<> AN EXAMPLE OF THE LENINGRAD NPP
Ivanov
V.K.1, Ilyin L.A.2, Tsyb A.F.1,
Kochetkov O.A.2,
Agapov A.M.3,
Panfilov A.P.3, Kisilev M.F.4, Kaidalov O.V.1,
Gorski A.I.1, Maksioutov M.A.1,
Soloviev V.Yu.2, Tukov A.R.2, Kozlov E.P.5,
Epikhin A.I.5
1 Medical Radiological
Research
Center of Russian Academy of Medical Sciences, Obninsk;
2 Institute of Biophysics,
Moscow;
3 Department of Safety,
Ecology and
Emergencies of Minatom of Russia, Moscow;
4 “Medbioextrem” at the
Ministry of
Health of Russia, Moscow;
5 Leningrad NPP, Sosnovy Bor
As of today it does not seem possible to assess objectively the causality between cancer diseases and professional exposure using clinical evidence only. The medical-dosimetric registries can do little to reveal statistically significant effects in the low dose range and do not provide unambiguous recommendations as to quantifying occurrence of malignant neoplasms in case of protracted exposure. The remaining option is to use the accumulated experience of large-scale epidemiological studies and existing recommendations for probabilistic assessment of possible occurrence of malignant neoplasms associated with professional activity for a given individual with allowance for his personal characteristics. In the paper the personnel of the Leningrad NPP that were under individual dosimetric monitoring is were grouped with respect to potential risk. The projection methodology recommended by UNSCEAR-94 is used. The source information is personalized data allowing for sex, age and year-by-year dynamics of dose accumulation. A total of 5086 males are considered, their average accumulated dose being 74 mSv. Since the large-scale epidemiological studies have not revealed radiation induction of cancer diseases in the low dose range, in the first stage the cohort of the Leningrad NPP workers with accumulated radiation doses more than 200 mSv was selected. For 289 persons who received accumulated doses more than 200 mSv (5.6% of the whole staff) the following distribution is derived: 138 workers had the attributive risk ranging from 5% to 10%, 7 workers - from 10% to 15%. The derived distribution can be used as a basis for administrative decision making. The proposed methodology for assessing individual potential risk of malignant neoplasms associated with professional activity can be used in the practice of NPP radiation safety services
Nowadays
it is impossible to solve the problem of unambiguous assessment
of causality between cancer morbidity and professional exposure using
only
clinical evidence. Medical-dosimetric registries can do little to
reveal
statistically significant effects in the low dose range and do not
provide
unambiguous recommendations as to quantifying occurrence of malignant
neoplasms
in case of protracted exposure. At the present time, a
medical-dosimetric
registry of nuclear industry workers is being formed [1]. We are hoping
for
additional information, which will appear with completion of the
registry
formation. This information will allow the specification of
regularities for
development of delayed effects of protracted exposure. So far, the only
option
for making probabilistic risk estimates is the use of accumulated
findings of
large-scale radiation-epidemiological studies and summarized
recommendations
derived from that information. In this connection estimates of risk
(probability) for development of malignant neoplasms associated with
professional practice of an individual with allowance for his/her
personal
characteristics are of great importance.
The
risk values of cancer morbidity obtained from a study of
professional exposure must not be treated as dogma. As of today many
issues
especially those related to protracted exposure with low doses are
still
unresolved. At the same time value of attributive individual risk (in
particular that caused by radiation) can be efficient tool for
formation of
groups of the personnel under potential risk, i.e. those who develop
cancer
possibly caused by their professional exposure.
In this
presented paper identification of groups under potential risk is
exemplified by the experience of the personnel of Leningrad NPP under
individual dosimetric monitoring.
Present-day
models for assessing radiation induced cancer morbidity and
mortality risks use mainly data derived from studies of the cohort of
survivors
of the atomic bombs dropped on Hiroshima and Nagasaki in Japan. To
study the
effects of these A-bombs on human beings, the Atmic Bomb Casualty
Commission (ABCC)
now Radiation Effects Research
Foundation (RERF), was set up in Japan.
The goal of the Foundation is to follow-up the cohort of the atomic
bomb
survivors in Hiroshima and Nagasaki till death. The cohort called
A-bomb Life
Span Study (LSS) was formed in 1950. Its size changed with time. As of
1990 it
totalled 86,572 individuals (the whole cohort includes 56% survivors)
with well
established radiation doses. In 36,459 cohort members radiation dose
was not
higher than 5 mSv, in more than 18 thousand individuals dose exceeded
0.1 Sv,
of them about 2 thousand had dose higher than 1 Sv. It should be noted
that
some of cohort members have dose above 4 Sv.
