保健医療科学

2018Vol.67 No.1 p.42－49

Topics: Lessons learned on public health from the Fukushima Daiichi Nuclear Power Plant accident＜ Review ＞
Results of the first-round thyroid examination of the
Fukushima Health Management Survey
Hideto Takahashi
Research Managing Director, National Institute of Public Health

Abstract
After the accident occurred at the Tokyo Electric Power Companyʼs Fukushima Daiichi Nuclear Power
Plant, the Fukushima Health Management Survey (FHMS) was initiated. The FHMS consists of a basic
survey and four detailed surveys: a thyroid ultrasound examination, a comprehensive health check, a mental
health and lifestyle survey, and a pregnancy and birth survey. In this article, we briefly summarized whether
an association exists between radiation exposure and the observation of thyroid cancer cases according
to the results of the first-round thyroid examination in the FMHS. Regarding this issue, Tsuda and hiscolleagues showed an association using an internal comparison (odds ratio (OR)=2.6, 95% confidence
interval (CI): 0.99-7.0) and an external comparison (incidence rate ratio =50, 95% CI : 25-90). However, forthis internal comparison, Ohira and his colleagues used two ways of objective classifications of districts inFukushima; (1) the group of municipalities of which proportion of the exposed external dose level of morethan 5 mSv was higher than or equal to 1% (≧1% of 5 mSv), the group of municipalities of which proportionof the exposed external dose level less than 1 mSv was higher than or equal to 99.9% (≧99.9% of 1m
Sv<99%), and others, and (2) the location groups applied by WHO. For the classification (1), they obtained
OR=1.49 (95% CI : 0.36-6.23) from the highest group to the lowest, which was similar to the results of theclassification (2). For the external comparison, Takahashi and his colleagues developed a cancer-progression
model with several sensitivities under non-accident conditions, and showed 116 cases were possible toobserve in Fukushima under non-accident conditions. Katanoda and his colleagues found an observed/
expected ratio of 30.8 (95%CI: 26.2-35.9) of the prevalence of thyroid cancer among residents aged ≦ 20
years (160.1 observed of cases and 50.2 expected cases), and a cumulative number of thyroid cancer
deaths in Fukushima Prefecture of 0.6 under age 40 with the same method. This large disparity implied thepossibility of over-diagnosis in thyroid examinations.
A researcher reported the results were unlikely to be explained by a screening effect, which implied
the association between thyroid cancer cases and external radiation exposure. However, subsequently,
a possibility that it might be a result of over-diagnosis of the thyroid examinations was pointed. And,no significant associations were found by applying objective classification of districts and by raising
comparability with the incidence data of whole Japan, respectively. In the Basic Survey of FHMS, only
individual external doses in the first four months after the accident has been observed. So neither external
dose after the four months nor internal dose was applied in these studies. Further studies are necessary to
clarify the existence of the association by applying the estimation of individual overall thyroid dose.
keywords: thyroid examination, Fukushima health management survey, thyroid cancer, radiation external
exposure, the first-round survey(accepted for publication, 15th February 2018)

Corresponding author: Hideto Takahashi
2-3-6 Minami, Wako, Saitama 351-0197, Japan.
Tel. +81-48-458-6148; Fax +81-48-469-0213; Email: takahashi.h.aa@niph.go.jp

42

J. Natl. Inst. Public Health, 67 (1) : 2018

Results of the first-round thyroid examination of the Fukushima Health Management Survey
2011. Each external radiation dose was estimated basedon the recorded movements of the respondents in the fourmonths after the accident. The overall effective response
proportion to the basic survey, for the entire population of
Fukushima Prefecture, was 27.6% (566,680 of 2,055,267) (31March 2017) as shown in Table 1. The individual externalradiation exposure was estimated using the external dose
estimation system for Fukushima residents by the National
Institute of Radiological Sciences (NIRS). The doses were
estimated by adding the product of the time data for the
residentsʼ behaviors and the dose rate to the map [2]. TheNIRS external system has applied the System for Prediction of Environmental Emergency Dose Information andmonitoring data reported officially by the government.

