Chernobyl Radiation-induced Thyroid Cancers in Belarus
Mikhail V. MALKO
Joint Institute of Power and Nuclear Research, National Academy of Sciences of Belarus
Krasin Str. 99, Minsk, Sosny, 220109, Republic of Belarus: mvmalko@malkom.belpak.minsk.by

Abstract
Assessment of incidence and mortality for thyroid cancers carried out for the Belarusian
population is described in the present report. It is found that in the period of 1987-2000 about 4,400
radiation-induced thyroid cancers appeared in Belarus: 692 cancers among children and 3,709
cancers among adolescents and adults. The number of lethal thyroid cancers in this period of timein Belarus was assessed as about 350 cases. The excessive absolute risk, EAR, of thyroid cancer
incidence assessed for the period of 1987-2000 on the basis of given data on the morbidity and theassessed collective thyroid dose of irradiation is (2.5 – 5.0) per 104 PYGy. The EAR value of
thyroid cancer mortality is assessed as (0.20 - 0.40) per 104 PYGy. The excessive relative risk, ERR,of thyroid cancer incidence is assessed as (11.2 – 22.4)/Gy. The radiation risks of thyroid cancers
found in the present report are higher than the risk coefficients established for atomic bomb
survivors that were irradiated with dose rates some thousand times higher than populations ofBelarus affected by the Chernobyl accident. The absence of marked latency period is anotherfeature of radiation-induced thyroid cancers caused in Belarus as a result of this accident.

INTRODUCTION
As a result of the Chernobyl accident a large amount of radioactive substances escaped from thedestroyed reactor. Especially high release was in case of volatile radionuclides. According to the existingassessments about 50% of the 131I isotope inventory came into the environment [1]. This caused theradioactive contamination of many countries of the Northern Hemisphere [2]. However, the main fractionof radioactive materials released by the accident deposited in Belarus. The total amount of the 131I
deposited in Belarus is assessed as approximately 3.3⋅1017 Bq or about 18.5% of the total amount of 131I
that escaped from the Chernobyl reactor [3]. Practically the whole territory of Belarus was contaminatedwith this and other iodine isotopes. This resulted in very high doses delivered to thyroid gland of a large
amount of people in Belarus. It is well known that the highest thyroid doses in Belarus were received by
the inhabitants of the south districts of the Gomel region and of the southeast districts of the Brest region[4]. According to the data of Yu.Gavrilin et al.[4], the arithmetic mean thyroid dose in the age group 0-6
years of the children living in settlements of the Khoiniki district settled in the 30-km zone was about 4.7
Gy and maximal doses were about 50 Gy. Very high thyroid doses were received also by the inhabitantsof Brest and Mogilev regions. This is the reason for a very high increase in the thyroid cancer incidence inBelarus after the Chernobyl accident, especially among the children of Gomel and Brest regions.
V.Kazakov, E.Demidchik and L.N.Astakhova [5] published the first reliable data on the significant
increase in the thyroid cancer morbidity among the children of Belarus after the Chernobyl accident.
However, specialists met these data with a big skepticism. There were serious reasons for this skepticism.
At first, the data of [5] have shown an unusual short latent period of about 2-3 years. At second, no similar
data about the increase in the incidence of thyroid cancer among the children of the affected areas of the
Ukraine and Russia were known at the time of publication [5]. This was very strange because all
specialists believed that the Ukraine was affected by the Chernobyl accident more significantly than
Belarus. It is now clear that such assumption was fully incorrect.

