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The Medical Implications of Nuclear War, Institute of Medicine. ~ 1986 by the National Academy of Sciences. National Academy Press, Washington, D.C. Expected Incidence of Cancer Following Nuclear War NIKOLAI P. BOCHKOV, M.D., and PER OFTEDAL, PH.D. Soviet Academy of Medical Sciences, USSR, and University of Osio, OsIo, Norway The carcinogenicity of ionizing radiation has been investigated in some detail both under experimental conditions and through direct observations of irradiated persons. The high incidence of occupational cancer in roent- genologists who did not suspect the treacherous properties of ionizing radiation has been well known since the 1920s. The data on radiation- induced cancer have been systematically examined by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), by the National Research Council Committee on Biological Effects of Ionizing Radiation (BEIR), and by the International Commission on Ra- diological Protection (ICRP). The calculation of expected incidence of cancer following nuclear war can be based on experimental data on radiation-induced cancer, obser- vations of occupational cancer, and the data from Hiroshima and Nagasaki. Data on cancer incidence in populations of Hiroshima and Nagasaki obtained during checkups of survivors of A-bombs, from analysis of case histories, and from pathological certificates allow one to draw objective conclusions on the cancer rate induced by radiation. The following results were obtained.~-3 An increase in leukemia incidence began in both cities about 3 years after exposure and reached a peak around 1951-1952. Later the leukemia rate among exposed persons declined. The rate in the Nagasaki-exposed survivors has not exceeded that of the control population since the early 1970s. There is still evidence of continuation of a slightly increased leu- kemia rate in Hiroshima among exposed survivors. There are complex 329

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330 HEALTH CONSEQUENCES OF NUCLEAR WAR differences between the types of leukemia in relation to age at the time of the bombing, city of exposure, and duration of the latent period fol- lowing exposure. Through 197S, total excess incidence of leukemia deaths due to radiation among all A-bomb survivors is estimated to be about 95 percent of the leukemia deaths not associated with radiation that is, the overall rate is nearly twice as high as it would have been without atomic radiation exposure. It is revealed that the younger the age at the time of the bomb, the greater the risk of leukemia during the early period and the more rapid the decline thereafter. A clear relationship between the incidence of leukemia and radiation dose is present for both cities, but the effect is more pronounced in Hiroshima than in Nagasaki. The lowest doses with a demonstrable leu- kemogenic effect appear to be in the 0.2-0.4 gray (Gy) (20-40 reds) range in Hiroshima. The marked difference between the Hiroshima and Nagasaki experi- ences has been attributed to the neutron component of the ionizing radiation in Hiroshima. However, the doses are now in the process of being re- evaluated. In regard to malignant solid tumors, the following conclusions can be made. Analyses of mortality have shown a significant excess of deaths from malignant solid tumors. The relative risk for various malignant tu- mors to be induced by radiation (2 versus 0 Gy) varies considerably by site. A significant increase is evident for leukemia; cancers of the lung, breast, and stomach; and multiple myeloma. It is also suggested, though not yet confirmed, that there is an increase in risk for cancers of the esophagus, colon, urinary tract, and salivary glands. Incidence data sug- gest that breast and thyroid tissues are especially sensitive to the carcin- ogenic effect of ionizing radiation. Death rates from all malignant tumors increased with dose in both Hiroshima and Nagasaki, but the increase with dose was higher in Hi- roshima than in Nagasaki. In most cases the relative risks for various organs are not significantly different from one another at the level of age of mortality. Radiation-induced solid tumors appear only after a latency period. The length of the latency period seems to decrease with dose increases. Ma- lignancies other than leukemia exhibit different latency patterns over time. Radiation-induced cancers do not become apparent until the usual cancer age is reached. For example, even for those individuals who had already reached the age for lung cancer at the time of the bombing, the shortest latency period was 10-15 years, and in this case, no shortening of the latency period was evident in the high-dose group.

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EXPECTED INCIDENCE OF CANCER FOLLOWING NUCLEI Wow 331 The process of radiation carcinogenesis could be modified by factors such as age at the time of the bombing; attained age; sex; and exposure to tobacco smoke, hormones, and the like. If age at death is fixed, the absolute risk is clearly greater for those who were younger at the time of exposure. The strongest effect among women has been an excess risk of breast cancer. The effect appears strongest for those exposed before the age of 20 years and less among women aged 20-39 at exposure, and it may not exist among women exposed at older ages. For women aged 20-30 at exposure, the minimal induction period appears to have been between 5 and 10 years. The increase in cancer mortality appears to be fairly general, including cancers of the lung, esophagus, stomach, colon, and urinary organs and multiple myeloma. The magnitude of the radiation effect varies by site. The excess risk of radiogenic breast cancer begins at ages when cancer rates normally become appreciable, and after 5-10 years among persons already at or near ages of appreciable cancer risk when exposed. Most solid tumors differ from breast cancer, however, in that an excess risk is seen among the oldest survivors as well as among those exposed at younger ages. The total excess cancer mortality from radiation-induced cancer through 1978 among all survivors is estimated to be 3.4 percent (340 excess deaths from radiation-induced cancers, compared with more than 10,000 not associated with radiation). The calculation of expected cancer incidence is based on the main conclusions of UNSCEAR, BEIR, and ICRP, which prognosticated the cancer incidence in case of nuclear war. According to these recommen- dations, radiation-induced carcinogenesis does not have a dose threshold, but the incidence dose rate is linear or linear-quadratic. It is necessary to stress that radiation exposure does not induce any specific "radiation- type" of cancer but just enhances the incidence of spontaneous malignant tumors. The principle of calculating the expected cancer incidence following nuclear war is not based on specific scenarios of nuclear war but draws on more general approaches based on radiation dose and the age structure of the population. The expected cancer rate in a nuclear war depends on the irradiation dose. In the case of modern nuclear war, the doses of gamma- and neutron- irradiation will be substantially greater than those in Hiroshima and Na- gasaki. A number of scenarios of nuclear wars have been published (from 1 to 10,000 megatons [Mt]) and respective levels of irradiation have been

