Analysis of Chromosome Aberrations in Human Lymphocytes after
Accidental Exposure to Ionizing Radiation
Galina SNIGIRYOVA and Vladimir SHEVCHENKO*
Russian Scientific Center of Roentgeno-Radiology, MOH RF,
Profsoyuznaya st. 86, GSP-7, 117997 Moscow, Russia: snigiryova@mrids.rssi.ru*N.I. Vavilov Institute of General Genetics, RAS,
Gubkin st. 3, 117809 Moscow, Russia: Shevchenko@vigg.ru

Abstract
Aim: Analysis of the results of cytogenetic examination and reconstruction of irradiation doses by thefrequency of chromosome aberrations in the liquidators of the consequences of the Chernobyl NPP
accident.
Materials and methods: Over 1500 people who worked in the zone of the accident predominantly in1986 and in 1987 have been examined since 1986. All of them were examined by means of theclassical cytogenetic method which makes it possible to assess the level of unstable chromosome
aberrations. 64 patients (including 12 professional workers of the Kurchatov Institute) were examinedby the FISH method which permits the estimation of the level of symmetrical translocations.
Results: The cytogenetic examination performed in 1986 revealed a high level of cells with dicentrics(exceeding 16-fold the control level) in the group of liquidators. In succeeding years this levelsignificantly decreased. However even in 15 years after the works in the zone of the accident werecompleted the frequency of cells with dicentrics in the group of liquidators significantly exceeds thecontrol level. Using the frequency of cells with dicentrics and the calibration dose-response curve, thedose of irradiation was determined for the group of liquidators examined in 1986. The average dose ofirradiation made up 0.14 Gy. The frequency of translocations was used to estimate the exposure dose
for the group of 52 liquidators. The average dose for the whole group was estimated to be 0.16 Gy.
For 18 patients in whom the frequency of translocations significantly differed from the control level
the individual doses of exposure were determined. The scatter in the doses was from 0.22 to 1.0 Gy.
In 1996, 22 workers of the Kurchatov Institute were examined. In most of them (13 patients) the
frequency of dicentrics in peripheral blood lymphocytes was significantly higher than the control
values. In five patients, cells with multiple chromosome aberrations were discovered. Three examined
workers were exposed to super-high doses. The frequency of dicentrics in peripheral blood
lymphocytes of these patients was 100 and even 1000 times higher than the control level. The dosesof irradiation estimated for five workers of the Kurchatov Institute by the frequency of translocations
using the calibration curve were in the range between 0.21 and 2.51 Gy. These doses were calculated
without correction coefficients, i.e. without taking into account the fact that those workers were forseveral years exposed to multiple irradiations at different dose rates. Consideration of these conditions
will lead to an increase of the estimated absorbed doses.
Conclusion: The cytogenetic methods are sufficiently sensitive for assessing the condition of the cellhereditary structures. Analysis of unstable chromosome aberrations is a decisive method inmonitoring of large groups of people exposed to radiation as a result of nuclear accidents. The data ofcytogenetic examination can be one of the criteria in the formation of groups with an increased risk ofdevelopment of different diseases. One of the most promising cytogenetic methods of biologicaldosimetry is the analysis of stable translocations by the FISH method. The lower limit of dose
estimation by the frequency of translocations is 20-25 cGy.