240

In order to explain an unusually high incidence of thyroid cancer among the children of Belarus somespecialists proposed an idea that this increase was caused by the improved screening in Belarus [6].
This situation changed only after the First International Conference on the RadiologicalConsequences of the Chernobyl Accident held in Mink in 1996 from 18 to 22 March. At the conference
new data on the thyroid cancer incidence in Belarus [7] as well as similar data on the increase of the
thyroid cancer incidence among the children of the Ukraine [8] and Russia [9] were presented. Data on the
thyroid cancer morbidity in these two countries differed only quantitative from the Belarusian data.
Analysis of the data on the thyroid cancer morbidity in three countries has shown that the highest
increase in the thyroid cancer incidence was registered in Belarus and the lowest in Russia. Such
difference was explained later on the basis of thyroid doses delivered to inhabitants of the affected areas ofBelarus, the Ukraine and Russia. It was shown that the highest irradiation doses of thyroid glands werereceived in Belarus and the lowest in Russia [3].
At the Minsk conference a number of evidences were given that indicated the association between theincrease in the incidence in thyroid cancer among the children of Belarus, the Ukraine and Russia andionizing origin. For example, it was shown that in Belarus thyroid cancer has mainly appeared in thechildren irradiated as a result of the Chernobyl accident. It was established that out of the total number of
390 thyroid cancers registered among the children of Belarus in 1986-1994 and the first 7 months of 1995
380 cases were diagnosed for the children born before the Chernobyl accident, 6 cases for the children
born at the time of the accident and only 4 cases for the children born after full disintegration ofradioiodine [7].
Other very important finding relates to the histological types of thyroid cancers in countries affectedby the Chernobyl accident. For instance, more than 90% of thyroid cancer in Belarus and the Ukraine
were papillary carcinoma as compared to 68% of the England and Wales tumors [10]. Within the papillarycarcinoma over 70% of thyroid cancers in the countries affected by the Chernobyl accident were of thesolid follicular type compared to only 40% in England and Wales [10].
At present, practically nobody doubts in the manifestation of radiation-induced thyroid cancers
among the children of the affected areas of the former USSR [11]. However, up to the present time
nobody recognizes the manifestation of radiation-induced thyroid cancers among adolescents and adults
despite of the existence of reliable data indicating the presence of radiation-induced thyroid cancers forother age categories than children [12].
The existence of such controversies requires careful studies of incidences of thyroid cancers bydifferent age categories in order to establish evidences on a link between radiation and the increased
morbidity of thyroid cancer. The present report describes the results of an analysis of incidences in thyroid
cancers among children, adolescents and adults of Belarus. The excessive absolute risk, EAR, and theexcessive relative risk, ERR, were also assessed in the report. Only data established for Belarus wereconsidered because the population of Belarus received the highest thyroid dose [3]. Due to this fact a moreclear manifestation of irradiation effects may be expected in Belarus. It is also important that the
epidemiological data established in Belarus are more comprehensive and accurate than in Russia and the
Ukraine. The present report is practically the further development of our previous studies [13,14].
MATERIALS AND METHODS
Data established by specialists of the Thyroid Surgery Registry of the Scientific and Practical Centre
for Thyroid Tumors of the Institute of Medical Radiology and Endocrinology of the Ministry for HealthCare of the Republic of Belarus [5,7,15-19] as well as data of the Belarusian Cancer Registry [20-28] andother official data [29-34] were used in the present report for an assessment of radiation-induced thyroid
cancer in Belarus. Results given in the present report were assessed for the mixed populations of children,
adolescents and adults of Belarus. In case of adults no separate consideration of persons involved in241mitigation work of the Chernobyl accident or so-called liquidators was performed. No separate assessment
for males and females was also conducted.
Monitoring of malignant neoplasms in Belarus began in 1950s [20-29]. Therefore, quite accurate data
exist at least from 1960s. However, until 1994 only sparse information on the incidence and mortality inseparate malignant neoplasms was published in the country. These were data on crude incidence andmortality in malignant neoplasms combined together and given for Belarus in total. Beginning from 1994
the Belarusian Cancer Registry publishes annually their statistical books that contain more detailed
information on the incidence and mortality in malignant neoplasms. This information is given for Belarus
in total as well as for separate administrative units of the country. Belarus is divided in 7 administrative
units: 6 regions (Brest, Vitebsk, Gomel, Grodno, Minsk, Mogilev) and the Minsk city which is the capitalof the country [29-34]. The smallest administrative unit is the Mogilev region with 1.2 million of people.
The largest unit is the Minsk city with the number of inhabitants about 1.7 million [29-34].
The excessive absolute risk, EAR, was estimated with the simplified expression:

EAR = (O − E ) / N PYSv .

(1)

Here O – observed number of thyroid cancer; E – expected number of thyroid cancer; NPYSv – number of
person⋅year⋅sievert under risk.
The value of NPYSv was assessed on the basis of equation:coll
N PYSv = H Th
⋅Y .

(2)

Here H Th − collective thyroid equivalent dose delivered in 1986; Y − number of years under risk after thecollaccident.
The excessive relative risk, ERR, was calculated by using of the expression:

ERR = (O / E − 1) / H ind .

(3)

Here H ind − average individual dose of irradiation.
The value of H ind can be determined with the formula:coll
H ind = H Th
/N .

(4)

