UNSCEAR

United Nations Scientific Committee
on the Effects of Atomic Radiation

EVALUATION OF DATA ON THYROID CANCERIN REGIONS AFFECTED BY
THE CHERNOBYL ACCIDENT
A white paper to guide the Scientific Committee’s
future programme of work

EVALUATING RADIATION SCIENCE FOR INFORMED DECISION-MAKINGEVALUATION OF DATA ON THYROID CANCERIN REGIONS AFFECTED BY
THE CHERNOBYL ACCIDENT
A white paper to guide the Scientific Committee’s
future programme of work

UN LOGO
UNITED NATIONSNew York, 2018

This document is available on the UNSCEAR website at www.unscear.org.
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This publication has not been formally edited.

CONTENTS
EXECUTIVE SUMMARY ......................................................................................................... v
I. INTRODUCTION .................................................................................................................. 1
II. RELEVANT PAST REPORTS OF THE COMMITTEE ...................................................... 1
III. RESULTS OF SCIENTIFIC RESEARCH SINCE THE UNSCEAR 2008 REPORT..................... 3
IV. UPDATE OF INCIDENCE RATES OF THYROID CANCER IN BELARUS,
RUSSIAN FEDERATION AND UKRAINE ........................................................................ 8
V. ASSESSMENT OF THE ATTRIBUTABLE FRACTION ................................................. 11
VI. CONCLUSIONS AND RECOMMENDATIONS............................................................... 12
ACKNOWLEDGEMENTS ....................................................................................................... 13
ANNEX ...................................................................................................................................... 15
REFERENCES ........................................................................................................................... 19iii

EXECUTIVE SUMMARY1

At its sixty-third session (27 June–1 July 2016), the Scientific Committee requested thesecretariat to prepare an evaluation of data on thyroid cancer in regions affected by the
Chernobyl accident. The Committee discussed a paper that recapitulated its previous findingson this matter, reported the latest data provided by the three most affected countries (Belarus,
the Russian Federation and Ukraine), summarized key literature of the past several years, andmade an assessment of the fraction of the observed incidence of thyroid cancer that could be
deemed attributable to radiation exposure of the thyroid:(a)Both the total number of cases and the crude incidence rate (number of cases per
100,000 person-years) basically increased monotonically over the period 2006−2015. The
total number of cases of thyroid cancer registered in the period 1991−2015 in males andfemales, who were under 18 in 1986 (for the whole of Belarus and Ukraine, and for thefour most contaminated oblasts of the Russian Federation), approached 20,000. This
number is almost three times higher than the number of thyroid cancer cases registered in
the same cohort in the period 1991−2005; 2(b)The observed increase in the incidence of thyroid cancer is attributable to a variety
of factors: increased spontaneous incidence rate with aging of the birth cohort, radiation
exposure, awareness of thyroid cancer risk after the accident, and improvement of
diagnostic methods to detect thyroid cancer;(c)
The Committee estimated that the fraction of the incidence of thyroid cancer
attributable to radiation exposure among non-evacuated residents of Belarus, Ukraine andthe four most contaminated oblasts of the Russian Federation, who were children oradolescents at the time of the accident, is of the order of 0.25. The uncertainty range of theestimated attributable fraction extends at least from 0.07 to 0.5;(d)
The increased incidence of thyroid cancer after the Chernobyl accident is a major
issue and needs further investigation to determine the long-term consequences of radiation
exposure. The ongoing epidemiological cohort studies and research on biomarkers forradiation-induced thyroid cancer may enhance the understanding of carcinogenesis afterradiation exposure and contribute to an improved estimation of the fraction of thyroid
cancer incidence attributable to exposure.

1

Based on the Official Records of the General Assembly, Seventy-second session, Supplement No. 46* (A/72/46*).
Sources and Effects of Ionizing Radiation: United Nations Scientific Committee on the Effects of Atomic Radiation
2008 Report to the General Assembly, vol. II, annex D (United Nations publication, Sales No. E.11.IX.3).

2v

EVALUATION OF DATA ON THYROID CANCER IN REGIONS AFFECTED BY THE CHERNOBYL ACCIDENTI.

1

INTRODUCTION

1.
The Committee has previously assessed in detail the radiation exposures that resulted
from the nuclear power plant accident at Chernobyl in 1986 and analysed the associated risks
and effects (UNSCEAR 2000 Report, annex J [U1]; UNSCEAR 2008 Report, annex D [U2];UNSCEAR 2012, annex A [U4]). At the sixty-third session (27 June–1 July 2016), theCommittee requested the secretariat to prepare a short paper for its next session (29 May–
2 June 2017) on the evaluation of data on thyroid cancer in regions affected by the accident at
Chernobyl.
2.
The primary objectives of the paper are: (a) to provide an authoritative report on thenumbers of thyroid cancer cases observed to date, primarily among people who were children
or adolescents at the time of the accident; and (b) to make an expert judgement of the fractionthat can be attributed to radiation exposure resulting from the accident. Secondary objectives
are to clarify, where possible, the scientific basis for and the reliability of risk projections,considering the levels and patterns of radiation dose to the exposed populations.
3.
This paper is based on data on annual incidence of thyroid cancer as submitted officially
under arrangements made by the secretariat with the representatives to the Committee of Belarus,
the Russian Federation and Ukraine. In addition, it presents an evaluation of key publications in
the peer-reviewed scientific literature up to December 2016, and an assessment of the fraction of
the incidence of thyroid cancer attributable to the radiation exposure caused by the accident.

II.