256

1. Introduction
It is already over several decades that the analysis of chromosome aberrations in peripheral blood
lymphocytes has been successfully used to examine people exposed to radiation during various accidents[1-9]. The results of cytogenetic analysis serve as a basis for estimating absorbed radiation doses and forpredicting possible negative effects of irradiation.
The traditional procedure in biodosimetric studies is the analysis of the frequency of chromosome
aberrations, namely, dicentrics or symmetrical translocations. The radiobiological basis for applying
cytogenetic indices in dosimetric studies is the existence of a relationship between the dose and the yieldof chromosome aberrations as well as the coincidence of dose-effect curves upon irradiation of blood cells
in vitro and in vivo. It is customary to assume (and it was corroborated in some investigations) [10,11] thatthe frequencies of dicentrics and translocations immediately after irradiation are practically the same.
Cells with dicentrics are good indicators of irradiation within short periods after radiation exposure.
With post exposure time such cells are eliminated from peripheral blood because their passage throughmitosis is impeded [12]. In connection with this, the analysis of the frequency of dicentrics is of low
efficiency for retrospective estimation of radiation doses. After a long time following the radiation
exposure as well as under conditions of long-term chronic or prolonged irradiation the accumulated doseis estimated using the analysis of stable chromosome aberrations – symmetrical translocations. In theopinion of some authors [5,6,13-15] the frequency of cells with translocations remains constant for a longtime after irradiation or, as other researchers [16,17] suppose, it may decrease, but with a considerablyslower tempo as compared to cells with dicentrics. The latter fact imposes some limits on the assessmentof radiation doses by the frequency of translocations but the importance of using cytogenetic methods inbiodosimetric studies does not become less. In emergency situations, when reliable methods of physicaldosimetry are often absent, the analysis of chromosome aberrations permits the degree of radiation
damage of the organism to be determined taking into account its specific individual features and, in thefirst turn, its individual radiosensitivity. Information on “biological dose” is of great prognostic value forassessing the remote effects of irradiation and for choosing adequate methods of therapy and prophylaxis.
In the case of large-scale cytogenetic monitoring in groups of people exposed to radiation as a resultsof radiation accidents the analysis of unstable chromosome aberrations assumes particular importance. It
allows sufficiently large contingents of exposed people to be examined without special financial
expenditures and without employing sophisticated techniques. The results of such monitoring make itpossible to obtain information on the effects of the radiation factors on the organism, on the degree ofgenome damage, and finally they can be used as criteria in the formation of groups of people with a highrisk of occurrence of different diseases, including oncological ones [18,19].
2. Subjects, materials and methods
2.1. Case reports of examined patients
The Chernobyl nuclear power plant accident is the most dramatic ecological disaster of the 20th
century. One of the largest categories of the population that has suffered most as a result of thiscatastrophe are the clean-up workers (liquidators). Beginning in 1986, the Russian Research Center ofRoentgeno-Radiology and N.I. Vavilov Institute of General Genetics have been carrying out joint
cytogenetic examinations of the liquidators [20,21]. Over 1500 persons have been examined by the present
time. All examined liquidators participated in the restoration works within the 30 km zone in 1986 and1987. Some of the liquidators came to work in Chernobyl in succeeding years, too, up to 1995. The total
term of working in the zone of the accident was from 2 weeks to 13 months. The works made by theliquidators were highly diversified. Among the examined patients there were specialists in the field of
dosimetry and decontamination, scientific workers, physicians, drivers, erectors, “sarcophagus” builders,helicopter pilots and unskilled labourers. Most of the liquidators were exposed to doses up to 1 Gy.
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Official data on irradiation doses are available for approximately 60% of the examined patients. It should
be noted that during the work in the zone of the accident the liquidators were exposed to additional factorsof the non-radiation nature, including acute and chronic psychoemotional overloads, changes in the
habitual stereotype of life and in the dietary regimen determined by the forced staying on the contaminatedterritory [22].
All people who participated in the restoration works were examined by the classical cytogenetic
method permitting us to estimate the level of unstable chromosome [23]. Part of the patients (52 persons)was cytogenetically analyzed by the FISH method which allowed estimating the level of stable
chromosome aberrations [24].
The obtained cytogenetic data were compared with the results of examination of the control groupcomposed of quite healthy people of analogous age having no occupational contacts with ionizing
radiation sources (118 persons for classical analysis and 15 persons for FISH analysis).
2.2. Lymphocyte culture and slide preparation
The venous blood was collected in sterile test-tubes with lithium heparinate. Lymphocyte enriched
plasma was obtained by centrifugation of whole blood for 40-50 min at 1000 rpm. After that, 0.5 ml of theblood plasma was mixed with 4.5 ml of RPMI-164 medium containing 15% fetal calf serum. 2,5% ofphytohaemagglutinin, 2 mM glutamine, antibiotics and 10 mM BrdU. The cultures were incubated at 370C
for 48 hours. Chromosome preparations were obtained according to standard procedures [23].
Slides used for analysis of unstable chromosome aberrations were kept for 5 days at room
temperature and then subjected to special fluorescence plus Giemsa (FPG-) staining. For analysis of stable
chromosome aberrations the slides were kept in a nitrogen atmosphere at –200C until the moment offurther treatment. A cocktail of probes was used for investigation with the FISH method: biotin-labelled
DNA probes for chromosome 1, 4 and 12 in combination with a digoxigenin-labelled pancentromericprobe. DNA probes for our work were kindly provided by the Laboratory of Cytogenetics of the Instituteof Radiobiology (GSF, Munich). Hybridization in situ with DNA probes was carried out by the method of[24]. Probes for chromosomes were identified using fluorescein isothiocyanate (FITC) – labelledstreptavidin and pancentromeric probes and 7-amino-4-methylcoumarin-3-acetic acid (AMCA) – labelled
antibodies. Propidium iodide (PI) was used as a counterstain in antifade solution. The analysis was
performed using a set of filters permitting a simultaneous observation of the fluorescence of FITC and PI
as well as AMCA and PI.
Genomic frequencies (FG) for symmetrical translocations were calculated from the frequencies ofpainted translocations (FP) for target chromosomes by inversion of the equation FP=2.05×fP(1- fP)×FG,
where fP=0.192 is the fraction of the DNA contained in the painted chromosomes.
2.3. Dose-response curves
The doses of irradiation received by the liquidators in the course of the restoration works in the zoneof the accident were determined, depending on the situation, either by the frequency of dicentrics or by thefrequency of translocations using the calibration dose-response curves obtained by us earlier [25].
An extensive experimental material was analyzed to construct calibration dose-response curves –blood samples from 5 donors were examined in the dose range from 0 to 4 Gy (dose rate – 0.1 Gy/min).
55007 cells were analyzed by the classical method (dicentrics) and 15561 cells were analyzed by the FISH
method (translocations). The calibration curve for the frequency of stable chromosome aberrations was
based on the frequency of translocations involving chromosomes 1, 4 and 12. Complete and incomplete
translocations were taken into account.
The data of cytogenetic analysis of dicentrics and translocations served as a basis for regression
equations characterized by a high degree of significance. α and β coefficients for dicentrics made up 0.015
258