Here N is the number of people with irradiated thyroid glands.
RESULTS AND DISCUSSION
Morbidity of thyroid cancers among children of Belarus
Thyroid cancer is a quite rare disease for children. According to the data of the Thyroid Surgery
Registry [15] in 1966-1985 (or during 20 years) only 21 cases of thyroid cancers were registered in thechildren of Belarus. This gives 1 case of thyroid cancer per year as a spontaneous morbidity of thyroid
cancer for the Belarusian children. The average number of children (less than 15 years old) in Belarus in
the pre-accidental period was about 2.37 million [29-34]. As can be calculated from this figures, thespontaneous morbidity of thyroid cancer for the children of Belarus before the Chernobyl accident wasapproximately 0.4 cases per million. This value is very close to the spontaneous morbidity of thyroid
cancer in the children of England and Wales [10] equal to approximately 0.5 cases per million. Data in
Table 1 taken from [17] demonstrate that before the Chernobyl accident only 3 possibilities wereregistered as the number of thyroid cancer incidence among the children. They were 0, 1 and 2 cases per
year. This means that the highest number of spontaneous thyroid cancers among the children of Belarus isequal to 2 cases in a year and the spontaneous incidence rate in thyroid cancers for the children of Belarus
is 1 ± 1 cases in a year.
The situation changed after the Chernobyl accident. Table 2 contains the numbers of thyroid cancers
registered annually in 1986-1997 [16]. Already in 1987, 4 cases of thyroid cancers were registered in the242Belarusian children. This number is by a factor of 2 higher than the maximal possible number of
spontaneous cases that can realize annually. The data in Table 2 demonstrate practically an absence of thelatency period for radiation-induced thyroid cancers among the children of Belarus. The first radiationinduced thyroid cancers among them, according to Table 2, manifested already in 1987 or 1 year after the
Chernobyl accident. This finding was unexpected for the majority of scientists because the believing in theexistence of the latency, at least 5 years long, was the common wisdom of radiation biology.
As can be seen from Table 2, the highest increase in the incidence of thyroid cancers among children
manifested in Gomel (53.1%) and Brest (23.5) regions and the lowest in Vitebsk region. These findings
correlate with the level of contamination of different Belarusian regions with iodine isotopes discharged tothe environment by the Chernobyl accident [35].
Our assessment demonstrates that 692 additional thyroid cancers in the children of Belarus wereregistered in 1987-2000 (see Table 3). The numbers of observed thyroid cancers in children in 1987-1997
were taken from the last row of the previous table.
The numbers of observed thyroid cancers in children in 1998-2000 were established by [19].
Data given in Tables 2 and 3 as well as in Fig. 1 show that the incidence in thyroid cancers among
the children of Belarus increased monotonously from 4 cases in 1987 to 91 cases in 1995, and thendecreased also monotonously to 31 cases in 2000.
The reason for the decline in thyroid cancers incidence after 1995 is very easy. It is caused simply by
the medical statistics used in Belarus [13,14]. All official data on cancer morbidity in Belarus are given for
two groups. The first group includes the data for those persons for which cancers as well as thyroid cancer
are diagnosed when they are less than 15 years old. The second group includes all persons that are older
than 15 years. Thus, when a person reaches 15 years at the time of the thyroid cancer diagnose, this person
will be considered as a person belonging to the group of adolescents and adults. This means that there is apermanent transition from the subgroup of children to the subgroup of adolescents and adults. As a resultof this transition, the children that received thyroid doses by the Chernobyl accident are being lost
Table 1. Incidence of thyroid cancer among children of Belarus before the Chernobyl accident.
Year

Male

1971
1972
1973
1974
1975
1976
1977
1978

0
0
1
0
0
2
1
0

Femal
e
0
0
1
2
1
0
1
1

Both

Year

Male

Female

Both

0
0
2
2
1
2
2
1

1979
1980
1981
1982
1983
1984
1985
1986

0
0
0
1
0
0
0
1

0
0
1
0
0
0
2
1

0
0
1
1
0
0
2
2

Table 2. Thyroid cancer among children of Belarus after the Chernobyl accident [16].
Region/Year
Brest
Vitebsk
Gomel
GrodnoMinsk
Mogilev
Minsk-city
Belarus total

86
0
0
1
1
0
0
0
2

87
0
0
2
1
1
0
0
4

88
1
0
1
1
1
0
1
5

89
1
0
3
2
1
0
0
7

90
7
1
14
0
1
2
4
29

91
5
3
43
2
1
3
2
59
243

92
17
2
34
4
4
1
4
66

93
24
0
36
3
4
7
5
79

94
21
1
44
5
6
4
1
82

95
21
0
48
5
1
6
10
91

96
25
0
42
5
5
3
4
84

97
13
0
37
3
6
4
3
66

Total
135
7
305
32
31
30
34
574

Table 3. Incidence of thyroid cancers among children of Belarus after the Chernobyl accident.
Year