RELEVANT PAST REPORTS OF THE COMMITTEE

4.
The Committee concluded in its UNSCEAR 2008 Report, annex D [U2] that the 1986
Chernobyl nuclear power plant accident resulted in the release of a significant amount ofradioactive material to the atmosphere leading to exposures of nearby populations mainly frombeta and gamma radiation. Many of the children and adolescents received elevated doses to thethyroid, almost entirely because they drank fresh milk containing 131I in the first few weeks
following the accident. The Committee estimated the average absorbed doses to the thyroid ofevacuated children and adolescents, and of non-evacuated children and adolescents (at the timeof the accident) in the so-called “contaminated areas” of the former USSR to be about
900 mGy and 170 mGy, respectively [U2]. Average doses to the thyroid of adults were lower,
and those of pre-school children were some 2 to 4 times greater than the population average.
The average dose to the thyroid of all evacuees was estimated to have been about 500 mGy(with individual values ranging from less than 50 mGy to more than 5,000 mGy). For the morethan six million residents of the contaminated areas of the former USSR, who were notevacuated, the average dose to the thyroid was about 100 mGy, while for about 0.7% of them,doses to the thyroid were more than 1,000 mGy.
5.
The background rate of thyroid cancer among children under age 10 was approximately
two to four cases per million per year. Since 1990–1991, a dramatic increase in the rate ofoccurrence of thyroid cancer was observed among members of the public who had been infants
or young children at the time of the accident. Among those exposed who had been under
14 years of age in 1986, there were 5,127 reported cases of thyroid cancer between 1991 and2005 (for those who had been under the age of 18 in 1986, there were 6,848 cases). Fifteencases had proved fatal. The observed pattern suggested that the dramatic increase in incidence
for the period 1991–1995 was associated with the accident. There was no evidence for a

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decrease in the annual excess incidence of thyroid cancer up to 2005 (the end of theobservation period for the report). For those born after 1986, there was no evidence for an
increase in the annual incidence of thyroid cancer [U2].
6.
The observed increased incidence has been confirmed in several case–control andcohort studies that have related the excess incidence of thyroid cancer to the estimatedindividual doses to the thyroid due primarily to the radioiodine released during the accident.
According to the estimates of Jacob et al. [J1], for the period 1986–2001, about 30% of theincidence of thyroid cancer in the whole of Ukraine—and about one half in the three northernoblasts (including Kyiv City)—was deemed attributable to radiation from the accident; inBelarus, about 60% of the incidence of thyroid cancer was deemed attributable to the accident.
Generally, the excess relative rate 3 (ERR) was higher for females than for males.
There was little suggestion of an increased incidence of thyroid cancer among those
exposed as adults in the general population. Among the recovery operation workers, elevatedrates of thyroid cancer compared to those in the general population have been reported, but noclear association with dose from external exposure has been found. In addition, no estimates ofdoses to the thyroid from inhaled radioiodine to those who worked on the Chernobyl site
between April and June 1986 have been available. The influence of annual screenings andactive follow-up of these cohorts make comparisons with the general population problematic [U2].
7.

8.
The Committee encouraged further investigation [U2] on the conclusion that iodine
deficiency might have influenced the ERR per unit dose to the thyroid for thyroid cancer
resulting from the incorporation of 131I released during the accident [C1, S2]. However, theconclusion was not statistically significant and the underlying mechanisms were unclear.
9.
Studies of large groups of people exposed to moderate and high doses may detect astatistically significant increase in the incidence of thyroid cancer. At lower doses, however,the necessary size of the study group to achieve sufficient power is larger than what could be
realized up to now [U4].
10.
The Committee in its UNSCEAR 2012 Report, annex B [U4] defined a scenario toestimate the uncertainty in the knowledge on thyroid cancer induced by radiation resulting
from the accident. In this scenario, Ukrainians, aged 10 years at the time of the accident,
received a hypothetical dose to the thyroid of 200 mGy from the incorporation of 131I. For
females, the estimated number of excess thyroid cancer cases during the lifetime of 10,000
exposed was 59 (95% credibility interval from 11 to 200). For males, the excess was about oneorder of magnitude lower.
11.
The Committee in its UNSCEAR 2013 Report, annex B [U3] referred to a cohort studyof 13,000 Ukrainians who were children or adolescents at the time of the accident. A firstultrasound screening of the thyroids was conducted in the period 1998–2000 and detected
45 thyroid nodules that were confirmed as malignant after fine needle aspiration. Doses to thethyroid of members of the so-called “Ukrainian-American (UkrAm) cohort” werereconstructed based on measurements of the 131I activity in the thyroid of individuals
performed within the first two months after the accident. The ERR per unit dose to the thyroid
was assessed to be 5.3 (95% CI: 1.7, 28) Gy−1.
3

The excess relative rate (ERR) is strictly a statistic calculated from observed frequencies/rates, while the excess
relative risk is a prospective estimate inferred from the data and reasoning [U4].

EVALUATION OF DATA ON THYROID CANCER IN REGIONS AFFECTED BY THE CHERNOBYL ACCIDENT

3

III. RESULTS OF SCIENTIFIC RESEARCH SINCE THE UNSCEAR
2008 REPORT
12.
Monitoring of thyroid cancer continues in the three countries, Belarus, the RussianFederation (four regions) and Ukraine, where there were higher levels of radionuclide
deposition after the Chernobyl accident. The data on the incidence of thyroid cancer have been
analysed in a large number of epidemiological studies published in peer-reviewed scientificjournals. While most of the available and ongoing studies address the incidence of thyroid
cancer in those exposed during childhood or adolescence, some studies address other age
groups in the exposed population. There is a discussion in the scientific literature about theeffects of “over-diagnosis” caused by the application of new highly sensitive screening
techniques.
13.
Further, substantial efforts have been undertaken to estimate the doses to the thyroid
and to quantify their uncertainties. More than 30 peer-reviewed publications have beenreviewed in detail during the preparation of this paper, including publications dealing with:(a) Theoretical modelling of the development of thyroid cancer;(b) Pathology and molecular biology of thyroid cancer;(c) Other thyroid diseases (not covered in this report);(d) Dose assessment methodologies and associated uncertainties.
A.