± 0.04 Gy-1 and 0.063 ± 0.003 Gy-2, respectively, and for translocations – 0.007 ± 0.003 Gy-1 and 0.017 ±0.002 Gy-2, respectively.
3. Results
3.1. Analysis of chromosome aberrations in the group of liquidators
3.1.1. Dicentrics
Table 1 presents the results of analysis of the frequency of unstable chromosome aberrations(dicentrics) in the group of liquidators for the period from 1986 to 2001. In the first year of examination
the average frequency of cells with dicentrics in the group of liquidators was 0.33 per 100 cells andexceeded 16-fold the analogous index in the control group (control – 0.02 per 100 cells). In a year thefrequency of cells with dicentrics in the examined group of liquidators decreased more than 2-fold andconstituted 0.14 per 100 cells. Further on, the frequency of cells with dicentrics continued decreasing but
not so sharply as in the first year after the accident. Through the whole period of observation, this index
remained to be significantly higher as compared to the control level. The frequency of cells with dicentrics
varied from 0.06 to 0.18 per 100 cells throughout the whole period of examination. Even 15 years after the
Chernobyl accident the level of this index exceeds the control. Thus, despite a very long period after theend of the clean-up works in the zone of the accident, we can observe a radiation-induced effect which ismost likely determined by the radiation damage of hematopoietic tissue stem cells leading to a gradualintroduction in peripheral blood of lymphocytes carrying chromosome aberrations.
For the group of liquidators cytogenetically examined in 1986 the dose of irradiation was estimatedusing the calibration dose-response curve for dicentrics. The average dose for the group made up 0.14 Gy.
3.1.2. Translocations
Table 2 presents the results of cytogenetic examination of 52 liquidators. They were examined
between November 1992 and July 1995. The average frequency of translocations (1.2 ± 0.16 per 100
cells) in the group of liquidators significantly (2.5-fold) exceeds the control level (0.47 ± 0.09 per 100
cells). In the subgroup of liquidators who worked several times in Chernobyl in the period from 1986 to
1995 the frequency of translocations exceeds 2-fold that in the subgroup of liquidators who worked only
in 1986. The differences between these subgroups of examined liquidators are statistically significant.
There were no differences in the frequency of translocations between the subgroup of liquidators (35patients) for whom we had official data on the irradiation dose (1.2 ± 0.19 per 100 cells) and the subgroup
Table 1. Cytogenetic results (classical method), pooled data for groups of liquidators.
Year ofexamination