Observed

Expected

1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
1987-2000

4
5
7
29
59
66
79
82
91
84
66
54
49
31
706

1
1
1
1
1
1
1
1
1
1
1
1
1
1
1

Observed Expected
3
4
6
28
58
65
78
81
90
83
65
53
48
30
692

SIR
4
5
7
29
59
66
79
82
91
84
66
54
49
31
50.4

permanently from the children’s subgroup. This transition causes the decrease in the incidence in thyroid
cancers among the children of Belarus with the time.
Thus, in order to establish real incidence rates of thyroid cancers for the children of Belarus one
needs to determine the number of irradiated children in the children’s subgroup. Such data are given in thethird column of Table 4. It is seen from this table that the number of irradiated children in Belarus
decreased from 2.303 million in 1986 down to 0.223 million in 2000.
The last group of the children with thyroid doses caused by iodine isotopes will move to thesubgroup of adolescents and adults in the first 3 months of 2001. This group will include only the childrenthat were irradiated in utero. It means that beginning from 2002 the incidence of thyroid cancers among
the children of Belarus will be the same level as before the Chernobyl accident, or 1 case annually.
For forecasting the number of thyroid cancers for 2001, the simplified procedure described in ourprevious publication [14] was used. It is based on a linear extrapolation between 2000 and 2002. The data
100

Number of cases

80

60

40

20

2003

2002

2001

2000

1999

1998

1997

1996

1995

1994

1993

1992

1991

1990

1989

1988

1987

1986

0

Years

Fig. 1. Number of thyroid cancer incidence among the children of Belarus.
244

Table 4. Assessment of the incidence rates of thyroid cancers among children of Belarus after the
Chernobyl accident.
Amounts ofTotal amounts
Incidence rateirradiatedObservedYearof children
Observed(per 100,000
children (inExpected(in millions)children)millions)
1986
2.303
2.303
2
1
0.0434
1987
2.315
2.152
4
3
0.1394
1988
2.325
1.999
5
4
0.2001
1989
2.337
1.858
7
6
0.3229
1990
2.337
1.715
29
28
1.6327
1991
2.335
1.581
59
58
3.6686
1992
2.328
1.446
66
65
4.4951
1993
2.327
1.328
79
78
5.8735
1994
2.307
1.198
82
81
6.7669
1995
2.273
1.062
91
90
8.4746
1996
2.217
0.910
84
83
9.1209
1997
2.154
0.758
66
65
8.5752
1998
2.086
0.597
54
53
8.8777
1999
1.961
0.379
49
48
12.6649
2000
1.898
0.223
31
30
13.4529
given in [28] shows that 31 cases of thyroid cancers were registered in 2000. Linear extrapolation between
31cases in 2000 and 1 case in 2002 gives 16 thyroid cancers in 2001 for the children of Belarus. Thisvalue is used in constructing Fig. 2.
The total number of thyroid cancers in the Belarusian children that can manifest in the period from1986 up to 2002 will be then equal to 722 cases. Subtraction from this number of the number of
spontaneous thyroid cancers that have to appear in 1987-2001 (15 cases) will give the number of radiation– induced thyroid cancers among the children of Belarus equal to 707 cases.
At the same time, we have to expect some contribution to the additional thyroid cancers that werecaused as a result of irradiation of their thyroid glands by other isotopes than isotopes of iodine. Thiseffect can give in reality some higher numbers of thyroid cancers in the children of Belarus after 2001 than
14

Incidence in 100,000

12
10
8
6
4
2
2000

1999

1998

1997

1996

1995

1994

1993

1992

1991

1990

1989

1988

1987

1986

0

Years

Fig. 2. Incidence rates of thyroid cancer among irradiated children of Belarus.