The
results of the follow-up processed from
1958 to 1987 allowed one to establish a statistically significant
association
of dose with morbidity rate [2]. From the paper it follows that the
association
found in the dose range above 0.2 Sv, is close to linear, excess
relative risk
(ERR) can be estimated as:
ERR(D)
= а × D,
(1)
where D
- radiation dose, Sv. Attributive risk AR is estimated as:
AR =
ERR/(1 + ERR).
Statistical
analysis of data on cancer morbidity in the LSS
cohort allowed one to determine а=0.63
Sv-1 for solid cancers in the cohort on average. If
radiation dose
is 1 Sv attributive risk is about 40%. Average radiation dose in the
LSS cohort
is 0.2 Sv, attributive risk for the cohort as a whole is 11%.
On the
basis of findings of LSS cohort the UN Scientific Committee on
the Effects of Atomic Radiation (UNSCEAR) worked out and recommended to
use the
model for radiation risks of cancer morbidity UNSCEAR-94 [3]. According
to the
model radiation-induced solid cancer morbidity in various specific
organs
following acute short-term exposure is presented as excess relative
risk ERR:
ERR(D,
g) = a×D×exp(b×(g-25)), t > TL,
(2)
ERR(D,
g) = 0,
t £ TL,
where
risk parameters а and b
depend on location of a malignant neoplasm; D
- radiation dose, Sv; g
- age at exposure; TL -
latent period (for solid cancers it is equal to
10 years). As the above form individual excess relative risk, ERR, does
not
depend on the background morbidity. For all solid cancers а has the following
values: men - 0.45 Sv-1, women - 0.77 Sv-1, b=-0,026
year-1 it does not depend on sex.
Figure
1 shows the association of attributive risk AR
with age at exposure of men who received a single radiation dose of 200
mSv. The relationship was calculated in
accordance with (2). It is seen that AR depends on age at exposure and
specific
organ. Increase or decrease of the risk value is associated with the
character
of the parameter b. The highest risks
are typical of cancer of bladder, lung and
liver (а is 1.0, 0.37 and 0.97
respectively). The smallest risks
are those of cancer of stomach and esophagus (а=0.16
and 0.23
respectively). The average attributive risk (AR) for all solid cancers
is about
5-6%. Maximal AR for all solid cancers following exposure to 200 mSv at
age of
20 years is 9%.
Fig. 1. Relationship between
attributive risk for cancer of different sites and
age at exposure
(men, radiation dose 200 mSv).
If we
assume that there is a threshold dose DT
of 200 mSv, then excess relative risk of
solid
cancer induced by exposure to doses below 200 mSv will not be or in
addition to
(2):
ERR(D,
g) = 0,
D £ DT.
Figure
2 shows the consequent relationship between attributive risk and
radiation dose to men of twenty years old at exposure.
The symbols used are the same as in Figure 1.
If radiation doses DT are
equal to 200 mSv, ERR=0 then AR=0.
It is evident from the figure that at dose 500 mSv attributive risk is
35% (in
the liver). For all solid cancers risk is not higher than 20%, it will
decrease
with age at exposure, as it is shown in the Figure 1. Minimal risks
(Figure 2)
for stomach and esophagus are less than 10%.
Fig. 2. Relationship between
attributive
risk for cancer of different sites and radiation dose
(men of 20 years age at exposure).
Figure
3 gives the relationship between attributive risk of solid
cancers development among men and dose with allowance for age at
exposure. It
is seen, the older were people at exposure the considerably less risk
they
have.
Fig. 3. Relationship between the
attributive risk for solid cancer, radiation dose
and age at exposure (men
of 20, 40,
60 years of age).
The
values of the individual radiation risks calculated with the model
are approximate. This is due to the following factors:
-
UNSCEAR-94 model for radiation risks is based on statistical data;
- type
of exposure (short-term, protracted);
-
possible inaccuracy at estimation of individual
radiation dose;
-
individual characteristic of a patient (region of
residence, life style, bad habits, occupation, etc.).
In
spite of some weaknesses of the model UNSCEAR-94, it is widely used
for assessing individual radiation risks of cancer development among
workers of
nuclear industry and population [4, 5].
Function
parameters of ERR are mainly derived from LSS cohort exposed to
high dose rate radiation and relatively high doses. Nowadays transition
to low
dose range is estimated with the use of dose and dose rate
effectiveness factor
(DDREF) [6]. Because of lack of reliable information necessary for
assessing
the factor it has big uncertainty and varies form 2 to 10 (2 is
recommended
value). We used factor of 2.
It is
evident that the association of cancer development among nuclear
workers with low dose and dose rate must be studied in order to
estimate
statistically significant limits of radiation risks. Though to detect
effect of
low dose and dose rate large arrays of radiation and epidemiological
information are required, we are able to make preliminary estimation
with the
use of available data. We can use this estimate for the concept
acceptable risk
within the framework of the ongoing debate on threshold or
non-threshold
effects of radiation with neglect of possible risk of delayed
consequences that
might occur from radiation doses below the specified dose limit of 0.2
Gy.