Numberof subjects

Number of cells
scoreddic per 100
cells ± SEM

1986
443
41927
0.33 ± 0.03
1987
280
44268
0.14 ± 0.02
1990
23
4268
0.10 ± 0.05
1991
110
20077
0.09 ± 0.02
1992
136
32000
0.14 ± 0.02
1993
75
18581
0.09 ± 0.02
1994
69
20879
0.16 ± 0.03
1995
110
30012
0.06 ± 0.01
1996
53
17960
0.07 ± 0.02
1997
126
63462
0.09 ± 0.01
1998
77
39426
0.09 ± 0.01
1999
41
22524
0.12 ± 0.02
2000-2001
28
14000
0.09 ± 0.02
Controls
118
48124
0.02 ± 0.01
dic, dicentric chromosomes; SEM, standard error of the mean.

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Table 2. Cytogenetic results (FISH method), pooled data for the total group and for subgroupsof liquidators.
Number ofTranslocations
Estimated dose
cells
(95% c.l.), Gy
FG ± SEM per 100 cells
scored
Total
52
44283
0.16 (0.11-0.21)
1.20 ± 0.16
Working only in 1986
35
28767
0.06 (0 – 0.14)
0.86 ± 0.13
Working 1986-1995 (several times)
17
15516
0.29 (0.25-0.34)
1.81 ± 0.35
With documented dose
35
29614
0.16 (0.11-0.21)
1.20 ± 0.19
Without documented dose
17
14669
0.16 (0.11-0.21)
1.20 ± 0.31
Controls
15
21953
0.47 ± 0.09
FG, genomic translocation frequency; SEM, standard error of the mean; c.l., confidence limits.
Group

Numberof subjectsof liquidators (17 patients) for whom such data were missing (1.2 ± 0.31 per 100 cells).
Using the frequency of translocations and the dose-response calibration curve, we calculated theirradiation doses for the group of examined liquidators. The average value of the calculated dose for thewhole examined group constituted 0.16 Gy. For the subgroup of liquidators who worked only in 1986 thevalue of the “biological dose” made up 0.06 Gy, and for the liquidators who worked in Chernobyl several
times for the period from 1986 to 1995 the value of the “biological dose” was 5 times higher – 0.29 Gy. It
is important to note that the obtained dose values are equivalent to the dose of a single acute exposure.
In 18 out of 52 examined liquidators the frequency of translocations significantly differed from thecontrol level. The individual data of cytogenetic analysis for these liquidators are presented in Table 3. It
also demonstrates the individual doses of irradiation calculated using the calibration dose-response curve.
For three examined liquidators the dose values were rather high. The dose calculated for patient L31 was 1
Gy. The dose recorded for this patient in the documents was 1.36 Sv. Patient L53 did not have official dataon the irradiation dose although he worked in Chernobyl several times during 1986-1995. The“biological” dose of irradiation for this patient made up 0.68 Gy. The dose of 0.5 Gy was also calculated
for liquidator L51 who worked at the reactor in 1986. Official data on the dose absorbed by him were also
Table 3. Cytogenetic results and biodosimetry estimates for 18 liquidators.
St per 100
NumberEstimated dosecells
of cells(95% c.l.), Gy
scoredFG ± SEM
L17
March, 1994
1297
0.31 (0.16-0.46)
1.9 ± 0.7
L19May, 1995
570
0.26 (0.05-0.47)
1.7 ± 1.0
L20
March, 1994
1314
0.43 (0.29-0.57)
2.4 ± 0.8
L23May, 1995
857
0.36 (0.17-0.54)
2.2 ± 0.9
L25
April, 1994
1161
0.40 (0.26-0.54)
2.4 ± 0.8
L27June, 1995
1688
0.34 (0.20-0.48)
2.0 ± 0.6
L28
June, 1995
395
0.39 (0.14-0.64)
2.4 ± 1.4
L30
June, 1996
797
0.25 (0.04-0.46)
1.6 ± 0.8
L31
April, 1994
473
1.00 (0.40-1.40)
7.3 ± 2.2
L33
June, 1995
1157
0.26 (0.11-0.41)
1.6 ± 0.7
L34
July, 1995
1097
0.22 (0.07-0.37)
1.4 ± 0.6
L41
March, 1994756
0.17 (0.00-0.40)
1.2 ± 0.7
L45
April, 1993
410
0.38 (0.13-0.64)
2.3 ± 1.3
L46
May, 1995
1459
0.23 (0.08-0.38)
1.5 ± 0.6
L47
February, 1993859
0.30 (0.10-0.50)
1.8 ± 0.8
L51
January, 1994
583
0.50 (0.31-0.69)
3.2 ± 1.3
L52
June, 1995
1209
0.29 (0.14-0.44)
1.8 ± 0.7
L53
February, 1994
1016
0.68 (0.55-0.81)
4.6 ± 1.2
St; symmetrical translocation, FG; genomic translocation frequency,
SEM; standard error of the mean, c.l.; confidence limits.
Case
no.