Epidemiological studies and analyses of radiation risk

14.
Brenner et al. [B4] evaluated the dose–response relationship for thyroid cancer among
12,514 members of the UkrAm cohort. The arithmetic mean of the dose to the thyroid from theincorporation of 131I was estimated to be 0.65 Gy. After the first screening (prevalence study[T2]), three biennial thyroid examinations were conducted between 2001 and May 2007 [B4].
All individuals with a nodule of ≥10 mm in its largest dimension, or a nodule of 5–10 mm withultrasound characteristics suggestive of malignancy were referred for fine needle aspiration. By
the end of 2008, a total of 65 thyroid cancer cases had been detected during the second, thirdand fourth screenings and pathologically confirmed after surgery. Of these, 61 were papillary,
three follicular, and one medullary thyroid cancer. Excess rates were estimated using Poisson
regression models. The linear dose–response model provided an adequate fit of relative rates indifferent dose groups. Inclusion of an exponential or a quadratic term did not improve the fit
significantly. The ERR model described the data better than the excess absolute rate (EAR)model. The ERR per unit dose to the thyroid was 1.91 (95% CI: 0.43, 6.34) Gy−1. It was higher
for residents of the Kyiv oblast (2.70 (95% CI: 0.27, 27.52) Gy−1) and particularly of theChernihiv oblast (4.07 (95% CI: 0.95, 16.80) Gy−1) than for residents of the Zhytomyr oblast(0.06 (95% CI: <−0.02, 1.08) Gy−1). The ERR per unit dose to the thyroid did not vary withtime since exposure, the use of iodine prophylaxis, the iodine status, sex, age, or tumour size.
The EAR per unit dose to the thyroid was 2.21 (95% CI: 0.04, 5.78) per 104 PY Gy [B4].
15.
Zablotska et al. [Z1] investigated the effects of screening, iodine deficiency, age at
exposure and other factors on the dose–response relationship. The so-called “BelarusianAmerican (BelAm) cohort” included Belarusians (aged 18 years or younger at the time of theaccident) for whom estimated doses to the thyroid due to 131I were available (based on
measurements on the thyroids of individuals and dosimetric data from questionnaires).
Examinations of the thyroids were performed for 11,903 individuals during the period 1996–
2001, and for 67 individuals during the period 2002–2004. The mean dose to the thyroid was

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0.56 Gy (range from 0.0005 to 32.8 Gy). The excess odds ratio (EOR) was modelled using
linear and linear–exponential functions. A significant dose–response relationship between thedose to the thyroid due to 131I and the incidence of thyroid cancer was observed; therelationship was linear–exponential over the full range of doses. For doses to the thyroid below
5 Gy, the dose–response relationship was linear, and the values of the EOR per unit dose were
estimated for pre-screening cases, cases detected by screening, and the combination of bothgroups (table 1).
Table 1. Excess odds ratio (EOR) per unit dose for prevalent thyroid cancer cases in a cohort ofBelarusians with doses to the thyroid of less than 5 Gy [Z1]Dose group (Gy)Cases

EOR per unit dose (Gy)−1Pre-screening<548

8.18 (95% CI: 2.11, 72.71)

Screening<585

2.15 (95% CI: 0.81, 5.47)

Screening<162

4.92 (95% CI: 1.32, 17.12)

Combined<5133

3.16 (95% CI: 1.49, 6.95)

Detection

16.
Ivanov et al. [I3, I4] analysed the incidence of thyroid cancer among members of the
population (aged 0 to 17 years at the time of the accident) in the most contaminated territoriesof the Bryansk, Kaluga, Oryol and Tula oblasts, which were affected by the Chernobyl
accident (cohort of 108,166 people; observation period 1991–2013). A total of 86 cases ofthyroid cancer were observed in children aged 0 and 4 years at the time of the accident (meandose to the thyroid of 326 mGy); 60.5% of the cases were deemed attributable to radiation
exposure. In the age group 4–9 years at the time of the accident (mean dose to the thyroid of
149 mGy), 78 cases of thyroid cancer were observed; 30% of them were deemed attributable toradiation exposure. In the age group 10–17 years, four out of 152 observed cases of thyroid
cancer (2.6%) were deemed attributable to radiation exposure. The value of the ERR for boys
was 2.9 times higher than that for girls (aged 0–17 years at the time of the accident). A
statistically significant decrease of the ERR with time since exposure (by a factor of 0.37 over
10 years) was observed for the whole cohort and for boys separately. As with the study ofJacob et al. [J2], the results of this study may be subject to ecological bias.
17.
Hatch et al. [H1] studied the health effects of in utero exposure to 131I following theChernobyl accident. The study included 2,582 children born during the two-month period after
26 April 1986. Of these, 1,494 were from the most affected oblasts of Ukraine. The estimatedaverage prenatal dose due to exposure to 131I was 72 mGy (range from 0 to 3,240 mGy).
Screening examinations included ultrasound and palpation techniques, clinical examinations
and interviews with the mothers. Based on a total of nine cases by 2015, the authors concluded
that an elevated risk of thyroid cancer might exist as a consequence of the exposure during
pregnancy. The magnitude of the risk was similar to that observed in the main cohort ofchildren exposed at 1–5 years of age. There was a significant dose–response association withthyroid nodules of 10 mm or larger.
18.
In a collaborative case–control study nested within cohorts of Belarusian, Russian andBaltic recovery operation workers, Kesminiene et al. [K2] evaluated the radiation-induced
incidence of thyroid cancer. The study included 107 cases and 423 controls. Doses to the thyroid
due to external exposure and intake of 131I were estimated for each cohort member. Most of theindividuals received low doses (median 69 mGy). A statistically significant dose–response
relationship was found with the total dose to the thyroid. The ERR at 100 mGy was 0.38 (95% CI:
0.10, 1.09). The risk estimates were similar when doses from 131I and external exposure wereconsidered separately, although for external exposure, the ERR was not statistically significant.