Since
the minimal latent period for the development of adult cancer
(other than leukemia) induced by ionizing radiation is 10 years one can
formulate the basic rule for the formation of group under potential
risk. With
allowance for protracted exposure one should take into account that
possible
development of cancer may be delayed in contrast to adverse
outcomes induced by acute exposure. In that case the
should be stricter criteria for assignment of nuclear workers to a
group of
potential risk. The suggested criterion is as follows: a worker is
assigned to
a group of potential risk if his accumulated dose is more than 200 mSv
and time
since beginning exposure is not less than 10 years.
The
database so formed of the personnel of Leningrad NPP stores
information about 5,938 men and women. The data are related to the
period from
1973 to 2003. Individual information is as follows: sex, registration
number
(badge number), year of birth, year of registration, year of discharge,
annual
radiation doses. It should be noted that the structure of Leningrad NPP
database differs from that of IPPE database [7] and corresponds to
Branch
Medical Dosimetric Registry of nuclear workers located at the Institute
of
Biophysics [1]. Data on actual cancer morbidity among the personnel of
Leningrad NPP are input into the database at the stage of their
collection and
processing. Information about the structure of the personnel of
Leningrad NPP
is processed and analyzed with the use of developed program package,
database
of personnel of the NPP being used as input parameter. The database
includes
information about discharged workers. Below we will analyze that
database
information which regards men being under individual dosimetric
monitoring
only.
The
coordination of the database
of Leningrad NPP personnel was followed by the formation of a working
database
of personnel (acting and acted, i.e. discharged people) under
individual
dosimetric monitoring. As shown in
Figure 4 the number increased almost linearly with time. As of 2003
this
working data base comprised 5,086 men.
Fig. 4. Growth the number of
personnel under individual dosimetric monitoring
from 1973 to 2003.
Dark area - the number of personnel in the group of potential risk
(accumulated dose with an allowance of 10-year latent period is more
than 200
mSv).
Fig. 5. Age distribution of
personnel as of 2003. Darkened area - employees
included
in group of potential risk (289 people).
The age
distribution of Leningrad NPP personnel (men) is presented at
Figure 5. Mean age is 44 years. Groups at potential risk include 289
men, their
age varies from 37 to 72 years (darkened part of the chart), and their
mean age
is 51 years. The group at potential risk includes 42 older employees
(mainly
retired employees men over 60 years).
The
distribution of the personnel, according to their individual dose
accumulated by 2003 is shown in Figure 6. The number of men whose
radiation
dose lay within the dose interval of 50 mSv is enumerated. This
distribution is
typical of nuclear workers. It is seen that about 4,000 employees
(first four
bars) have accumulated dose less than 200 mSv (see also Figure 2). About one worker in five (20% only) had a
dose above 200 mSv. Maximal radiation dose totals 934.9 mSv, the mean accumulated dose is 74 mSv. The mean annual dose is 2.5 mSv. For
comparison, in reference [5] an annual accumulated dose of 10 thousand
English
employees of nuclear industry been taken on from 1971 to 1976 totalled
130 mSv
in 1986. Dark color was used for showing the dose distribution of the
personnel
from groups under potential risk (period of work more than 10 years +
accumulated dose over 200 mSv).
Fig. 6. Distribution of personnel
of
Leningrad NPP according to dose accumulated by 2003.
Darkened area - personnel included in the group of potential risk
(289 people with doses over 200 mSv, record of service more than 10
years).
It is
evident that when planning actions towards elimination of
consequences of radiological accidents formation of groups of potential
risk
should meet specified quantitative criteria associated with risks. On
the other
hand, economical capacity of the industry or specific enterprise
intended for
health care and welfare of the personnel included in the groups should
be taken
into consideration as well. Those employees who worked with harmful and
who
have an ascertained cancer, are assumed to have a radiation induced
disease and
included in the group of potential risk may claim for indemnity for the
detriment
of their health. This may be fixed by law.
Below
are given prognostic estimations of baseline and radiation risks
of solid cancers for personnel of Leningrad NPP which illustrate how
information about employees of nuclear industry can be exploited for
management
of risks of radiation-induced diseases. Russian data on cancer
morbidity in
1996 were used for calculation of annual number of sporadic cancer
cases [8].
Cancer morbidity rates depend considerably on region of Russia. Thus,
average
rate of solid cancer incidence in Russia as whole was 268.2 cases per
100
thousand men in 1996. The highest rate (336.5 cases) was registered in
Saratov
region and the smallest, 119.1 cases - in Chukotka. The difference is
quite
high. There is lack of information on personnel migration preceding the
start
work at Leningrad NPP. This should be taken into consideration when
predicting
risks. The database of personnel
should, in the future, include
information about exposure (professional, for medical purposes, etc.)
of an
employee to radiation before his starting work at a given enterprise.