245the spontaneous incidence before the accident. However, this number can not differ very strong from theforecasted number of 707 cases.
As can be seen from Fig. 2, the incidence rate of thyroid cancers in the children of Belarus increases
practically linearly after the Chernobyl accident. It increased from 0.0434 cases per 100,000 children in
1987 up to 13.5 cases per 100,000 children in 2000 or by a factor of 310 (see the last column of Table 4).
For calculation of incidence rates of thyroid cancers given in the last column of Table 4, the numbers ofchildren in the third column and the numbers of additional thyroid cancers in the fifth column were used.
Assessment of the incidence rate of thyroid cancers in the children of Belarus carried out on the
population data presented in the second column of Table 4 gives for 2000 1.58 cases per 100,000 children.
This number is 8.5 times less than the real incidence rate of thyroid cancers in 2000 among the children
affected by the Chernobyl accident. It is clear that the method of medical statistics used in Belarus gives
an apparent decrease of thyroid cancers manifested in children after 1995. This apparent decrease iscaused by the transition of persons that reach 15 years to the sub-population of adolescents and adults. We
assessed that in 1996-2000 approximately 400 cases of thyroid cancers manifested in persons that wereless than 15 years at the time of the Chernobyl accident went to the sub-population of adolescents andadults of Belarus. We assessed this number of thyroid cancers (400 cases) by use of the following
expression:
Y=13.5•T – 26,833(5)
It was developed on the basis of incidences of thyroid cancers in the children of Belarus in 19891996 by using of the least square method.
Addition of 400 cases to 706 cases (see the last value in the second column of Table 3) gives theapproximate total number of thyroid cancers manifested in Belarus in 1987-2000 in those persons thatwere children at the time of the Chernobyl accident. It is equal to 1106 cases instead of 706 cases
considered by specialists. This simplified analysis demonstrates what incorrect conclusions can be drawn
about the development of the morbidity of thyroid cancers in the children of Belarus.
Morbidity in thyroid cancers among adolescents and adults of Belarus
The described above transition of the children with thyroid doses from the children’s subgroup into
subgroup of adolescents and adults decreases the number of radiation-induced thyroid cancers among thepersons that are considered as children according to rules of the medical accounting. At the same time thistransition causes an additional increase in the incidence in thyroid cancers by adolescents and adults ofBelarus after the accident. However, the transition between different subgroups can not explain a very
high increase in the incidence of thyroid cancers among adolescents and adults of Belarus after the
Chernobyl accident (see Fig.3). The upper curve in Fig.3 presents the registered number of thyroid cancers
in adolescents and adults. Here the data for 1977 –1997 were taken from [7,16]. The observed numbers of
thyroid cancers in 1998-2000 were estimated on the basis of the data of the Belarusian Cancer Registry[26-28].
According to [26] the incidence of thyroid cancer among the Belarusian population was 7.5 cases per
100,000 persons in 1998 and the total number of people in Belarus was 10.2 million. By using these data
one can calculate the number of thyroid cancers registered in Belarus in 1998 equal to 765 cases. This
number is a sum of thyroid cancers in children and in adolescents and adults. Subtraction of the number ofchildren’s cancers (54 cases in 1998 [17]) from the total numbers of thyroid cancers gives the number ofthyroid cancers in adolescents and adults equal to 711 cases. This value is shown in the second column ofTable 6. The similar method was used for assessment of number of thyroid cancers registered byadolescents and adults of Belarus in 1999 and 2000.
Two other curves in Fig.3 give assessed spontaneous or expected cases in the group of adolescentsand adults of Belarus.
246

Observed

Expression (7)Expression (6)1000

Number of cases

800

600

400

200

1999

1997

1995

1993

1991

1989

1987

1985

1983

1981

1979

1977

0

Years

Fig. 3. Incidence of thyroid cancers among adolescents and adults of Belarus in 19772000 (absolute numbers).
The expected numbers of thyroid cancers in adolescents and adults of Belarus combined together canbe assessed by using of the formula [13]:(6)
Ej=E0·(1+a)j,
Where E0 = spontaneous morbidity of adolescents and adults in 1977;
a = is a constant showing the annual increase in the incidence of the thyroid cancer, it is about
0.02 (2% increase annually);j = is the number of the consequent year beginning with j =0 for 1977 (j = 1 for 1978 and so on);
Ej = spontaneous morbidity of adolescents and adults in the year j.
Data calculated with this expression are presented in Fig.3 (the lower line in figure).
The last expression was established in [13] on the basis of data on thyroid cancer incidences inadolescents and adults of Belarus [7] that were registered before the Chernobyl accident. It was used for
calculation of the expected numbers of thyroid cancers among adolescents and adults of Belarus that arealso shown in the third column of the Table 5.
Comparison of the data calculated by using of the expression (6) with the observed data presented in
the second column of Table 5 demonstrates a very good agreement for 1977-1985 and a quite significant
disagreement for 1986 and 1987. We believe that inefficient screening of thyroid cancers among
adolescents and adults before the Chernobyl accident causes this disagreement. The following arguments
support this idea.
As can be seen from Table 5, the number of registered thyroid cancers in adolescents and adults
abruptly increased in 1986 and 1987, and then changed only very insignificant from 1987 to 1988. The
probable reason for such change in the incidence in thyroid cancers among adolescents and adults ofBelarus can be a more careful investigation of this category of people by specialists or an improved
screening.
Table 3 demonstrates that in case of children a clear manifestation of additional thyroid cancers
began in 1987 and a marked increase in the incidence occurred only from 1990 or 3 years after the
Chernobyl accident. It is difficult to believe that in a case of adolescents and adults a significant
manifestation of radiation-induced thyroid cancers began already in 1986. In order to correct the possible
effect of inefficient screening in a case of adolescents and adults we modified the expression (6) byinserting of the coefficient c that takes into account the screening effect. This modified expression is given247

below:
Ej=E0·c·(1+a)j,(7)

We determined the coefficient c by dividing of the observed number in 1987 of thyroid cancers inadolescents and adults by the number calculated with the expression (6). By correcting this way the
expected incidences of thyroid cancers in adolescents and adults are shown in the fourth column of Table
5. The middle curve in Fig. 3 presents values calculated with the last expression.
We applied the same method for the possible screening effect to assessment of spontaneous
morbidity of thyroid cancers in adolescents and adults of Belarus for the whole period after the Chernobyl
accident. The corrected numbers of expected thyroid cancers among this category of the Belarusian
population are given in the third column of Table 6.
Data on numbers of thyroid cancers registered by adolescents and adults of Belarus in 1986 – 1997
given in the second column of Table 6 were taken from the report of [16]. The number of thyroid cancers
in this column for 1998-2000 were calculated on the basis of the morbidity rate in thyroid cancer of thewhole Belarusian population and a number of people in the country that are presented in the reports of theTable 5. Incidence of thyroid cancers among adolescents and adults of Belarus in 1977-1988.
Year

