Chapter 2

 

PROBLEMS OF NUCLEAR AND RADIATION SAFETY:
CANCER RISKS FROM LOW DOSE EXPOSURE

 

 

One of the key goals for development of the nuclear power and industry in the future is objective evaluation of possible health effects for the personnel and general public. These issues became the subject of active debate in the last few years with appearance of data on human health effects which were obtained in the USSR and USA in the 40s and early 50s during the studies towards creating nuclear weapons and were unavailable up till recently. Another factor which influences public opinion is the epidemiological studies after the atomic bombing of Hiroshima and Nagasaki after many years, and the assessments of radiological consequences of the Chernobyl accident.

On December 20th 1951, a nuclear reactor in Idaho produced useful electricity for the first time. Following this, the first nuclear power plant was put in operation in Obninsk (USSR) in 1954. In the recent 30-40 years the development of nuclear power became critical for the prosperity of developed countries.

Nevertheless estimation of the risk of induction of cancer at low radiation doses (0-0.1 Sv) remains a high priority. In Figure 1 we show the risk evaluated from Japanese data.

 

Figure 1. Cancer risk at low doses evaluated from Japanese data (Preston, 2000).

 

 

Let us focus on the most important outcomes of the large-scale epidemiological studies undertaken in the latest years.

 

Hiroshima and Nagasaki

 

The large-scale epidemiological studies of the cohort of 86.5 thousand individuals (Hiroshima and Nagasaki) covering the period 1950-1990 showed an increase in cancer incidence rate as a function of radiation dose received. These studies were based on using a linear non-threshold model for the whole range of radiation doses (0-4 Sv). On the other hand, the data regarding low doses (0-0.1 Sv) published in the last few years provide no evidence to an increase in cancer incidence rate among those exposed to radiation at such levels (Figures 1-3) (Pierce et al., 2000; Little and Muirhead, 1998).

Figure 2. Risk of mortality from solid cancers at low doses (Little, 1998).

 

 

Figure 3. Risk of mortality from leukemias at low doses (Little, 1998).

 

 

 

Chernobyl: prognosis and factual data of the National Registry.

 

Although 15 years have passed since the Chernobyl accident, the largest in the history of nuclear power,  the prognosis of long-term (stochastic) radiation effects remains a high priority. Here we are basically dealing with a probability of causation of cancers in emergency workers and residents of the contaminated areas.

Let us consider a simple example. At the 49th session of the UNSCEAR (Vienna, May 2000) it was concluded that the mean external radiation dose for the Chernobyl emergency workers was 0.1 Gy (10 Rems). As of today, the National Radiation and Epidemiological Registry contains individual medical and dosimetric information for 180 thousand of emergency workers. If we use the linear non-threshold model of dose-response relationship to predict long-term cancer risks (i.e. presumably any small dose can cause cancer with extremely low probability), the number of cancers due to exposure to be expected in 180 thousand of emergency workers is estimated at about 1 thousand cases.

At the same time, with the assumption (based on the above data for Hiroshima and Nagasaki) that radiation carcinogenesis is absent for low doses (0-0.1 Gy) (and cancer only arises above the threshold of 0.1 Gy), the number of radiation induced cancers among emergency workers will become much less and the group of workers with a radiation risk will actually include only emergency workers who received external doses more than 0.2 Gy (the 1986 emergency workers only). This conclusion can be of major medical as well as social and psychological significance.

Today, 15 years after the Chernobyl accident it may be too early to make inferences about possible late health effects of exposure for emergency workers and the public. Yet, the data of the National Registry gathered to date provide evidence that there is an increase in cancer incidence rate among emergency workers with external radiation doses more than 0.1 Gy. This is shown in Figures 4-6.

Figure 4. Dose-response relationship for the incidence rate of solid cancers among
emergency workers (National Registry, 2001)

 

Figure 5. Risk of induction of solid cancers (National Registry, 2001).