I．Introduction
The Great East Japan Earthquake occurred on 11
March 2011. Tokyo Electric Power Companyʼs Fukushima
Daiichi Nuclear Power Plant was damaged by a tsunami,
which caused radiation emission outside of the plant. The
Fukushima prefectural government charged Fukushima
Medical University with planning and conducting a survey,
named the Fukushima Health Management Survey (FHMS),
to relieve anxiety and promote the long-term health of theresidents of Fukushima prefecture. The FHMS consists of abasic survey and four detailed surveys: a thyroid ultrasoundexamination, a comprehensive health check, a mental healthand lifestyle survey, and a pregnancy and birth survey. In
this article, we briefly summarized the association between
radiation exposure and the observation of thyroid cancer
cases in the FHMS from the results of the first-round thyroid examination in FMHS.

2. Thyroid ultrasound examination (preliminary baseline screening) [3]
The target population is the residents of Fukushima Prefecture aged 0-18 years as of 11 March 2011 (born between
2 April 1992 and 1 April 2011). The thyroid ultrasoundexamination was planned with a cohort study design [4]in which the subjects were monitored routinely (at 2-year
intervals in those ≦20 years old, and at 5-year intervals in
those >20 years old), because the Chernobyl accident resulted in an excess emergence of thyroid cancer four years
after the accident. The examination consists of (i) a primary
complete survey and secondary confirmatory examinations;(ii) a detailed ultrasound examination [3]; and (iii) fine-needle aspiratory cytology (FNAC). In the primary examination, ultrasonography was applied to examine the thyroid
gland, and examination specialists assessed the image on
the basis of the following criteria. Criteria A: Those with

1. The Basic Survey (radiation dose estimation) [1]
The Basic Survey is a questionnaire survey which targets
all the residents (approximately 2,050,000) as of 11 March
Table 1 Response proportions to the Basic Survey
Response
proportion (%)
Targets
Original
questionnaire
Simplified
questionnaire
Responders

Total

2055267100.0493538

24.0

73142

3.6

56668027.6

As of 31 March 2017
Modificationof
ofTableTable11fromfromthetheBasic
BasicSurvey
Survey[1][1](radiation(radiationdose
doseestimation)
estimation)
Modification

Table 2 Primary thyroid examination screening
Targetpopulation(N)

TargetParticipants Proportion
Participants Proportion
population(N)(%)(N)(%)(N)

Results

Results

N
(%)

A1 N
(%) (%)

A2
A1
(%)
(%)

BA2
(%)
(%)

C
B
(%) (%)C(%)

FY201147769
FY2011

41810
47769

87.541810

41810
87.5
(100.0)

26375
41810
(63.1)(100.0)

15214
26375
(36.4)(63.1)

221
15214
(0.5)
(36.4)

0 221(0.0) (0.5)

0
(0.0)

FY2012161123
FY201213933716112386.513933713933786.5(100.0)76194
139337(54.7)(100.0)

62155
76194
(44.6)(54.7)

987
62155
(0.7)
(44.6)

1 987(0.0) (0.7)

1
(0.0)

FY2013

158757FY201311932615875775.2
11932611932675.2(100.0)52036
119326(43.6)(100.0)

66205
52036
(55.5)(43.6)

1085
66205
(0.9)
(55.5)

0 1085(0.0) (0.9)

0
(0.0)

Total

367649Total

30047336764981.730047330047315460530047381.7(100.0)(51.5)(100.0)143574154605(47.8)(51.5)

2293
143574(0.8)(47.8)

1 2293(0.0) (0.8)

1
(0.0)

Number and proportion of participants with nodules/cysts
ParticipantsParticipants
Nodules≧5.1 mm

Results
NodulesResults
Cysts

Cysts

≧5.1
≦5.0mmmm≦5.0mmmm ≧20.1≧20.1mmmm≦20.0≦20.0 mm

FY2011FY2011 219 (0.5)
41810
41810

(0.5)
230219(0.6)