Observed

1977
1978
1979
1980
1981
1982
1983
984
1985
1986
1987
1988

121
97
101
127
132
131
136
139
148
162
202
207

Calculated values
Expression (6)Expression (7)
121
166
123
169
126
172
128
176
131
179
134
183
136
187
139
190
142
194
145
198
147
202
160
206

Table 6. Incidence of thyroid cancers among adolescents and adults of Belarus after the Chernobyl
accident.
ObservedYear
Observed
Expected
SIR
Expected
1987
202
202
0
1
1988
207
206
1
1.005
1989
226
210
16
1.076
1990
289
214
75
1.350
1991
340
219
121
1.552
1992
416
223
193
1.865
1993
512
227
285
2.256
1994
553
232
321
2.384
1995
531
237
294
2.241
1996
568
241
327
2.357
1997
641
246
395
2.606
1998
711
251
460
2.833
1999
847
256
591
3.309
2000
891
261
630
3.414
1987-2000
6934
3225
3709
2.150

248

Table 7.Additional incidence of thyroid cancers in Belarus in 1987-2000.
Year

Children

1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
1987-2000

3
4
6
28
58
65
78
81
90
83
65
53
48
30
692

Adolescents andadults
0
1
16
75
121
193
285
321
294
327
395
460
591
630
3709

All ages
3
5
22
103
179
258
363
402
384
410
460
513
639
660
4401

Belarusian Cancer Registry [26-28].
As can be seen from Table 6, the first radiation-induced thyroid cancer in the group of adolescentsand adults of Belarus, according to our assessment, manifested already in 1988 or 2 years after the
Chernobyl accident. This means that the minimal latency period of radiation-induced thyroid cancers by
this category is about 2 years and not 10 years later as could be expected on the basis of an experience
accumulated previously.
The early manifestation of radiation-induced thyroid cancers in Belarus shows clear an absence of thesignificant latent period in radiation-induced thyroid cancer in Belarus. This is not surprising. Such
possibility was forecasted a long time ago by J.Gofman [36]. He stated in his book “Radiation and Human
Health” [36] that the latent period of some cancers can depend on the number of persons in the group
under study. In a case of a small group it can be required very long time in order to get provable differencewith a control group. On the contrary, in a case of very large group of irradiated persons the provabledifference can be reached in a very short time. Such situation had the place in Belarus where about
10,000,000 persons were irradiated.
The total number of additional thyroid cancers among adolescents and adults manifested in Belarus
in 1987 - 2000 calculated by using the method described here is equal to 3,709 cases (see Table 6).
Assessment of additional thyroid cancers in Belarus
Table 7 contains the numbers of additional thyroid cancers manifested in Belarus in 1987-2000
estimated in previous sections of the present report. The total number of additional thyroid cancers
appeared in the country after the Chernobyl accident is 4,401: 692 cases in children and 3,907 cases inadolescents and adults.
Mortality from radiation-induced thyroid cancers in Belarus
The similar procedure as used by assessment of the thyroid cancer incidence can be used forassessment of the number of people in Belarus that died from thyroid cancer caused by the Chernobyl
accident.
Fig. 4 shows the dynamic in the crude mortality rate resulted from the morbidity in thyroid cancers.
The data shown in this figure were taken from the reports of the Belarusian Cancer Registry [20-28]. They
give the mortality of thyroid cancers for the whole Belarusian population (children, adolescents and249

Mortality rates (per 100,000 persons)Observed

Expected

1.2

1

0.8

0.6

2000

1999

1998

1997

1996

1995

1994

1993

1992

1991

1990

1989

1988

1987

0.4
Years
Fig. 4. Mortality rates of thyroid cancers in the Belarusian population.
adults) determined as a number of cases in 100,000 persons. The upper (solid) line gives the observedmortality rates. The lower (dotted) line shows the “spontaneous” mortality rates determined by us. We
assessed the “spontaneous” mortality assuming that the mortality rate in 1987 did not differ from thespontaneous mortality and that the annual increase in the spontaneous mortality was in 1987-2000
approximately 2%. Using these data as well as the data on the number of people in Belarus in 1986-2000[29-34] we assessed the additional number of fatal thyroid cancers in 1987-2000 as approximately 351
cases.
Radiation risks
Excess absolute risk (EAR) for thyroid cancer incidence:coll
equal to 127⋅104 man⋅Gy was given as the total collective thyroid dose for the
The value of H Th
Belarusian population in our previous report [3] The assessment of the collective equivalent thyroid dosein the report [3] was carried out assuming the ratio of activities of 131I to 137Cs (recalculated on the 26
April of 1986) in Belarus equal to 20. This is derived from the data on the total discharge of 131I and 137Cs
collequal to 121⋅104 man⋅Gy.
in the environment [3]. In present report we used the corrected value of H Th

