Association between the detection rate of thyroid cancer and the external radiation dose-rate after the nuclear power plant accidents in Fukushima, Japan

Abstract

A thyroid cancer ultrasonography screening for all residents 18 years old or younger living in the Fukushima prefecture started in October 2011 to investigate the possible effect of the radiological contamination after the Fukushima Daiichi Nuclear Power Plant accidents as of March 12 to 15, 2011. Thyroid cancer in 184 cases was reported by February 2017. The question arises to which extent those cancer cases are a biological consequence of the radiation exposure or an artefactual result of the intense screening of a large population.Experiences with the Chernobyl accident suggest that the external dose may be considered a valid surrogate for the internal dose of the thyroid gland. We, therefore, calculated the average external effective dose-rate (μSv/h) for the 59 municipalities of the Fukushima prefecture based on published data of air and soil radiation. We further determined the municipality-specific absolute numbers of thyroid cancers found by each of the two screening rounds in the corresponding municipality-specific exposed person-time observed. A possible association between the radiation exposure and the thyroid cancer detection rate was analyzed with Poisson regression assuming Poisson distributed thyroid cancer cases in the exposed person-time observed per municipality.The target populations consisted of 367,674 and 381,286 children and adolescents for the 1st and the 2nd screening rounds, respectively. In the 1st screening, 300,476 persons participated and 270,489 in the 2nd round. From October 2011 to March 2016, a total of 184 cancer cases were found in 1,079,786 person-years counted from the onset of the exposure to the corresponding examination periods in the municipalities. A significant association between the external effective dose-rate and the thyroid cancer detection rate exists: detection rate ratio (DRR) per μSv/h 1.065 (1.013, 1.119). Restricting the analysis to the 53 municipalities that received less than 2 μSv/h, and which represent 176 of the total 184 cancer cases, the association appears to be considerably stronger: DRR per μSv/h 1.555 (1.096, 2.206).The average radiation dose-rates in the 59 municipalities of the Fukushima prefecture in June 2011 and the corresponding thyroid cancer detection rates in the period October 2011 to March 2016 show statistically significant relationships.

Figure 1

Target municipalities (n = 59) of the 1st round of the Fukushima Health Management Survey partitioned into 3 organizational strata; dark: FY 2011 area, No. 1–13; medium: FY 2012 area, No. 14–25; light: FY 2013 area, No. 26–59; the red indexed municipalities are subject to an average dose-rate greater than 2.0 μSv/h; see Table 1 for a list of the municipalities by consecutive number; http://fmu-global.jp/?wpdmdl=1563.

Figure 2

Flow chart of the thyroid cancer examination in the FHMS.^[28] Decision criteria: A1: no nodules or cysts; A2: nodules ≤5.0 mm or cysts ≤20.0 mm; B: nodules ≥ 5.1 mm or cysts ≥ 20.1 mm, some A2 test results may be re-classified as B results when clinically indicated; C: need for immediate confirmed examination.

Figure 3

Time schedule of the 1st and the 2nd thyroid cancer screenings in units of 1 month relative to the date of the beginning of the nuclear accidents in the FDNPP in March 11, 2011; dark: 1st screening round, light: 2nd screening round; see Table 1 for the municipalities by index number.

Figure 4

Contour plot of the dose-rate [μSv/h] on a log₁₀ gray level scale for 1710 dose-rate measurements taken at locations (white dots) in the Fukushima prefecture in June 2011; the red indexed municipalities are subject to an average dose-rate of greater than 2.0 μSv/h, see the UNSCEAR 2013 report.^[68]

Figure 5

A. Association of the dose-rate [μSv/h] with the Cs-137 activity [kBq/m²] for 1710 positive radiation readings; B. Association of the dose-rate [μSv/h] with the I-131 activity [kBq/m²] for 418 positive radiation readings; C. Mean dose rate in the municipalities by their distances from the FDNPP; see the UNSCEAR 2013 report.^[68]

Figure 6

A. Thyroid cancer detection rate by dose-rate and Poisson regression line in the Preliminary Baseline Screening Program in all 59 municipalities; B. data restricted to 53 municipalities with dose-rate < 2.0 μSv/h; one outlying data point not shown for prefecture No. 10 Kawauchi Mura with 1 TC in 280 1st round participants.

Figure 7

A. Thyroid cancer detection rate by dose-rate and Poisson regression line in the First Full-Scale Screening Program in all 59 municipalities; B. data restricted to 53 municipalities with dose-rate < 2.0 μSv/h.

Figure 8

A. Thyroid cancer detection rate by dose-rate and Poisson regression line (Preliminary Baseline Screening and First Full-Scale Screening Programs combined) in all 59 municipalities; B. data restricted to 53 municipalities with dose-rate < 2.0 μSv/h; one outlying data point not shown for prefecture No. 10 Kawauchi Mura with 1 TC in 280 1st round participants and no TC in 213 2nd round participants.

Figure 9

Comparison of the MEXT/UNSCEAR dose-rate and the questionnaire-based FHMS dose in 51 Fukushima municipalities with dose-rate < 1.5 μSv/h,^[68,93] see also Table 3 and http://www.pref.fukushima.lg.jp/uploaded/attachment/201726.pdf.

Figure 10

Thyroid cancer detection rate in 100,000 person-years (1st and 2nd screening rounds combined) by the natural log of the mean dose-rate in the 10 decile categories and Poisson trend with 95%-prediction bands for the mean; see Table 4.