EVALUATION OF DATA ON THYROID CANCER IN REGIONS AFFECTED BY THE CHERNOBYL ACCIDENT

5The ERR was similar for microcarcinomas and larger size tumours, and for tumours with andwithout lymph-node involvement. Although recall bias and uncertainties in doses could haveaffected the magnitude of the risk estimates, the findings of this study contribute to a bettercharacterization of the risk of thyroid cancer after radiation exposure in adulthood.
19.
Kaiser et al. [K1] included in their study all 115 papillary thyroid cancers reported for
the UkrAm cohort that occurred before the screenings, or were detected during the first four
screenings. The analysis was based on likelihood functions expressing the different state ofknowledge for the prevalent and incident 4 thyroid cancer cases. For an attained age of 27, theERR per unit dose to the thyroid was estimated to be 1.6 (95% CI: 0.67, 2.6) Gy−1, in good
agreement with the result of Brenner et al. [B4] for the incidence in the period 2001–2008. The
data do not permit the identification of effects due to age at exposure, age attained or time since
exposure. However, a model with a constant EAR per unit dose, which best described the data,
resulted in an ERR that depended on the age attained. According to the EAR model, the ERR
decreased with increasing age attained. However, the estimate obtained from this model has avery large uncertainty for age below 20 (figure I).
Figure I. Excess relative rate (ERR) per unit dose to the thyroid for cases of thyroid cancer in the
UkrAm cohort and among the survivors of the atomic bombings in JapanThe triangle with uncertainty range gives the result for the prevalent cases detected in the first screening [T2], thesquare with uncertainty range gives the result for incident cases detected in the second to fourth screenings [B4].
The dashed line with darker shading and the dotted line are for papillary thyroid cancer cases detected in all four
screenings including those detected among cohort members before the first screening as derived with amechanistic and a descriptive model, respectively. Results derived from the life span study of survivors of theatomic bombings in Japan (continuous line with lighter shading) are given for comparison. The circle withuncertainty range relates to the ERR for an overexpression of the CLIP2 gene. The ERRs are sex-averaged and
95% confidence ranges are displayed (figure from Kaiser et al. [K1])4

Prevalence is the number of cases either reported before a screening (pre-screening cases), or identified during thefirst screening, while incidence is the number of new cases during a given period, e.g. identified in a screening of
persons, who were free of the disease at the previous screening.

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20.
In summary, the cohort studies in Ukraine and Belarus provide evidence for an ERR
per unit dose to the thyroid of the order of 2 Gy−1 for the period 2001–2008. There areindications that the ERR was higher during earlier periods after a minimum latent period of afew years.
21.
Ecological studies of the incidence of thyroid cancer related to the average regionaldose to the thyroid, sex, age at the time of the Chernobyl accident and calendar year arecontinuing in Ukraine and Belarus. It has been shown that the increase in the incidence rate ofthyroid cancer among persons under 18 years old in 1986 was significantly higher in the six
oblasts of Ukraine, where the average regional dose to the thyroid was more than 35 mGy, thanin the rest of Ukraine [F2]. In subsequent work [B1], the authors provided evidence of a similareffect in those who were adults in 1986. The authors also noted that no significant increase in
the incidence rate was observed in persons born after 1986 in these six oblasts.
22.
The data in the state registers of Belarus for the period 1978–2015 have also shown asignificant increase in the incidence of thyroid cancer in the total population of the country
since 1991, which reached a plateau around 2001 and has not declined since then [B3]. The
highest incidence was registered in the Gomel and Mogilev regions, where levels of radioactive
contamination were higher than in other areas. A strong correlation between the incidence of
thyroid cancer and doses to the thyroid was observed. An increased incidence of thyroid cancer
occurred also in children and adolescents, especially males, who were evacuated from theseareas. No significant increase in the incidence was observed in persons born after 1986.
However, both the Ukrainian and Belarusian studies [B1, B3, F2, T3] could not be used toobtain reliable estimates of radiation risk coefficients as they were not free from ecologicalbiases and limitations.
23.
Jacob et al. [J3] made calculations predicting the prevalence of thyroid cancer in thefirst screening in Fukushima Prefecture based on the evidence available from ultrasonicsurveys performed in the UkrAm cohort after the Chernobyl accident. In view of the improved
technology of ultrasonography and the related detectability of smaller tumours, the detection
rate in the Fukushima study was assessed to be a factor of about three higher than in the
UkrAm cohort. Based on data from the life span study (LSS) of the survivors of the atomic
bombings of Hiroshima and Nagasaki, most of the long-term thyroid cancer risk attributable to
the radiation exposure was predicted to accumulate over several decades with the risk during
the first years after exposure being a relatively small component of this. Furthermore, thecontribution of the first 10 years to the risk over 50 years was predicted to decrease withincreasing age at exposure. According to the models applied, the long latency was especiallyexpressed for females: for an age at exposure of 1 year, less than 1% of the attributable risk
accumulated during 50 years after exposure would be contributed by the first 10 years.
B.