However, new experimental data [36] established on the basis of the isotope 129I demonstrates apossibility of incorrectness of the data on release of the isotope 131I accepted in the report [3]. The authors[37] give a value of the ratio of activities of 131I to 137Cs (recalculated on the 26 April of 1986) equal to 10±3.1. The collective equivalent thyroid dose assessed using this value is equal to 60.5⋅104 man⋅Gy. The
coll
can be considered as the lower limit of the collective equivalent thyroid dose of thelast value of H Th
coll
Belarusian population caused by the Chernobyl accident. It is by a factor of 2 less than the value of H Th

equal to 121⋅104 man⋅Gy, which can be considered as the upper limit of the collective equivalent thyroidcolldoses of the Belarusian population. The real value of H Th
can be between these two assessments.

Inserting these values and the number of additional thyroid cancers that was assessed in this work
(4,401 cases) into the expression (1) gives the value of EAR for the incidence of additional thyroid
cancers in Belarus in 1987-2000 to be (2.5-5.0)/104 PYGy.
This value of the excessive absolute risk agrees quite well with EARs established in other studies.
250

For example, according to [37], the value of EAR for inhabitants of the Marshall Islands, which had astrong internal irradiation of thyroid gland, is estimated as 1.1 per 104 PYGy [38]. In the study ofconsequences of the Utah 131I fallout [39] the value of EAR equal to 3.3 per 104 PYGy was evaluated.
According to [40], the excessive absolute risk of radiation-induced thyroid cancers established for mixed
population of atomic bomb survivors is 1.1 per 104 PYGy (95% CI=0.55-1.75).
The last value is by factor 2-4 less than the excessive absolute risk of radiation thyroid cancers
assessed in this report for the Belarusian population. This discrepancy can not arise from anoverestimation of the number of additional thyroid cancers or from an underestimation of the collective
dose of thyroid gland irradiation used in the present report. We draw this conclusion because our value ofthe excessive absolute risk is in a qualitative agreement with the value of EAR established for Russianliquidators [12]. According to Ivanov et al [12], the excessive absolute risk of radiation-induced thyroid
cancers estimated for this category of irradiated people for period of 1986-1995 is 1.31 per 104 PYGy
(95% CI=0.55-1.75). The last value is by factor 1.8-3.6 less than our values of the EAR and close to thevalue established for atomic bomb survivors. However, Russian liquidators form a special group of people,
93 % of which consist of males that were in average 33.4 years old during their period of duty in the 30km zone [12]. Therefore, the excessive absolute risk of a mixed population that contains approximately
equal fractions of males and females has to be at least 2 times higher than the excessive absolute risk
established by Ivanov et al [12] for Russian liquidators.
Thus the high value of the EAR assessed in the present report for the Belarusian population is quite
reasonable. This is an unexpected result because it is generally accepted that radiation risks of cancers
decline with decrease of dose rates. The hypothesis about existence of such dependence between radiation
risks and dose rates was proposed in 1980 by the USA National Council on Radiation Protection andMeasurements (NCRP) [41]. In order to take into account this effect the NCRP introduced a special doserate-effectiveness factor (DREF). According to the NCRP assessment, values of DREF vary from 2 to 10
for different types of cancers. The United Nations Scientific Commission on Effects of Radiation(UNSCEAR) [42] and the International Commission on Radiological Protection (ICRP) [43] also
recognize reduction in a carcinogenic efficiency of ionizing radiation by decrease of a dose rate ofirradiation. As an evidence of this effect in case of radiation-induced thyroid cancers, the ICRP considers
the findings of Holm [44]. This author showed that a chronic irradiation of thyroid gland with 131I is 4
times less effective than acute irradiation with x-rays.
The irradiation of thyroid gland of atomic bomb survivors occurred in approximately 1 minute. Incase of liquidators of the Chernobyl accident as well as populations of the former USSR that were affectedby this accident the irradiation of thyroid gland lasted at least 1 month or 43,000 minutes. This means thatdose rates of the thyroid gland irradiation of liquidators and affected populations of the Soviet Union wereabout 40,000 times less than dose rates in case of atomic bomb survivors. It is clear that in case ofcorrectness of the NCRP hypothesis the values of EAR for the Belarusian population and liquidators haveto be less than the EAR value for atomic bomb survivors. Our data and the data of [12], however,demonstrate a fully disagreement with this expectation. Such disagreement is not the unique feature ofradiation-induced thyroid cancers in Belarus. Ivanov et al [45,46] found that radiation risks of solidcancers other than thyroid cancers for Russian liquidators are also by some factors higher than in case of
atomic bomb survivors. The authors [45,46] carried out their study by using the cohort method similar to
the method used by specialists of the Radiation Effects Research Foundation (RERF) and are quite
reasonable. Similar findings were also found in the report [47].
Excess relative risk (ERR) for thyroid cancer:
By using the data on observed and expected incidences of thyroid cancers in children, adolescentsand adults, the averaged value of the excessive relative risk, ERR, was also assessed on the basis of251expression (3). The values of O and E were obtained from the data shown in Tables 3 and 6. The value of