230
1 (0.6)15139 1(36.2)

15139 (36.2)

FY2012FY2012 973139337
139337(0.7)
119326FY20131083119326
(0.9)

(0.7)
730973(0.5)

730
9 (0.5)62267 9(44.7)

62267 (44.7)

FY20131083
753(0.6)(0.9)

753
2 (0.6)66493 2(55.7)

66493 (55.7)

Total

Total 2275300473
2275
(0.8)
300473
(0.8)
1713
(0.6)

1713
12(47.9)
12 (0.6) 143899

143899 (47.9)

Modification of Table 1 and Table 2 from Thyroid Ultrasound Examination (Preliminary
Baseline Screening) [3]

J. Natl. Inst. Public Health, 67 (1) : 201843

Hideto Takahashi
test results of A1 (no nodules / cysts) and A2 (nodules ≦5.0 mm or cysts ≦20.0 mm) were recommended for watchful waiting until they undergo the next screening. Criteria
B: Those with B (nodules ≧5.1 mm or cysts ≧20.1 mm)
were advised to take the Confirmatory Examination. Criteria C: Those with immediate needed for a confirmatory
examination [3]. In secondary confirmatory examinations,
we conducted ultrasonography, blood tests, urine tests, and
FNAC if it was necessary for those with B or C test results.
We recommended medical follow-up for those requiring iton the basis of their confirmatory test results.
3. Results of the first-round thyroid examination [3]According to the primary examination, the participation
proportion was 81.7% (300,473 of 367,649) as shown in Table 2. The number of participants with A1 or A2 test results
was 298,179 (99.2%), the number with B results was 2,293(0.8%), and only 1 had C results. According to the confirmatory examination, 2,293(92.9%) were recommended forfurther testing and 2,090 of them (98.1%) participated in it.
Of 2,090 participants, the number of those who met A1 or
A2 primary diagnostic criteria (including those with otherthyroid conditions) were 132 with A1 and 579 with A2, respectively. They were advised to take their next regularlyscheduled examination (Full-scale thyroid screening program). 1,379 (66.0%) participants with neither A1 nor A2
results were recommended to have medical follow-up after
6 to 12-months, or were advised to take their next regularlyscheduled examination. Of 1,379 participants, 547 (39.7%)
underwent FNAC.

II．Findings from first-round thyroid examination of the FHMS
In February 2018, more than 25 articles based on the
FHMS data were published. They are listed at http://
fmu-global.jp/publications-n/, which contained four articles
that explored the association between radiation exposureand the emergence of thyroid cancer. Here we summarized
them.
1. Study by Shimura and colleagues [5]This study showed that the proportion to detect thyroid
nodules and cancer increased in age, but showed a peak at
11 to 12 years in cysts, and that the prevalence proportionof thyroid nodules and cancers were different in sex, but its
difference of cysts was small. They summarized this resultsby the description of the age and sex distributions of the
thyroid ultrasound examinations of children and adolescentsin the FHMS. In this study, 294,905 participants aged≦18
years at the time of the accident received primary thyroid