Blood sampling

260

Documented
dose, Sv
0.14
0.02
0.17
0.29
0.11
0.80
0.25
0.50
1.36
0.60
0.26
-

Working time(months)
1986
1987 >1987
1.3
2.0
1.3
1.5
2.5
2.5
4.5
2.0
0.5
4.8
1.0
3.0
1.0
1.0
3.0
1.0
1.2
3.0
1.9
6.5
2.0
6.0
1.0
2.0
0.3
1.4
1.0
1.5
1.5
3.0
3.0
2.5

missing.
For most of the examined liquidators (15 patients) the individual doses were estimated in the range
from 0.22 to 0.43 Gy. For those liquidators whose irradiation dose was recorded in official documents the“biological doses” were in agreement to the data of physical dosimetry (L23, L28, L34). However, it
should be kept in mind that in certain cases the obtained doses might be slightly underestimated (L27, L30,
L31, L33) because the doses were calculated on the basis of the calibration curve generated for irradiation
at a dose rate of 0.1 Gy/min under conditions of an acute single exposure. In fact, many of the clean-up
workers could be exposed to prolonged irradiation at a lower dose rate. An illustrative example isliquidator L27 who himself is a specialist in the field of dosimetry and used an individual dosimeter for
the whole period of working in Chernobyl. By his measurements, his irradiation dose is 0.83 Sv. The dose
calculated by the frequency of translocations made up 0.34 Gy.
The results obtained clearly demonstrate that the analysis of translocations by means of the FISH
method is a sufficiently sensitive technique for retrospective assessment of the radiation effect in theliquidators participating in the clean-up works after the Chernobyl accident. Unfortunately, it is difficult tofind a scientific meaning in comparing the data of physical and biological dosimetry since it is known thatin many cases the data of physical dosimetry (from the point of view of official dosimetry) did notcorrespond to the real doses that might be received by the liquidators during their work in the zone of theaccident. In connection with this, the results obtained by means of biological dosimetry acquire particularimportance and, namely, they help in assessing possible unfavorable impacts of radiation on the humanorganism.
3.2. Cytogenetic examination of the workers of I.V.Kurchatov Institute of Atomic Power participating in
the liquidation of the consequences of the Chernobyl NPP accident
In 1996 a group of 22 workers of the Kurchatov Institute was subjected to cytogenetic examination.
In the first days after the accident they began research work in the zone of the accident. The main tasks ofthis work were to establish the causes of the accident, to examine the condition of the reactor, nuclear fuel,structural constructions of the 4th block and to study the radioactive conditions in the zone of theChernobyl NPP and on the controlled territory. All of the examined patients were exposed to low-dose
chronic irradiation in the course of their professional activity due to contacts with different sources ofradiation. Most of them started working under unhealthy occupational conditions in the early 60ies. The
official annual dose up to 1986 did not exceed 0.05 Gy. 18 persons from the examined group worked in
Chernobyl in 1986 and in 1987. Four out of the examined patients carried out research works at thenuclear plant till the middle 90ies. For 19 persons the official doses of irradiation ranged from 0.08 to 0.52
Table 4. Personal data for the examined group from the Kurchatov Institute.
Case
Number ofWorking time atDocumentedNo.
cells scored
Chernobyl
dose (cSv)*
K3
65
1986; 1981-1991
8.57
K4
57
May, 1986
24.85
K6
56
November, 1986
27.27
K8
49
June, 1986
24.45
K9
44
September, 1986
12.60
K12
48
October, 1986
10.71
K13
64
July, 1986
1.55
K14
51
October, 1986
4.63
K15
56
October, 1986
41.69
K20
49
1986-1996
1710**
K21
63
1986-1994
360**
K28
44
1986-1996
1160*** Doses accumulated during working in Chernobyl.
** This dose was obtained from the paper of A.V. Sevan’kaev et.al., 1995.