Molecular biology and pathological studies

24.
In the past few years, there have been publications that aim to improve the scientificunderstanding of the development of thyroid cancer. These publications include studies oftheoretical modelling of the development of thyroid cancer, and the pathology and molecularbiology of thyroid cancer and of other thyroid diseases. The Chernobyl Tissue Bank (CTB)
was established in response to the scientiﬁc interest in studying the molecular biology ofthyroid cancer after the Chernobyl accident [T1]. The project began collecting a variety of
biological samples from patients on 1 October 1988, and has supplied material to 23 research
projects in Japan, the United States and Europe. Several research groups performed molecularEVALUATION OF DATA ON THYROID CANCER IN REGIONS AFFECTED BY THE CHERNOBYL ACCIDENT

7

biological analyses of normal and thyroid cancer tissues from patients who had been exposed
during childhood to radiation due to the Chernobyl accident and from non-exposed patients[A1, B4, D2, L1, S1]. A brief overview of possible biomarkers for radiation-induced cancer,
which have been identified, is presented here, but further validation is required.
25.
Abend et al. [A1] evaluated differential gene expression in thyroid tissue in relation to
the dose from exposure to 131I received as a consequence of the Chernobyl accident. They
analysed 63 paired RNA specimens from papillary thyroid carcinoma (PTC) that werediagnosed between 1998 and 2008 in the UkrAm cohort. The specimens were taken from fresh
frozen tumour and normal tissue provided by the CTB and satisfied the necessary quality
control criteria. This study was among the first to provide direct human data on long-term
differential gene expression in relation to individual doses to the thyroid due to exposure to 131I
and to identify a set of genes that may be important in radiation carcinogenesis.
26.
Kaiser et al. [K1] attempted to model molecular data for PTC in the areas affected by
the Chernobyl accident making assumptions about the mechanisms for cancer development.
They found that an overexpression of the CLIP2 gene could be distinguished statistically forradiation-associated thyroid cancers occurring before age 20 compared to those thyroid cancers
that were not radiation-associated. The best estimate of the ERR per unit dose for the CLIP2
overexpression was comparable to the ERR per unit dose for thyroid cancer, and its credibility
interval was smaller (figure I). Future research to find and model predictive biomarkers forradiogenic thyroid cancer is warranted; it may shed new light on the pathogenesis.
27.
Evidence from pathological analysis showed [Y1] that paediatric PTC was the mostprevalent type of thyroid cancer in all studied periods. In recent years, 43.6% of detected PTCs
were <10 mm, which reflects the improvement in diagnostic methods. Almost one third of all
PTCs were fully encapsulated tumours with minimal invasive properties. A large proportion ofthem had dominant solid growth patterns. With increasing time after the accident, PTCs with adominant papillary pattern exhibited more pronounced invasive properties, but, in general,
PTCs became less aggressive, which is an important sign for the postoperative prognosis.
C.

Dosimetric studies

28.
Drozdovitch et al. [D3, D4, D5, D6] described the opportunities and limitations ofmethodologies used to quantify the absorbed dose to the thyroid gland from various routes of
exposure. Because of the short half-lives of the relevant radioisotopes of iodine, measurements
covered only a fraction of the population residing in the contaminated regions. A uniform
method for dose reconstruction for the population of Belarus and contaminated oblasts of theRussian Federation was described; and the uncertainties of this approach were assessed using
Monte Carlo simulation methods. The assessment of doses to the thyroids of individuals took
into account the knowledge of their whereabouts and dietary habits (obtained by personal
interviews), and information on radionuclide deposition density available for each settlement
where they resided after the accident. The dose to the thyroid decreased with increasing age:the median doses were 0.38, 0.37, 0.13, 0.041, and 0.015 Gy for the age groups <2, 2–4, 5–9,
10–14, and 15–18 years, respectively. The doses to the thyroids of young children were foundto be higher than those to adolescents. For the same intake of 131I, a child’s thyroid receives ahigher radiation dose because the same amount of energy is deposited in a smaller thyroid
mass. In addition, dairy products, which were the main source of intake of 131I, were reported tobe consumed in higher quantities during childhood. The study included an uncertainty analysis:
the geometric standard deviation of the dose to the thyroid varied from 1.7 to 4.0 with medianvalues of 2.3 and 2.1 for members of the Belarusian and Russian cohorts, respectively. The

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ratios of the “measured” and estimated doses to the thyroid due to 131I were rather wide, bothfor low and high doses. The mean ratio of dose to the thyroid estimated using the model to that
estimated from direct measurements on the thyroid was found to be 2.0 ± 2.2 and the median of
the ratios was found to be 1.2. This wide distribution of the ratios shows the relatively large
uncertainties involved in the model calculations, which are due to uncertainties in the choice ofvalues for the parameters used in the dosimetry models and in data on individual habits that
had been obtained more than 10 years after the accident.
29.
Likhtarov et al. [L2] described efforts to improve the estimates of doses to the thyroidfor 13,204 members of the UkrAm cohort. All cohort members had at least one direct
measurement on the thyroid taken between 30 April and 30 June 1986. These cohort members
resided at the time of the accident in the northern parts of the Kyiv, Zhytomyr, or Chernihiv
oblasts, which were the most contaminated territories of Ukraine. Doses to the thyroid of thecohort members, which had been estimated following the first round of interviews, were reevaluated following the second round of interviews. The revised doses to the thyroid ranged
from 0.35 mGy to 42 Gy, with 95% of the doses between 1 mGy and 4.2 Gy, with anarithmetic mean of 0.65 Gy, and a geometric mean of 0.19 Gy. Many of the revised estimatesof doses to the thyroids of individuals were substantially different from those obtained
following the first round of interviews. The responses to the improved questionnaire used in
the second round of interviews made it possible to perform a more realistic risk analysis.
30.
Skryabin et al. [S3] published a study, which was aimed at determining the volume of
the thyroids of children and teenagers (5–16 years of age) living in the Gomel and Mogilev
oblasts. The mass of the thyroid is a parameter that accounts for around 50% of the uncertainty
in the dose to the thyroid. For a given age and sex, the differences in the dose to the thyroid
between children from the Gomel and Mogilev oblasts did not exceed 12%, which is relatively
small compared to the differences between individuals in the general population. For childrenof a given age, the values showed a distribution characterized by a geometric standard
deviation of 1.25 to 1.4.