44

ultrasound examinations from October 2011 through March
2014, and 2032 subjects participated in secondary confirmatory examination in the first round of the FHMS. The
evaluation of the age- and sex-specific prevalence and sizeof thyroid cysts, nodules, and cancers showed that thyroid
cysts, nodules, and cytologically suspected cancers were
found in 68,009, 1415, and 38 male subjects and in 73,014,
2455, and 74 female subjects, respectively.
2. Study by Tsuda and colleagues [6]The first-round of screening included 298,577 examinees.
In this study, the data up to December 31, 2014, were usedand compared with the Japanese annual incidence and theincidence in four areas in Fukushima Prefecture. The highest incidence rate ratio (IRR), with their assumption of alatency period of 4 years, was estimated (IRR = 50, 95%
confidence interval (CI): 25-90) in the central middle district of the prefecture compared with the Japanese annual
incidence of the national cancer registry (NCR) (external
comparison). The prevalence proportion of thyroid cancer(605 per million, 95% CI : 302-1082) and the prevalenceodds ratio (OR=2.6, 95% CI : 0.99-7.0) were compared withthe reference district in Fukushima Prefecture (internal
comparison). They concluded that an excess of thyroid cancer had been detected by ultrasound among children andadolescents in Fukushima Prefecture within 4 years of therelease, and was unlikely to be explained by a screening effect.
3. Study by Katanoda and colleagues [7]After Tsudaʼs paper was published, Katanoda and his
coauthors calculated the observed/expected (O/E) ratio of
thyroid cancer prevalence for the residents aged≦20 years.
The observed prevalence was the number of thyroid cancer
cases detected by the thyroid examination in FHMS until
April 2015. They calculated the expected prevalence as the
cumulative incidence as determined by a life-table method
using the national estimates of the thyroid cancer incidence
rate in 2001-10 and the population of Fukushima Prefecture.
Under the assumption that there was neither a nuclear
accident nor a screening intervention, an O/E ratio of 30.8
(95% CI: 26.2-35.9) was determined as shown in Table 3.
The cumulative number of thyroid cancer deaths under age
40 in Fukushima Prefecture was 0.6. Combined with the existing knowledge regarding the effect of radiation on thyroid
cancer, their descriptive result of disparities between themsuggested the possibility of over-diagnosis.
4. Study by Ohira and colleagues [8]Applying a cross-sectional study design with 300,476
participants aged 18 years and younger who underwent

J. Natl. Inst. Public Health, 67 (1) : 2018

Results of the first-round thyroid examination of the Fukushima Health Management Survey
Table 3 Observed and expected thyroid cancer prevalence in Fukushima Prefecture, as of the end of 2014
Sex
Observed (age at screening ≦20b)

Number ofPercentage among

O/E

malignant casesa

target population

ratio

Males

54.8

0.032

Females

105.3

0.064

Total

160.1

0.047

95%CI

Expected (attained age ≦20)
Based on average incidence ratec

Based on average incidence rated

Males

1.2

0.001

46.1

34.5-59.8

Females

4.0

0.002

26.6

21.7-32.0

Total

5.2

0.002

30.8

26.2-35.9

Males

1.3

0.001

41.4

31.0-53.7

Females

5.9

0.004

17.9

14.6-21.6

Total

7.2

0.002

22.2

18.9-25.9

O/E, observed/expected; CI, confidence interval
a: Including suspected malignancy.
b: Corrected for age-specific screening rate.
c: Calculated using the national incidence rate between 2001 and 2010.
d: Calculated using the national incidence rate extrapolated to 2014 using the average annual percent change
between 2001 and 2010.
Reprint of Table 1 from Katanoda K, et al. (2016) [7]

Table 4 Age- and sex-adjusted ORs and 95% CIs of thyroid cancer according to location group by first
4-month external radiation doses estimated by The Fukushima Health Management Survey
Group Aa)

Group Bb)

Group Cc)

N

4192

213564

82720

Women, %

50.5

49.4

49.8

Age at the time of the nuclear accident, y (SD)

9.4 (5.4)

9.0 (5.1)

8.6 (4.8)

Age at the time of the screening, y (SD)

10.2 (5.4)

10.6 (5.1)

11.2 (4.9)

0.8 (0.6)

1.7 (0.7)

2.6 (0.5)

Duration from the nuclear accident to the time of the screening,y (SD)
No. of cases

2

76

34

Prevalence proportion per 100000 people

47.7

35.6

41.1

Crude OR (95%CI)

1.16 (0.28-4.83)

0.87 (0.58-1.30)

Reference

Age- and sex- adjusted OR (95%CI)d)1.49 (0.36-6.23)

1.00 (0.67-1.50)

Reference

Multivariable-adjusted OR (95%CI)e)

1.01 (0.22-4.63)

0.82 (0.51-1.34)