261

Sv. The values of these doses were obtained from the dosimetric register of the Kurchatov Institute. Three
persons (K20, K21, K28) from the examined group were exposed to super-high doses. The individual
characteristics of the examined group are presented in Table 4. The publication about these persons was in
the journal Radiation Protection Dosimetry [26].
Table 5 presents the individual data of cytogenetic examination by means of the classical method(analysis of the frequency of unstable chromosome aberrations). The values of the frequencies ofdicentrics and centric rings (markers of radiation exposure) in 13 examined workers of the Kurchatov
Institute significantly exceed the control level. In 5 patients (K4, K5, K20, K21, K28) the blood contained
cells with multiple chromosome aberrations (with dicentric chromosomes). Taking into consideration thatthe examined persons could be irradiated from different radiation sources, it can be suggested that theappearance of such cells in their blood is the result of the action on the organism of high-LET ionizing
radiation
In three patients from the examined group who were exposed to super-high doses (K20, K21, K28)
the frequency of dicentrics and centric rings was 100 and even 1000 times higher than the control level.
These workers were subjected to cytogenetic examination several times in the period from 1996 to 2001.
Here we present only the results of cytogenetic analysis obtained in 1996. It should be noted that by the
moment of examination there were no health complaints from these patients.
Table 6 presents the individual results of cytogenetic analysis performed with the use of the FISH
Table 5. Cytogenetic results (classical method) for the workers from the Kurchatov Institute.
Case No.

Number ofcells scoreddic+ Rc per 100
cells ± SEM

Cdr per 100
cells ± SEM

K2
871
0.69 ± 0.28 *0.69 ± 0.28 *
K3
1300
0.08
0.08
*
K4
1200
0.42 ± 0.28
0.25 ± 0.14 **K5600
0.33 ± 0.24
0.33 ± 0.24 *K6
1004
0.40 ± 0.24 *0.30 ± 0.17 *
K7
500
0
0
K8
1300
0.08
0.08
K9
550
0
0
K10
1300
0.08
0.08
*
K12
800
0.38 ± 0.22
0.38 ± 0.22 *
K13
800
0
0
*
K14
1000
0.20 ± 0.14
0.20 ± 0.14 *K151000
0.30 ± 0.17 *0.30 ± 0.17 *
K16
1000
0
0
*
K17
1000
0.20 ± 0.14
0.20 ± 0.14 *K18
1000
0.30 ± 0.17 *0.30 ± 0.17 *
K22
1133
0
0
*
K25
1000
0.50 ± 0.22
0.50 ± 0.22 *
K27
1000
0.10
0.10
K20
279
20.50 ± 2.70 *18.00 ± 2.50 *K21830
2.80 ± 0.60 *2.80 ± 0.60 *K28
500
20.40 ± 2.00 *19.20 ± 1.90 **Control
48124
0.02 ± 0.01
0.02 ± 0.01 *dic; dicentric chromosomes, Rc; centric rings, Cdr; cells containing dic+Rc,
SEM; standard error of the mean (only for cases with more than one aberration),* significantly different from control (p<0.05).