IV. UPDATE OF INCIDENCE RATES OF THYROID CANCER IN
BELARUS, RUSSIAN FEDERATION AND UKRAINE
31.
A compilation of incidence rates of thyroid cancer between 1982 and 2015 among those
exposed under the age of 18 in Belarus, the Russian Federation (the Bryansk, Kaluga, Orel andTula oblasts) and Ukraine is presented in the annex. It updates table D11 of the UNSCEAR
2008 Report, annex D [U2] with information broken down by age at exposure and for
subsequent time periods (usually 5-year intervals).
32.
Both the total number of cases and crude incidence rate per 105 person-years basicallyincreased monotonically during the last decade (2006–2015). The total number of cases ofthyroid cancer registered in the period 1991–2015 in males and females who were under 18 in
1986, for the whole of Belarus and Ukraine and for the four most contaminated regions of theRussian Federation, exceeded 19,000 (table 2). This number is 2.8 times higher than thenumber of thyroid cancer cases registered in the same cohort in the period 1991–2005 [U2]. On
average, the registered numbers of thyroid cancer for females were about four times higher
than for males.

EVALUATION OF DATA ON THYROID CANCER IN REGIONS AFFECTED BY THE CHERNOBYL ACCIDENT

9Table 2. Total number of cases of thyroid cancer registered in 1991–2015 among those who were
under 18 at the time of the accident
GenderBelarus

Russian Federation(Bryansk, Kaluga, Orel and Tula oblasts)

Ukraine

Total

Females

4 546

1 504

9 393

15 443

Males

1 360

334

2 096

3 790

5 906

1 838

11 48919 233Total

33.
The observed increase in the incidence of thyroid cancer was influenced by various
factors: an increased spontaneous incidence rate with adulthood, radiation exposure, andimprovement in diagnostic methods. Discerning the effect of exposure to ionizing radiation
contributing to this complicated situation requires both careful epidemiological analysis andbasic research into the processes of molecular biology.
34.
The sharp increase in the incidence rate of thyroid cancer among children from about
five years after the accident is evident in figure II. The incidence rate of thyroid cancer
among Belarusian children up to 10 years old at the time of diagnosis was higher in the
period 1991–1995 by about one order of magnitude compared with the incidence rate inother 5-year periods. Both before this period (i.e. before the minimum latent period of 4–5
years) and after this period (i.e. when the cohorts did not include those who were children in1986), an increase in the incidence rate has not been observed.
Figure II. Incidence rate of thyroid cancer in Belarus for children under 10 years old at diagnosis [D1]35.
A similar, though less pronounced phenomenon, can be observed in cohorts ofBelarusian adolescents (aged 10–19 years at diagnosis); see figure III. The incidence rate ofthyroid cancer began to increase during the period 1991–1995, reached a peak during theperiod 1996–2000 and decreased from the period 2001–2005. After 2005, the incidence rate
remained higher than that prior to the accident. This apparently reflects the effect of thescreening regime. Nevertheless, the incidence rate was significantly lower than in the period
1996–2000 (10–15 years after the accident).

10

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Figure III. Incidence rate of thyroid cancer among adolescents (age at diagnosis 10–19) inBelarus [D1]36.
Figure IV shows the increase in incidence rates of thyroid cancer with time among
those exposed as children and adolescents in Belarus at the time of the accident. There is noevidence for a decrease in the excess incidence of thyroid cancer up to 2015. Part of theincrease is related to the normal age pattern of spontaneous disease occurrence whereas anotherpart can be deemed attributable to the radiation exposure from the accident.
Figure IV. Incidence rate of thyroid cancer among those in Belarus aged under 18 years at thetime of the accident [S4]

EVALUATION OF DATA ON THYROID CANCER IN REGIONS AFFECTED BY THE CHERNOBYL ACCIDENT

11

V. ASSESSMENT OF THE ATTRIBUTABLE FRACTION37.
The screening in the cohort studies had a significant effect on the absolute incidence
rate of thyroid cancer. The crude incidence rate in the UkrAm cohort in the period 2001 to May
2007, for example, was about 100 per 105 PY resulting from 65 cases among 12,514 cohort
members for an average time of about 5 years between the detection of the new cases and thefirst screening [B4]. According to the present update of incidence rates of thyroid cancer in theregions affected by the accident, the incidence rate in the same birth cohort and calendar time
period in the Ukraine was about 3 per 105 PY (see annex). In spite of the large effect on the
crude incidence rates, there is no evidence that the ERR per unit dose to the thyroid is
significantly influenced by screening. In fact, the confidence bands shown in figure I for thetwo cohorts, UkrAm and LSS, essentially overlap even though they were subjected to very
different screening conditions.
38.
An assessment of the fraction of thyroid cancer incidence deemed attributable toradiation due to the accident has been made here based on estimations of average doses to thethyroid, and preconditions that (a) the dose–response relationship in the dose range covering
the bulk of the doses to the thyroid of the population group of interest is linear, and (b) that theestimates of the ERR per unit dose derived from the cohort studies are applicable to the
population. Precondition (a) might not be true: although there was no evidence for a departureof the dose response from linearity in the UkrAm cohort [B4], there was evidence for a down
bending in the BelAm cohort [Z1]. Detailed information on the dose distribution in the
population would be necessary to base the estimation of the attributable fraction on a nonlinear dose response.
39.
As noted before, cohort studies of thyroid cancer among Belarusians and Ukrainians,
who were children or adolescents at the time of the accident, indicate an ERR per unit dose to
the thyroid of about 2 Gy−1 for the period 2001–2008, and a higher value for earlier times after
the minimum latent period of a few years. This is consistent with the result of about 6–8 Gy−1
in a case–control study of cases in the period 1992–1998 [C1].
40.
Estimates of the ERR for thyroid cancer per unit dose to the thyroid from the studies ofthe survivors of the atomic bombings in Japan showed a decrease with increasing age attained[F1, J3] (see also figure I). For those exposed as children or adolescents and for age attained ofabout 27, the average age of the UkrAm cohort in the period 2001–2008, the best estimate of
the ERR per unit dose of about 2–5 Gy−1 was derived, depending on age at exposure [F1]. Thisvalue is somewhat higher than that from the cohort studies in Belarus and Ukraine, although
the difference is not significant.
41.
The average dose to the thyroid of evacuated children and adolescents, and of nonevacuated children and adolescents (at the time of the accident) in the contaminated areas of
the former USSR was estimated to about 900 mGy and 170 mGy, respectively [U2]. If it wereassumed that for those having been children or adolescents at the time of the accident, the ERR
per unit dose was 2 Gy−1 for the period 2001–2008, then the ERR for the evacuated and nonevacuated would be estimated to be slightly less than two and about 0.3, respectively.