The
calculated annual number of sporadic solid cancer cases among
different groups of the Leningrad NPP personnel from 1974 to 2003 is
given in
Figure 7. The increase in sporadic
cancer
cases is caused by an increase in average age and number of the
personnel. At
the start of Leningrad NPP operating the number of men been under
individual
dosimetric monitoring totalled 300 men,
the average age was 25 years, and the morbidity rate was 20
cases per
100 thousand people. Curve 1 of figure 7 shows the number of cancer
cases among
all employees of the NPP (128 cases for 30 years).
The predicted number of cases in 2003 is 17.
As it was already mentioned diseases associated with radiation can
occur in
people who were exposed 10 and more years ago. It is obvious that the
exposure
history of any employee who previously worked with radiation sources,
including
any service as a clean up worker following a radiological accident
should be
taken into account. At present time such complete information of such
type has
not been collected. So in our discourse
below we will assume that personnel did not operate with radiation
sources
before their work at the NPP. In 2003 cancer detected in people taken
on after
1993 cannot be associated with radiation regardless of the radiation
dose he
received.
So with an allowance for 10-year latent period the numbers of employees
under
radiation risk decrease, the number of cancer cases which could
associate with
decreases. In curve 2 of figure 7 the
number of solid cancer cases with an allowance for latent period (total
number
for the whole period - 79).
Fig. 7. Expected annual number of
sporadic solid cancer cases among different
groups
of the Leningrad NPP personnel. 1 - all personnel; 2 - personnel taken
on until
1993;
3 - the same as in 2, but for personnel with accumulated dose more than
200
mSv.
The
group at potential risk which meets all specified criteria has been
formed since 1987. In 2003 it included 289 people (5.7% of all
personnel).
Changes of the group number are shown as dark area in Figure 4. Curve 3
in
Figure 7 shows the number of cases detected in the group of potential
risk.
Predicted number of sporadic cancer cases in the group of potential
risk for
the whole period is 8, in 2003 - 1-2 cases.
Because
radiation can affect cancer morbidity in minimum 10-year latent
period it is necessary to calculate individual attributive risk for the
whole
period (in this case till 2013). The value of risk is likely to be
considered
as a benchmark criteria for formation of risk groups. Figure 8 shows
distribution of members of groups of potential risk made by attributive
risk of
solid cancer in 2003 left) and in 2013 (right). Sporadic mortality was
not
taken into account for calculation of attributive risk. When
attributive risk
was calculated the DDREF was assumed to be 2. It is seen that in 2003
of 289
people 144 employees had an attributive risk less than 5%, 138 people
had risk
from 5 to 10%, and 7 people had risk from 10 to 15%. In 2013 the number
of the
group at potential risk will total 515 people and the number of
employees with
risk from 10 to 15% will total 22 people. Nobody will have an
attributive risk
higher than 15%.
Table 1
give information about the distribution of the number of
Leningrad NPP personnel been in group of potential risk made according
to value
of attributive risk. It is seen that 50% employees had attributive risk
higher
than 5%; in 2013 51.6% employees will have the risk of such value. In
2003 2.4%
employees have risk of 10%, their number will be 4.3% of the total
number of
the group of potential risk.
Fig. 8. Distribution of personnel
included in the group at potential risk (GPR)
according to attributive risk for solid cancers as of 2003 (left) and
2013
(right).
Number of people is numerated.
Table 1
Dynamics of change of
personnel number included in group of potential
risk
with attributive risk exceeding assigned value
|
Attributive risk |
Percentage of the
personnel of the group of potential risk |
|
|
2003 |
2013 |
|
|
>5% |
50.0 |
51.6 |
|
>10% |
2.4 |
4.3 |
|
>15% |
0.0 |
0.0 |
Conclusions
1. At
the present time accumulated information
resulted from large-scale radiation epidemiological studies allows one
to standardize radiation safety with an
allowance for attributive risk.
2.
Since radiation actually does not cause a solid
cancer in people with doses less than 200 mSv in 10-year latent period
for
radiation-induced cancer notion of group at potential risk was
introduced. Idea
of formation of that group is interpreted as follows: an employee
belongs to
group at potential risk if his integrated absorbed dose is more than
200 mSv
and he has been exposed to radiation due to professional activities no
less
than 10 years.
3. With
the use of personnel of Leningrad NPP (men)
under individual dosimetric monitoring as an example we have shown that
at
present time about 5.7% of that personnel meet criteria for group at
potential
risk. To 2013 the group will include about 10% of men under individual
dosimetric monitoring. Attributive risks of personnel included in group
of
potential risk do not exceed 15%.
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