Referencea) The group of the proportion of exposed external radiation of 5 mSv or more is more than or equal to 1%.
b) The group of the proportion of exposed external radiation of 5 mSv or more is less than 1 % and of 1 mSV or less is less than 99.9%.
c) The group of the proportion of exposed external radiation of 1 mSv or less is more than or equal to 99.9%.
d) Adjusted for age at the thyroid examination and sex.
e) Adjusted for age at the thyroid examination, sex, and duration from the nuclear accident to the thyroid examination.
95% CI=95% confidence interval, OR=odds ratio, SD=standard deviation

Reprint of Table 2 from Ohira T, et al. (2016) [8]thyroid examinations between October 2011 and June 2015,
Ohira and his colleagues used two ways of objective classifications of districts in Fukushima; (1) the group of municipalities which proportion of the exposed external dose levelof more than 5 mSv was higher than or equal to 1% (≧1%of 5m Sv), the group of municipalities which proportion of
the exposed external dose level less than 1 mSv was higher
than or equal to 99.9% (≧99.9% of 1m Sv<99%), and others, and (2) the classification by WHO [9]. The prevalence

proportions of thyroid cancer for the three groups (1) were47.7/100,000 for the highest dose area, 35.6/100,000 for the
middle dose area, and 41.1/100,000 for the lowest dose area,
respectively. It showed that the highest and the middle doseareas had OR=1.49 (95% CI: 0.36-6.23) and 1.00 (0.67-1.50),
respectively, using the lowest dose areas as a reference(internal comparison) as shown in Table 4 by using logistic
regression models adjusting for age and sex. For the classification (2), the groups were characterized as relatively

J. Natl. Inst. Public Health, 67 (1) : 201845

Hideto Takahashi
Table 5 Expected detected cases determined by the model with several sensitivity values
Table 5 Expected detected cases determined by the model with several sensitivity values
Sensitivity(primary mass

Sensitivity

Males(Observed cases n=39)

Females(Observed cases n=77)(secondary
confirmation
examination)

Expecteddetected

95%CI

Expecteddetected

1.0

1.0

49.3

35.5-63.0*

141.1

117.8-164.4

0.9

1.0

44.3

31.3-57.4*

127.0

104.9-149.1

0.9

0.9

39.9

27.5-52.3*

114.3

93.4-135.3

0.9

0.8

35.5

23.8-47.2*

101.6

81.9-121.4

0.8

1.0

39.4

27.1-51.7*

112.9

92.1-133.7

0.8

0.9

35.5

23.8-47.2*

101.6

81.9-121.4

0.8

0.8

31.5

20.5-42.5*

90.3

71.7-108.9*

0.7

1.0

34.5

23.0-46.0*

98.8

79.3-118.3

0.7

0.9

31.0

20.1-42.0*

88.9

70.4-107.4*

screening)