12

WHITE PAPER

42.

The attributable fraction, AF, is conventionally derived as
AF = ERR/(1+ERR)

Based on the estimates given above, the fraction of the thyroid cancer cases in the period
2001–2008 attributable to the radiation exposure caused by the accident is assessed to be about
0.6 and 0.25 for the evacuated and non-evacuated children and adolescents, respectively. Thereis no clear evidence for a value of the ERR per unit dose during other periods. However, there
are indications that it was higher in earlier periods and will be slightly lower at later periods.
Thus, the estimates of the attributable fraction of 0.6 and 0.25 may be considered to be roughestimates for the whole period 1991–2015.
43.
The uncertainty in the estimates is large. Based on only the statistical uncertainty of the
ERR per unit dose in the study of Brenner et al. [B4], the estimate of the 95% credibility
interval of the attributable fraction for the non-evacuated group would range from 0.07 to 0.5.
However, the interval is wider due to other uncertainties including those related to theestimation of average dose, the assumption of a linear dose–response relationship, and thetransfer of the result from the UkrAm cohort to the whole population. The width of thecredibility interval estimated here—more than a factor of seven (0.5/0.07) between the lower
and upper boundaries—is consistent with that obtained in a previous assessment of theCommittee of the risk of thyroid cancer among a hypothetical group of Ukrainians exposed to
131
I at the age of 10 [U2].

VI. CONCLUSIONS AND RECOMMENDATIONS
44.
Various epidemiological studies have shown that the thyroid gland is highly susceptible
to the carcinogenic consequences of external exposure to radiation during childhood. Studies ofthyroid cancer statistics after exposure to radioiodine released during the Chernobyl accident
resulted in comparable estimates of the ERR per unit dose. There are indications of a possible
biomarker for radiation-induced thyroid cancer at ages below 20, but independent confirmationis necessary. In the absence of a biomarker, it is impossible to distinguish a radiation-related
thyroid cancer from one that develops from other causes. An observed thyroid cancer in an
individual among the population of those exposed as children or adolescents at the time of theaccident cannot therefore be unequivocally attributed to radiation exposure at the present time.
45.
The Committee has estimated the relative fraction of the incidence of thyroid cancer—
among non-evacuated residents of Belarus, the four contaminated oblasts of the RussianFederation and Ukraine who were children or adolescents at the time of the accident—
attributable to radiation exposure. For the period 2001–2008, the fraction was assessed to be of
the order of 0.25. It was probably higher in the first 10 years following a minimum latent
period of a few years. Experience from the LSS indicates that it would be smaller after 2008,
although the decrease would be expected to be moderate.
46.
The uncertainty in the estimated attributable fraction of 0.25 ranges at least from 0.07 to
0.5, which is consistent with that obtained in a previous assessment of the Committee [U2] ofthe uncertainty related to limitations of the knowledge of the risk of thyroid cancer after
exposure to ionizing radiation.

EVALUATION OF DATA ON THYROID CANCER IN REGIONS AFFECTED BY THE CHERNOBYL ACCIDENT

13

47.
Despite the efforts made during the past decade to better understand the risk ofradiation-induced thyroid cancer, there are still open questions that require continued follow-up
of the health status of the affected populations, as well as basic scientific research on theunderlying processes of cancer development. Key scientific questions to be resolved throughfuture research include the continuing need (a) to quantify the risk of thyroid cancer after
exposure of children to 131I with doses to the thyroid below 500 mGy and for adults at evenhigher doses; (b) to investigate how long any radiation-induced risk of thyroid cancer will
persist and how it will reduce with time; (c) to improve the understanding of the effects ofconfounding factors, such as the influence of iodine deficiency on the risk of thyroid cancer;and (d) to identify biomarkers for radiation-induced thyroid cancer.

ACKNOWLEDGEMENTS
The secretariat is grateful to M. Balonov (Russian Federation), D. Bazyka (Ukraine),
Y. Demidchik (Belarus), V. Ivanov (Russian Federation), P. Jacob (Germany), A.N. Stazharau(Belarus) and W. Weiss (Germany) for their contributions to this white paper.