 

Figure 6. Risk of induction of leukemia (National Registry, 2001).

 

 

Registries of nuclear industry workers

 

The Japanese Registry in Hiroshima and Nagasaki has been functioning for nearly 50 years and, in principle, should provide sufficient information to the ICRP about radiation carcinogenesis on the basis of direct epidemiological studies. At the same time, in the last 15-20 years epidemiological registries of nuclear workers were established in a number of the developed countries (USA, UK, France, Canada, Japan). Why? This brings us back to the key problem of the modern radiation epidemiology, namely low doses and cancer risks at protracted exposure (for professional workers). It should be remembered that the Japanese risks were derived for acute radiation exposure as a result of the atomic bombing, whereas in Russia the dose limits for nuclear industry workers are set to be, on the average, 20 mSv per year over any 5 consecutive years, but not more than 50 mSv per year (NRB-99).

In 2000 the WHO published the monograph by leading experts in radiation epidemiology concerning estimation of cancer risks (ISBN 92 832 1275 4). This book includes, among other things, the state-of-the-art data on occupational cancer incidence among nuclear industry workers. The mean cumulative doses among nuclear workers are estimated to range from 10 to 128 mSv for different Registries. A statistically significant radiation risk was revealed in a number of studies. Yet, it is still an open question whether extrapolation of the derived risks to low radiation doses using the terms of the non-threshold model is legitimate. The joint study (Cardis et al., 1995) of the occupational cohort of nuclear workers of the USA, UK and Canada revealed a statistically significant difference in the number of expected and observed leukemias only for the dose range 400 mSv and more.

Thus, the present-day epidemiological studies of the cohorts of nuclear workers in the leading countries (Table 1) provide no basis for saying that there are proven statistically significant risks of radiation carcinogenesis at low radiation doses (0-100 mSv) (IARC, 2000).

 

Estimation of radiation risks of cancer among nuclear workers of Russia

 

Further development of nuclear power and industry in Russia will depend, to a great extent, on objective evaluation of possible health consequences and public awareness of the problem. These tasks, in turn, require undertaking complex epidemiological studies that would be mostly oriented at normal (emergency-free) operation of nuclear facilities and health impact for the personnel and general public.

 

 

 

Table 1

Cancer risks for the nuclear industry workers

 

Study

Cohort, persons

Mean dose, mSv

Relative risk

all cancers

Leukemia

trend/statistical significance

trend/statistical significance

UK

Sellafield (Douglas et al., 1994)

14 282

128

Yes

No

Yes

Yes

Atomic Energy Authority
(Fraser et al., 1993)

39 718

40

Yes

No

No

No

Committee on Nuclear Weapons
(Beral et al., 1988)

22 552

8

Yes

Yes

No data

National Registry for Radiation Workers (Muirhead et al., 1999)

124 743

30

Yes

No

Yes

No

Pooled analysis (Carpenter et al., 1994)

75 006

56

Yes

No

Yes

Yes

USA

Hanford (Gilbert et al., 1993)

44 154

23

No

No

No

No

Facilities X-10 and H-12 in Oak Ridge (Frome et al., 1997)

28 347

10

Yes

Yes

No

No

NPP in Oak Ridge (Wing et al., 1991)

8 318

17

Yes

Yes

Yes

No

Pooled analysis (Gilbert et al., 1993)

44 943

27

No

No

No

No

Canada

Atomic Energy Authority
(Gribbin et al., 1993)

8 977

15

Yes

No

Yes

Yes

International studies

Pooled analysis (Cardis et al., 1995)

95 673

40

No

No

Yes

Yes

 

 

 

In the last few years data were made available on statistically significant cancer risks among the «Mayak» plant workers who received fairly high cumulative radiation doses in the late 40s-early 50s (external gamma and internal alpha irradiation due to incorporated plutonium) (Koshurnikova et al., 2000). At the same time, the number of studies (and consequently publications in this country and abroad) regarding radiation risks in the workers of the Minatom of Russia who received low cumulative radiation doses (0-100 mSv) remains extremely limited (Figure 7).