cases

cases

95%CI

0.7

0.8

27.6

17.3-37.9

79.0

61.6-96.5*

0.6

1.0

29.6

18.9-40.2*

84.7

66.6-102.7*

0.6

0.9

26.6

16.5-36.7

76.2

59.1-93.3*

0.6

0.8

23.7

14.1-33.2

67.7

51.6-83.9*

0.5

1.0

24.6

14.9-34.4

70.6

54.1-87.0*

0.5

0.9

22.2

12.9-31.4

63.5

47.9-79.1*

0.5

0.8

19.7

11.0-28.4

56.5

41.7-71.2**Observed cases are included in the 95% CI of the expected cases.
Modified from Takahashi H, et al. (2017) [10]95%CI=95% confidence interval
Modification of Table 4 from Takahashi H, et al. (2017) [10]highest dose area, middle dose area, relatively lowest dosearea with OR=1.50 (95% CI: 0.37 - 6.15) and OR=1.01 (95%
CI: 0.69 - 1.47), respectively. From these results, the external radiation dose was not associated with thyroid cancer
prevalence among Fukushima children within the first 4
years after the nuclear accident.
5. Study by Takahashi and colleagues [10]In this study, a common model applicable to any regionin Japan under non-accident conditions was built, and the
expected prevalence was estimated using simulation of
the sensitivity of the first-round thyroid examination. To
adjust for age, a cancer-progression model that was an extension of Day and Walterʼs was assumed. By minimizing
the weighted root mean squared error between the averageage-specific thyroid incident rates from 2001 to 2010 in the
NCR and those determined by the model, the expected
detectable prevalent cases were obtained by the modelwith their examination-participation proportions, and several sensitivities were simulated. The simulation results
showed that the numbers of observed prevalent cases were
within the 95%CI of the expected prevalent cases with several sensitivities in each gender as shown in Table 5. These
results implied that the number of observed thyroid cancer
cases can be detected by the FHMS first-round thyroid
screening at several sensitivities under non-accident conditions. They also estimated that the median sojourn times
between the detectable incidence and clinical incidence
were 34 years (males) and 30 years (females), respectively.

46

III．Summary
With regard to the association between radiation exposure and cancer detection, Tsuda and colleagues showedthat the maximum prevalence proportion of thyroid cancer
among areas in Fukushima expressed as a ratio with thereference district in Fukushima (internal comparison) was
estimated to be 2.6. They also showed that the incidence
rate was 50 times as high as that estimated by the national
cancer registry (external comparison). This was a shocking
result, but there were questions about the legitimacy of theestimation methodology. Nine scientific letters were immediately sent in response to the article to question the justification of their results [11-19]. For example, the results
might have arisen due to screening effect or over-diagnosis,
and thus did not reflect the association between radiation
exposure and the incidence. And they pointed several technical questions in their article with regard to the process of
estimation.
The first was their procedure by which the districts weregrouped. Whether were the districts classified objectively
or not? The more frequently they tried grouping, the more
often statistical significance they would find (i.e., a type I
error). Second, whether was their comparison of the data
between FHMS and NCR valid? The prevalence proportion(with the unit of no-dimension) was obtained from the thyroid examinations in FHMS, but the incidence rate (withthe unit of 1/time) was obtained from NCR. To unify theunit, they applied a basic formula: P (prevalence proportion)

J. Natl. Inst. Public Health, 67 (1) : 2018

Results of the first-round thyroid examination of the Fukushima Health Management Survey=I (incidence rate)×D (disease duration), which holds ina steady-state disease pool [20]. They assumed 4 years
for the disease duration (their latent duration) of childhood thyroid cancer, corresponding to the time between
the Fukushima accident and thyroid cancer detection, forwhich the maximum duration was 3 years and 10 months.
The point is whether they did check “steady-state diseasepool” or not. Furthermore, the data applied by them wereobtained from the first-round survey, which was the baseline study of the cohort study of the thyroid examinations in
FHMS. According to the evidence from the Chernobyl accident, thyroid cancer in children and adolescents increased
5 years after the accidents [21-22], but the first-round thyroid examination of the FHMS was conducted in the first 3
years. It is likely that the cancer detected in the first-roundexamination does not reflect the incident cases caused by
the radiation exposure, but seems the original prevalent
cases observed independently of the exposure.
Ohira and colleagues showed the results when the firstquestionable point was corrected by applying two ways ofobjective classifications of districts. Both results showed nosignificance between the external radiation and thyroid cancer prevalence among Fukushima children within the first 4
years after the nuclear accident.
Takahashi and colleagues showed the results when thesecond questionable point was corrected by raising thecomparability of both data as modifying three points, touse a common index for adjusting units between data of
FHMS and NCR, to develop a cancer-progression model for
adjusting age, and to simulate the sensitivity of the thyroid
examinations. They successfully built a cancer-progression
model of thyroid cancer based on Japanʼs NCR data under
non-accident conditions, they were able to adjust the units
and age between data, and they simulated sensitivity for thecomparison. The results implied that the detected thyroid
and its suspected 116 cases in the first-round screening examination was possible to observe with several sensitivities
under non-accident conditions.
Katanoda and their colleagues compared NHMS with
NCR. They applied a cumulative incidence rate for unit
unification. Under the assumption that there was neither anuclear accident nor a screening intervention, two results,
namely an O/E ratio of 30.8 (95% CI: 26.2-35.9) and thecumulative number of thyroid cancer deaths in Fukushima
Prefecture being 0.6 under age 40, implied the possibilityof over-diagnosis in the thyroid examination. There is another report over-diagnosis was considered by the disparityof longitudinal tendency between incidence rate (rapid
increase) and mortality rate (stable) of thyroid cancer inSouth Korea, 1993-2011 [23].
Since the thyroid examinations in FHMS started, peo-ple have been worried that many cases were detected by
the thyroid examinations. For this situation, a researcher
reported the results were unlikely to be explained by ascreening effect, which implied the association between
thyroid cases and external radiation exposure. However, subsequently, a possibility that it might be a result ofover-diagnosis of the thyroid examinations was pointed.
And, no significant associations were found by applying objective classification of districts and by raising comparability
with the incidence data of whole Japan, respectively. In the
Basic Survey of FHMS, only individual external doses in
the first four months after the accident has been observed.
So neither external dose after the four months nor internal
dose was applied in these studies. Further studies are necessary to clarify the existence of the association by applying
the estimation of individual overall thyroid dose.