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332 HEALTH CONSEQUENCES OF NUCI:FAR WAR calculated for different explosions and heights above certain localities.4 The main conclusion from all scenarios is the following. The survivors near the target areas of a conflict involving 10,000 Mt of nuclear explosives will be exposed at least to doses of 0.5 to 1.0 Gy (50-100 reds) and greater, which by far exceeds those to which the inhabitants of Hiroshima and Nagasaki were exposed. Naturally, the proportion of exposed persons will be a great deal higher. Practically all the nuclear war survivors throughout the world will be irradiated. Even in territories distant from the explosion sites the people will be exposed to radiation from radioactive fallout at doses up to 0.1 Gy (10 reds) and higher. Complex interacting radiation factors and limited understanding of the operative dose-effect relations preclude exact predictions of the postwar incidence of cancer. The calculation of cancer risk following nuclear war was made for target areas and for areas exposed to global fallout.3 First, with regard to target areas, the cancer frequencies have been calculated by making assumptions about the distribution function of the exposure rates, the survival from the blast, and the distribution of shelters. In Table 1, one can see a calculated distribution, by dose, of the sur- viving population. After attenuation for death from acute symptoms, the assumptions yield a 43 percent total survival rate. A reduction factor of 0.55 has been applied to convert average surface exposure to average organ dose. The mean dose among survivors is in this way calculated to be 0.58 Gy. Obviously, choosing other values for the various parameters would change the estimates to some extent. It is possible to modify the function affected according to new information and Gus increase the reliability of the estimates. TABLE 1 Calculated Exposure Distribution of Survivors of Blast and Acute Radiation Effects Bone marrow and Average organ dose Exposure (R) Percentage exposure (R) red Gy .. 0- 9 21 2.7 1.5 0.015 10- 49 27 22 16 0.16 50- 99 14 77 42 0.42 100-199 18 140 75 0.75 200-299 13 240 131 1.31 300-399 1.5 310 173 1.73 >400 5.5 470 256 2.56 Total 100.0 105 58 0.58

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EXPECTED INCIDENCE OF CANCER FOLIO WING NUCLEI We 333 TABLE 2 Estimated Cancer Deaths per Million Survivors, Using 1980 BEIR Risk Equations and Projection Models (all ages combined) Projection Model . Absolute Risk Males Females Relative Risk Total expected, all cancers Leukemia Expected Estimated excess Linear model No. (% increase) No. (% increase) Linear quadr. All other cancers Expected Males Females 180,000 147,000 9,860 3,280 (33) 3,120 (39) 8,018 2,180 (27) 2,070 (26) 170,400 139,400 170,400 139,400 Estimated excess: Linear model No. 5,330 8,530 24,500 28,200 (% increase) (3.1) (6.1) (14) (20) mean 5 17 Linear quadr. No. 4,790 7,430 21,900 24,300 (% increase) (2.8) (5.3) (13) (17) mean 4 15 SOURCE: World Health Organization (1984, p. 157).3 Reprinted with permission. Table 2 indicates that cancer mortality owing to local fallout would be greatly increased among the survivors of a nuclear war.3 The increased risk of cancer would be far from being the most horrible consequence of the disaster. Depending on the risk model used and the method of pro- jection beyond the 30 years of present follow-up, the excess mortality would be around 5 percent, or 17 percent of the normal cancer burden. In other words, if 15 percent of some present populations today could normally be expected to die of cancer, taking into consideration a 10-year latency period, about 16-18 percent of the surviving population would die of cancer. The most noticeable oncological effect would be that of leukemia. The excess risk would be relatively high compared with the normal risk, and it would occur within 2-30 years after nuclear war. However, the total number of deaths from radiation-induced leukemia would be large. Radiation-induced solid tumors tend to occur at ages at which such cancers normally occur; that is, radiation causes more cancer deaths to occur, but not at earlier ages than usual. Because most cancer deaths occur