262

method among 12 workers of the Kurchatov Institute. The frequency of symmetrical translocations in fivecases significantly exceeds the control level (0.47 per 100 cells). The table also gives the values ofindividual doses calculated on the basis of cytogenetic indices. The dose values for patients K4 and K14
turned out to be higher than those presented in Table 4. One of the causes seems to be the fact that before
starting working in the zone of the accident at the Chernobyl NPP these persons worked for a long time
under unhealthy occupational conditions. For instance, patient K4 worked with the sources of ionizing
radiation since 1962. By 1986 his total accumulated dose constituted 29 cSv. Patient K14 worked withfluorine-containing substances for more than 10 years.
For three workers of the Kurchatov Institute (K20, K21, K28) exposed to super-high doses of ionizing
radiation the biological doses calculated by the frequency of translocations significantly differ from thosepresented in Table 4. The causes of such discrepancy can be summarized as follows. First, the doses were
determined using the calibration curve obtained under conditions of acute single irradiation. In reality,
these specialists worked in the zone of the accident for several months almost every year over 10 years. In
addition, they were repeatedly exposed to high-dose radiation. The total irradiation dose for these patientswas accumulated under conditions of fractionated exposure. Taking into account the character ofirradiation, it is more desirable to use the linear dose-response relationship for dose calculations [8]. In thiscase the irradiation dose can be determined by the following formula:D = (Y – Y0) / α,where Y is the frequency of measured translocations, Y0 – the frequency of spontaneous translocations, α- the linear coefficient.
The doses calculated according to this formula for the examined patients K20, K21 and K28 made up
10.3, 3.1 and 19.1 Gy, respectively. The values of these doses are already quite comparable with the doses
presented in Table 4. It should be noted, however, that such estimates are rather rough. A more precise
assessment of doses needs a detailed analysis of the history of irradiation of these persons during theirwork in the zone of the accident at the Chernobyl nuclear power plant.
Another important point to be mentioned is that the cytogenetic examination was carried out 10 years
after the beginning of exposure. During that time two opposite processes took place in the blood of theirradiated patients: gradual elimination of cells with unstable chromosome aberrations (dicentrics, centricrings) in which stable translocations were also detected, and a simultaneous accumulation in peripheral
blood of cells with chromosome aberrations due to additional irradiation. There is no doubt that this fact
Table 6. Cytogenetic results (FISH method) for the workers from the Kurchatov Institute.
Number of
Estimated dose
FG ± SEM
cells scored(95% c.l.), Gy(ST per 100 cells)
K3
1135
NS
0.57 ± 0.38
K41134
0.54 (0.40-0.68)
3.32 ± 0.94*
K6
1043
NS
0.91 ± 0.53
K8
1062
NS
0.88 ± 0.50
K9
843
NS
1.13 ± 0.66
K12
1271
NS
0.50 ± 0.35
K13
1000
0.33
NS
K14
1350
0.21 (0.06-0.36)
1.38 ± 0.57*
K15
1571
NS
1.00 ± 0.44
K20
2169
1.78 (1.69-1.87)
22.20 ± 1.00*
K21
2031
0.88 (0.78-0.98)
6.96 ± 0.60*
K28
9502.51 (2.38-2.64)
40.90 ± 2.10*
Control
21953
0.47 ± 0.09
FG; genomic translocation frequency, ST; symmetrical translocations,
SEM; standard error of the mean (only for cases with more than one aberration),c.l.; confidence limits, * significantly different from control (p<0.05).

Case No.