ANNEX
Incidence rates of thyroid cancer in 1982–2015 among those exposed under the age of 18 in Belarus, Russian Federation (Bryansk, Kaluga, Orel andTula oblasts) and Ukraine
Update of table D11 in the UNSCEAR 2008 Report, annex D [U2]Belarus [I2, S4]
Sex

F
0–4
M

F
5–9
M

F
10–14
M

F
15–18
M

F
Total
(0–18)
M

Parameter

Calendar year periods
1982–1985

1986–1990

1991–1995

1996–2000

2001–2005

2006–20102011–2015

11

155

258

209

276

437

Crude rate per 10 PY

0.57

8.02

13.35

10.82

14.52

23.97

Number of cases

9

103

146

79

79

101

Crude rate per 10 PY

0.45

5.21

7.38

3.99

4.01

5.25

Number of cases

9

108

91

169

288

403

Crude rate per 10 PY

0.50

6.02

5.07

9.42

16.57

21.31

Number of cases

10

66

42

59

64

106

Crude rate per 10 PY

0.54

3.59

2.29

3.21

3.66

5.45

Number of cases

8

67

131

202

298

398

Crude rate per 10 PY

0.46

3.85

7.52

11.60

17.46

23.05

Number of cases

7

12

43

60

77

101

Crude rate per 10 PY

0.39

0.67

2.39

3.34

4.67

5.88

Number of cases

15

57

109

223

309

358

Crude rate per 10 PY

1.11

4.21

8.05

16.46

18.23

21.16

Number of cases

5

9

33

40

64

76

Number of cases
5

5

5

5

5

5

5

5

Crude rate per 10 PY

0.37

0.66

2.41

2.93

4.0

4.73

Number of cases

2

43

387

589

803

1 171

1 596

Crude rate per 105 PY

0.04

0.64

5.72

8.71

11.88

16.64

22.38

Number of cases

1

31

190

264

238

284

384

0.02

0.45

2.75

3.82

3.44

4.10

5.34

5

Crude rate per 10 PY

EVALUATION OF DATA ON THYROID CANCER IN REGIONS AFFECTED BY THE CHERNOBYL ACCIDENT 15

Age at exposure(years)Age at exposure(years)

Sex

F
0–4
M

F
5–9
M

F
10–14
M

F
15–18
M

F
Total
(0–18)
M

Parameter

Calendar year periods
1982–1985

1986–1990

1991–1995

1996–2000

2001–2005

2006–20102011–2015

0

1

13

36

46

57

96

Crude rate per 10 PY

0.0

0.12

1.5

3.9

5.2

6.3

11.5

Number of cases

0

0

12

26

24

21

15

Crude rate per 10 PY

0.0

0.0

1.3

2.8

2.7

2.2

1.7

Number of cases

1

2

20

37

52

117

133

0.15

0.24

2.4

4.3

6.4

13.4

16.5

Number of cases5

55

Crude rate per 10 PY
Number of cases

0

1

6

10

14

21

19

Crude rate per 105 PY

0.0

0.12

0.69

1.1

1.7

2.4

2.3

Number of cases

0

3

24

48

108

131

178

Crude rate per 10 PY

0.0

0.36

3.0

6.0

13.9

15.3

22.5

Number of cases

0

1

9

9

10

19

40

Crude rate per 10 PY

0.0

0.12

1.1

1.1

1.2

2.3

5.3

Number of cases

1

8

43

61

87

91

126

0.18

1.2

7.2

10.0

13.9

14.4

21.5

5

5

5

Crude rate per 10 PY
Number of cases

0

1

8

14

17

20

20

Crude rate per 105 PY

0.0

0.15

1.3

2.1

2.6

3.3

3.6

Number of cases

2

14

100

182

293

396

533

Crude rate per 10 PY

0.09

0.44

3.2

5.7

9.5

12.1

17.7

Number of cases

0

3

35

59

65

81

94

0.0

0.09

1.1

1.8

2.0

2.5

3.1

5

5

Crude rate per 10 PY

16 WHITE PAPER

Russian Federation (Bryansk, Kaluga, Orel and Tula oblasts) [I1, I2]

Ukraine [B2, I2]
Age at exposure(years)

Sex

F
0–4
M

5–9
M

F
10–14
M

F
15–18
M

F
Total
(0–18)
M

Calendar year periods
1982–1985

1986–1990

1991–1995

1996–2000

2001–2005

2006–20102011–2015

6

85

202

254

420

957

Crude rate per 10 PY

0.1

0.9

2.2

2.9

4.6

11.0

Number of cases

9

55

91

103

103

183

0.1

0.6

0.9

1.1

1.1

2.0

1

20

106

181

326

601

1 091

0.01

0.2

1.2

2.0

3.9

7.0

14.0

7

40

57

74

121

218

0.1

0.4

0.6

0.8

1.4

2.8

Number of cases
5

5

Crude rate per 10 PY
Number of cases
5

Crude rate per 10 PY
Number of cases
5

Crude rate per 10 PY
Number of cases

9

35

113

252

496

693

1 148

Crude rate per 105 PY

0.1

0.4

1.2

2.8

5.7

8.3

14.9

Number of cases

7

18

34

55

99

140

248

Crude rate per 10 PY

0.1

0.2

0.4

0.6

1.1

1.7

3.4

Number of cases

15

54

176

277

403

624

988

Crude rate per 10 PY

0.3

0.8

2.6

4.0

5.4

9.7

16.9

Number of cases

7

15

37

53

74

121

190

0.1

0.2

0.5

0.7

1.0

2.0

3.5

Number of cases

25

115

480

912

1 479

2 338

4 184

Crude rate per 105 PY

0.1

0.3

1.4

2.7

4.4

7.2

13.9

Number of cases

14

49

166

256

350

485

839

0.05

0.1

0.5

0.7

1.0

1.5

2.8

5

5

5

Crude rate per 10 PY

5Crude rate per 10 PY

EVALUATION OF DATA ON THYROID CANCER IN REGIONS AFFECTED BY THE CHERNOBYL ACCIDENT 17

F

Parameter

EVALUATION OF DATA ON THYROID CANCER IN REGIONS AFFECTED BY THE CHERNOBYL ACCIDENT

19

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V.18-02117

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