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334 HEALTH CONSEQUENCES OF NUCLEAR WaR among the elderly, the effect of a 5 percent excess mortality or 17 percent increase in cancer mortality would not have a marked effect on the average life span.3 Much less refined calculations (based on the risk factor estimated by UNSCEAR, 1.25 x 10-4 per red) give practically the same increase in cancer risk, namely 4.3 percent excess mortality. This calculation was made for the oncological effects of local fallout on the population. Other scenarios for other areas would produce different values. However, in view of the generalized character of the assumptions, it seems that there would be no major modifications in the conclusions drawn. As far as the global fallout is concerned, the estimates have been based on the general assumption of a total bomb yield of 10,000 Mt. The mean effective dose equivalent to the population of the world would be 0.1 Gy per person. The collective dose could be found by multiplying the number of individuals exposed by the mean dose. The distribution of the fission products would be nonuniform and would result in the following doses: bone marrow, 17 red; bone cells, 19 red; lung, 16 red; other sensitive organs, 10 red. The risk of cancer according to the ICRP data for an individual would be as follows: leukemias 3.5 x 10-4; osteosarcomas, 1.5 x 10-4; lung cancer, 3.0 x 10-4; cancer of other organs, 20.5 x 10-4. The rate of natural occurrence of cancer in the population of an indus- trialized country is 15 percent. The global fallout of fission products from blasts of 10,000 Mt would increase the cancer rate in the surviving world population by slightly more than 1 percent. Chazov and colleagues4 presented the estimates of late radiation con- sequences for the population in the form of the expected incidence of malignant tumors developed in various organs and tissues with a fatal outcome. As one can see from Table 3, local radioactive fallout can give rise to malignant tumors induced by ionizing radiation that is expected to kill 21 million; of these, approximately 3 million will die of leukemia, 3.6 million of mammary gland cancer, and more than 4.6 million of thyroid gland cancer. The risk factors normally used by the ICRP are based on fatal cancers only. In view of the reduction in the efficiency of health services to be expected in a postwar world, the inclusion of normally nonfatal cancers might be relevant. This leads to an approximate doubling of the risk, owing mainly to the large contribution from thyroid cancer. Note, how- ever, that if the risk factors are used that include the normally nonfatal component which might to some extent become fatal under postwar

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EXPECTED INCIDENCE OF CANCER FOLLOWING NUCLEI We 335 TABLE 3 Expected Incidence of Malignant Tumors with Fatal Outcome as a Result of the Effect of Local and Tropospheric Fallout Type of Late Consequences Risk Factor of Development of Tumor per rem Number of Cases, million Leukemia 2.010-6 2.83 Mammary gland cancer 2.5-10-6 3.60 Thyroid gland cancer S.O.10-6 4.62 Lung cancer 2.0~10-6 2.88 Malignant tumors in other organs and tissues 5.0 10-6 7.20 SOURCE: Chazov et al. (1984, p. 202).4 Reprinted with permission. conditions this is then at variance with the usual definition of effective dose, which refers to the induction of fatal cancer. In summary, a general nuclear war would presumably expose popula- tions of industrial and densely populated areas around the world to levels not less than 1.0 Gy.3 The rest of the world would be exposed to delayed fallout. Based on a total explosive force of 10,000 Mt. survival in the target areas would be about 50 percent. It might be expected that there would be 100 million survivors in each of the target areas of North Amer- ica, Western Europe, the USSR, and various scattered smaller areas. About 400 million survivors would be irradiated with doses leading to a 17 percent increase of the present cancer incidence, from 15 percent to about 18 percent. This means that about 12 million cases of cancer due to radiation would arise in target areas. In the rest of the world an increase of about 1 percent from 15 percent to about 15.2 percent might lead to some 7 million extra cases. Cancer induction would thus add to the suffering of the postwar world. The general health detriment implicit in such an in- crease in cancer frequency would, under ordinary circumstances, be re- garded as gravely significant. In conclusion, we would like to stress the following. When cancer develops in new victims, life becomes very difficult for all survivors. They begin to fear the fatal end. Naturally, immediate casualties after nuclear attack will be much greater than oncological consequences, but if even one child develops cancer or leukemia, this will not lighten the burden of responsibility on those who might want to launch a nuclear war. As the great Russian humanist Fydor Dostoyevsky said, "No goods of civilization are worth the tears of a single tortured child."

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336 HEALTH CONSEQUENCES OF NUCLEAR WAR NOTES Committee for the Compilation of Materials on Damage Caused by the Atomic Bombs in Hiroshima and Nagasaki. 1981. Hiroshima and Nagasaki: The Physical, Medical and Social Effects of the Atomic Bombings. Tokyo: Iwanami Shoten. (London: Hutchinson; New York: Basic Books). 2Kato, H., and W. J. Schull. 1982. Studies of the mortality of A-bomb survivors. Mortality, 1950-1978: Part I. Cancer mortality. Radiation Res. 90:395-432. 3World Health Organization. 1984. Effects of Nuclear War on Health and Health Services. Geneva: World Health Organization. 4Chazov, Y. I., L. A. Ilyin, A. K. Guskova. 1984. Nuclear War: The Medical and Biological Consequences. Moscow: Novesti Press.