Conflict of Interest
There are no conflict of interest.

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Results of the first-round thyroid examination of the Fukushima Health Management Survey

福島県民健康調査甲状腺検査先行検査の結果について
高橋秀人
国立保健医療科学院統括研究官

抄録
福島東京電力原子力発電所事故後,福島県民健康調査（FHMS）がスタートした．この調査は基本調査，甲状腺検査，健康診査，こころの健康度・生活習慣に関する調査，妊産婦に関する調査から構成されている．この論文では，放射線被ばくと甲状腺がんとの関連が存在するかどうかについての検討を先行検査（ 1 巡目検査）の結果から簡潔にまとめる．津田らの研究は県内の地域間比較（オッズ比OR=2.6，95％ 信頼区間（CI）: 0.99-7.0）と日本全体の発生状況との外的比較（罹患率比（IRR）= 50，95％ CI : 25-90）を示し，関連性の存在をアピールした．しかし地域間比較については大平らが 2 通りの客観的な分類として，（1）5 mSvより高い外部線量の割合が 1 ％以上である市町村からなるグループ, 1 mSvより低い外部線量の割合が99.9％以上である市町村からなるグループ,その他）と,（2）WHOにもって用いられた地域，をそれぞれ用いた．分類（1）では，外部線量の最も高い群の最も低い群に対するオッズ比OR=1.49（95％ CI : 0.36-6.23）を得，これは分類（2）でも同様であった．外的比較については，高橋らが，事故がない仮定のもとで，がんの進展モデルと甲状腺検査の感度を用いて，事故がない状況であっても福島県において116人の患者を検出しうることを示した．片野田らの研究では福島県における事故後の累積罹患率の期待度数（5.2人）と観測度数（160.1人）の比30.8（95％ CI: 26.2-35.9）と累積死亡数（40歳以下で0.6人）の大きな乖離から，甲状腺検診の過剰診断の可能性を示唆している．
このように，今回の放射線事故に関する放射線被ばくと甲状腺がんとの関連については，はじめに関連が示唆された結果が発表されたものの，それは過剰診断の可能性により生じている可能性が指摘され，その後客観的な分類，比較可能性等を考慮した研究により，これらの関連は否定されている．
しかし，県民健康調査の甲状腺検診では事故後 4 か月間の外部被ばく線量の値のみが得られており，そのため事故後 4 か月以降の外部被ばく線量や内部線量はこれらの研究では用いられていない．個人
個人の総被ばく線量の推定値を用いて関連の有無を明らかにする研究がさらに必要とされている．
キーワード：甲状腺検査，福島県民健康調査，甲状腺がん，放射線外部被ばく，
1巡目検査（先行検査）

J. Natl. Inst. Public Health, 67 (1) : 201849