263

considerably impedes obtaining reliable information on irradiation doses. However, a rough estimation ofabsorbed radiation doses, and, what is more important, with regard to the individual peculiarities of an
irradiated organism, can be made.
The data obtained by the frequency of translocations (Table 6) permitted us to analyze thedependence between the level of individual values and the documented dose for the examined persons.
These data are presented in Fig. 1. The results of examination of patients K20, K21 and K28 are omitted
here. The dotted line indicates the linear-quadratic curve and the unbroken line indicates its linear
component. It should be noted that for all patients (except one) a good correlation between the frequencyof translocations and the officially registered irradiation dose is observed. The values of translocation
frequencies for two patients lie higher than the linear-quadratic calibration curve. It can thus be suggested
that those workers could be exposed to higher doses than those recorded in the documents. In three of theexamined workers with documented dose larger than 0.2 Sv, the value of the translocation frequency lies
considerably below our linear-quadratic calibration curve but in the range of its linear part. Assuming thatthe official dose in this worker is close to the real one, this fact, as has been mentioned above, shows anevidence that irradiation with rather low dose rates or fractionated exposures produce cytogenetic damages
less effective than single acute irradiation.
It is important to mention that the dependence of the translocation frequency on the exposure dose isalso observed for persons in whom the dose values were not determined because of insignificant
differences between the cytogenetic indices and the control. With a low level of translocations inperipheral blood lymphocytes an analysis of extensive material (a large number of cells) is required inorder to obtain reliable data. Unfortunately, it is often impossible in connection with large expenses. The
lower limit of dose estimation by the frequency of translocations, as shown by our studies, is 20-25 cGywith 1000 – 1500 cells analyzed. To estimate a dose below this threshold it is necessary to significantly
increase the number of cells to be analyzed. The data presented in Table 6 and in Fig. 1 support thisconclusion.
3.00

Fp ( translocations per 100 cells)

2.50

2.00

1.50

1.00

0.50

0.00
0.00

0.20

0.40

0.60

0.80

1.00

1.20

Documented dose, Sv

Figure 1. Dose-response relationship of translocation frequencies (Fp) with
documented doses (explanatory notes are in the text).
2644. Conclusion
The results of cytogenetic examination of the “liquidators” permit the following conclusions to be
made.
The cytogenetic methods (classical and FISH methods) are sufficiently sensitive for assessing thecondition of the cell hereditary structures.
It was found that the level of cells with dicentrics in the examined liquidators in the first year after thework in the zone of the accident significantly exceeded the control level. With time, years after theradiation exposure, this level gradually decreased but it did not reach the control level even within 15
years after the accident. The average frequency of cells with dicentrics in the recently examined group ofliquidators made up 0.09 % exceeding 5-fold the control values.
Using the frequency of cells with dicentrics, an average dose for the group of liquidators examined in1986 was determined. The calculated dose constituted 0.14 Gy. Retrospective estimation of irradiation
doses (both individual and for the groups) on the basis of analysis of cells with dicentrics does not seem
possible. It is first of all due to the elimination of such cells from the peripheral blood of the examinedpatients.
However, in the course of large-scale monitoring in groups of people who suffered after accidents or
emergency situations, analysis of unstable chromosome aberrations by means of the classical cytogenetic
method is decisive as it does not require large funds and technical resources. During retrospective
examination of the liquidators the analysis of cells with unstable chromosome aberrations (dicentrics andcentric rings) can be used predominantly for bioindication of the mutagenic action of ionizing radiation.
The results of such analysis can be considered as a criterion in the formation of groups at risk with regard
to the development of different diseases and primarily oncological ones
The FISH method is the most promising cytogenetic method in biological dosimetry. It allows one toanalyze the frequency of cells with stable chromosome aberrations – symmetrical translocations. The
data presented here suggest that this method can be used rather extensively for estimating radiation doses.
In most cases the doses calculated by the frequency of translocations are in good agreement with the dataof physical dosimetry. Some underestimation of biological doses is primarily due to the fact that thecalibration dose response curves for the frequency of translocations were obtained under definite
conditions – single acute irradiation. Under real conditions the liquidators were, as a rule, exposed tochronic or prolonged radiation. In connection with this, in determining irradiation doses it is necessary to
introduce the correction coefficient which makes it possible to take into account the real conditions of
exposure.
The results of cytogenetic examination by means of the FISH method presented in this work suggest
the existence of irradiation levels below which the dose estimation by the frequency of translocations
seems to be unlikely. The causes of such limitation can be quite diverse: interindividual variability of thetranslocation frequency, ability to repair radiation damage, the volume of analyzed material (the numberof analyzed cells). According to our data, the real lower threshold of dose estimation by the frequency oftranslocations is about 20-25 cGy. It seems reasonable in future to carry out cytogenetic examinations
among the liquidators of the accident at the Chernobyl nuclear power plant using not only the classicaland FISH methods but also new molecular-genetic methods permitting the estimation of remote genetic
effects of radiation and their comparison with detected structural damage of the cell genome.
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