Figure 7. Cumulative radiation doses of the IPPE workers (mSv).

 

 

 

The National Registry has competed the studies aimed at estimating radiation risks of cancer induction in the workers of the Institute of Physics and Power Engineering (IPPE) at Obninsk,  and the nuclear industry workers engaged in mitigation of the consequences of the Chernobyl accident (Ivanov et al., 2001a) (Figures 8,9).

 

 

 

Figure 8. Cancer risk for employees of the IPPE.

 

 

 

Figure 9. Risk of developing cancer in the emergency workers –

employees of the nuclear industry.

 

 

These studies did not show any excess in cancer incidence rate above the spontaneous level (males in the country) in the IPPE workers subjected to individual dosimetric monitoring (about 77% of individuals have cumulative radiation doses up to 100 mSv).

Of the Chernobyl emergency workers, roughly 22 thousand are workers of the nuclear industry. The mean external radiation dose for them resulting from involvement in the activities in the 30-km zone was 50 mSv. For this group of emergency workers one should take into account their exposure to radiation prior to the Chernobyl accident. At the same time, the cancer incidence rate in the emergency workers, employees of the nuclear industry, remains lower that for males of the country in general (Ivanov et al., 2001b).

Thus, considering the above (Hiroshima, Nagasaki, Chernobyl, the registries of nuclear workers) it may be concluded that the currently available epidemiological data provide no evidence of proven radiation carcinogenesis in the region of low doses (0-100 mSv). Also, it is obvious that activities in this study area should be intensified to obtain objective data and work out optimum norms of radiation safety.

 

References

 

Cardis E., Gilbert E.S., Carpenter L., Howe G., Kato I., Armstrong B.K., Beral V., Cowper G., Douglas A., Fix J., Fry S.A., Kaldor J., Lave C., Salmon L., Smith P.G., Voelz G.L., Wiggs L.D. Effects of low doses and low doses rates of external ionizing radiation: cancer mortality among nuclear workers in three countries//Radiation Research. - 1995. - V. 142. - P. 117-132.

IARC Monographs on the evaluation of carcinogenic risks to humans, Vol. 75, Ionizing radiation, Part 1: X- and gamma (g)-radiation and neutrons. - Lyon: IARC, 2000. - P. 197.

Ivanov V.K.(a), Tsyb A.F., Rastopchin E.M., Gorsky A.I., Maksioutov M.A., Vayzer V.I., Suspitsin Y.V., Fedorov Y.V. Cancer incidence among nuclear workers in Russia based on data from the Institute of Physics and Power Engineering: a preliminary analysis//Rad. Research. - 2001. - N 155. - P. 801-808.

Ivanov V.K.(b), Ilin L.A., Tsyb A.F., Tukov A.R., Gorski A.I., Maksioutov M.A., Matyash V.A. Radiation epidemiological analysis for cancer incidence among nuclear workers taken part recovery operations following the accident at the Chernobyl NPP //Medical Radiology and Radiation Safety. - 2001. - V. 46, N 4. - P. 40-45 (in Russian).

Koshurnikova N.A., Gilbert E.S., Sokolnikov M., Khokhryakov V.F., Miller S., Preston D.L., Romanov S.A., Shilnikova N.S., Suslova K.G., Vostrotin V.V. Bone cancers in Mayak workers//Radiation Research. - 2000. - V. 154, N 3. - P. 237-245.

Little M.P., Muirhead C.R. Curvature in the cancer mortality dose response in Japanese atomic bomb survivors: absence of evidence of threshold//Int. J. Radiat. Biol. - 1998. - V. 74, N 4. - P. 471-480.

Pierce D.A., Preston D.L. Radiation-related cancer risks at low doses among atomic bomb survivors//Radiation Research. - 2000. - V. 154. - P. 178-186.