V.K.1, Ilyin L.A.2, Tsyb A.F.1,
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
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 . 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 . 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 . 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
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) . 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  and corresponds to Branch Medical Dosimetric Registry of nuclear workers located at the Institute of Biophysics . 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
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  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
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 . 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
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.
Dynamics of change of
personnel number included in group of potential
with attributive risk exceeding assigned value
Percentage of the personnel of the group of potential risk
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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