H ind was calculated by inserting the collective equivalent dose of the thyroid cancer irradiation of thecollequal to (60.5-121)⋅104 man⋅Gy into expression (4) and the number ofBelarusian population H Th

population in Belarus equal to 10 millions of people. Thus, the ERR value of (11.2-22.4)/Gy was obtained
for the mixed population in Belarus.
The assessed value of ERR is one order in magnitude higher than the value 0.84/Gy established foratomic bomb survivors [40]. However, it agrees very well with the excessive relative risk found forchildren and adolescents of the affected areas of Belarus, the Ukraine and Russia in the report [48].
According to authors [48], the excess relative risk per unit dose in the study area is equal to 90/Gy. One
needs to remember that the ERR value for children and adolescents is by some factors higher than for themixed population. Therefore, we can state that the excess relative risk assessed in our report for the mixed
Belarusian population agree quite well with the data of authors [48]. This allow us to conclude that ourvalue of the ERR for the incidence of radiation-induced thyroid cancers in Belarusian population is quite
accurate.
Excess absolute risk (EAR) for thyroid cancer mortality:
The excessive absolute risk of mortality resulting from the radiation-induced thyroid cancers was also
assessed. The same method as in case of the incidence in radiation thyroid cancers was used for thispurpose. It was found that the excessive absolute risk of mortality in radiation-induced thyroid cancers for
the Belarusian population is equal to (0.20-0.40) per 104 PYGy.
This value agrees quite well with the excessive absolute risk of the death from radiation-induced
thyroid cancer assessed by E.Gilbert [49]. The EAR of this author is about 0.125 - 0.25 per 104 PYGy.
However, the quality of our assessed data on mortality from radiation-induced thyroid cancers is not sogood as in case of data on the incidence in thyroid cancers. Therefore, we consider our assessment on themortality for radiation thyroid cancers in Belarus as a qualitative assessment.
CONCLUSIONS
1. The morbidity and the mortality from radiation-induced cancers in Belarus as a result of theChernobyl accident have been assessed for the period of time 1987-2000. According to our assessment,
692 radiation-induced thyroid cancers among the children of Belarus appeared in 1987-2000. The total
number of children's thyroid cancers that will manifest in 1987-2001 can be about 707 cases. The
number of the radiation-induced thyroid cancers manifested among adolescents and adults of Belarus
in 1987-2000 as a result of the Chernobyl accident was assessed as 3,709 cases. The total number ofradiation-induced thyroid cancers appeared in this period of time is about 4,401 cases. The number ofthe people that died in Belarus in 1987-2000 from radiation-induced cancer was assessed as 351 cases.
2. The low latency period of radiation-induced thyroid cancers among the Belarusian population was
found: about 1 year in case of children and about 2 years in case of adolescents and adults.
3. The excessive absolute risk, EAR, of the radiation-induced thyroid cancers manifested in Belarus after
the Chernobyl accident is assessed as (2.4 – 4.8) per 104 PYGy. This value is higher than the excessive
absolute risk established for atomic bomb survivors that were irradiated with dose rates many
thousands times higher than dose rates of the Belarusian population.
4. The excessive relative risk, ERR, of the incidence in radiation-induced thyroid cancers manifested in
Belarus after the Chernobyl accident is assessed as (11.2-22.4)/Gy. These values are one order higher
than the excessive relative risk established for atomic bomb survivors.
5. The high value of the radiation risks of the thyroid cancers incidence as well as the absence ofsignificant latency of radiation-induced thyroid cancers by the Belarusian population indicate an252

irrelevancy of radiation risks established from atomic bomb survivors for the Belarusian population.
6. The excessive absolute risk of the mortality in Belarus in 1987-2000 from the radiation-induced
thyroid cancers was assessed as (0.20 - 0.40) per 104 PYGy.
ACKNOWLEDGMENT
The author of the paper expresses his acknowledgment to Dr. Nadya Ryabakon for the assistance inpreparing of the manuscript.
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