected by radioactive material from the Fukushima Nuclear Power Plant accident. The major findings of this study are that 99 % of the total participants were classified with a status of ‘A’, 1 % classified with a status of ‘B’, and no participants were classified with a status of ‘C’ (requiring immediate further examinations). In addition, 56.5 % of the total participants had a status of ‘A2’, having thyroid nodules with a diameter of ≤5.0 mm or thyroid cyst with a diameter of ≤20.0 mm. Most ‘A2’ cases involved thyroid cysts with a diameter of ≤20.0 mm. The results of the current study, thus, provide valuable information regarding the actual frequency of ultrasonographically detected thyroid nodular lesions in the general population of Japanese children.

We paid particular attention to the accuracy of the ultrasound examinations. Due to recent advances in ultrasonographic technology, imaging quality has dramatically improved. We used the same high-quality ultrasonography equipment as that used in the Fukushima Health Management Survey. Thyroid ultrasound specialists, such as certified sonographers, conducted the examinations. We judged the thyroid findings according to the same classification as that used in the Fukushima Survey based on the guidelines of the Japan Association of Breast and Thyroid Sonology. These factors are strengths of the current study.

We do not think that the participation bias largely affected the frequency of thyroid nodules and cysts because the overall participation rate was relatively high. However, several limitations possibly exist in further applying these results to Japanese general populations aged <18 years. In our study population, the group aged between 3 and 5 years was much smaller than the other age groups, and there were slightly more females than males. Because the frequency of thyroid nodules and cysts is generally higher in females than in males and increases with age [7], the frequency found in this study might be higher than those seen in the similar populations. Inter-observer differences in the ultrasound examinations, iodine consumption, socio-ecological and educational backgrounds, and the lack of information regarding individual family and past histories must also be taken into consideration. Therefore, careful interpretation is required when comparing the frequency of thyroid nodular lesions between different populations. Further detailed analyses will be conducted in a future study.

Conclusion

In summary, the current study investigated the frequency of ultrasonographically detected thyroid nodular lesions in Japanese children aged between 3 and 18 years in the general population of three prefectures (Aomori, Yamanashi and Nagasaki). Of the 4,365 children, 4,321 participants (99 %) were classified with a status of ‘A’ (indicating that no further examinations were required) and 44 participants (1 %) were classified with a status of ‘B’ (indicating the presence of nodules with a diameter of ≥5.1 mm or cysts with a diameter of ≥20.1 mm). No participants were classified with a status of ‘C’ (indicating the presence of thyroid findings that required immediate further examinations). Although careful interpretation is required, these results would be useful for evaluating thyroid nodular findings on ultrasonography in Japanese children.