Risk Factor for Radiation-Induced Leukaemia Among Entrants to Hiroshima (Enclosure a)

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Risk Factor for Radiation-Induced Leukaemi- n u Entrants to Hiroshima A large number (over 40,000) of persons entered Hiroshima after the bombing for rescue work or in search of relatives. These entrants were exposed t o the x - r a y s from radioactivity induced in the soil and buildings by the neutrons emitted at fission, and to a lesser extent to fall-out radiation. Hirose (1968) and Watanabe (1974)reported a significant increase in leukaemia hc!de=ce among the early entrants, here defined as,those who entered the city during the first three days. This paper is an attempt to estimate.the leukaemia risk factor in this population. . A. Estimate of averaze dose Although no attempt appears t o have been made to determine the doses received by individual entrants, it is possible to make a fairly close estimate of the average dose received by them. For this purpose the following assumptions a r e made: (a) all early entrants entered Hiroshima 6 hours after the explosion; (b) they all remalned in the city for at least 4 days (which in practice means an infinite period); (c) they all stayed in an area within a radius of 1000 metres from the (d) within that area they were uniformly distributed; (e) the r i s k of leukaemia is linearly related to the dose. Assumr=tfoa (a) foliows from tho information that a fire raged in the city during the first 6 houm, making it impossible for anyone to enter within a 1000 metres. , Assumptions @) and (c) are most unlikely. An estimate made on these will therefore give an upper limit to the dose, or a lower Iimit to the risk factor. . Assumption (d) seems plausible. Assumption {e)is justified even for low LET radiation at low doses, because the quadratic term is then small by comparison (Brown, 1977). The average dose Is calculated from 5 r Z f $ f , where $r is the fraction of the total early entrant population, who were in a ring at a distahce Y from the hypocentre ($fqz j), and & is the cumulative dose at that distance.

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assumptions made, the actual value is likely to be nearer the upper end of the range than the lower. The risk factor used by ICRP is based on observations from two groups of In the first group only part id body exposure occurred and this may carry P lower r i s k than that based on the assumption that the risk is proportional to the fraction of bone marrow exposed. With regard t o the A-bomb aurvivors, it is known that the incidence rate p e r rad of cancers of the thyroid, breast and lung is laver among them than in other irradiated poyulations by factors ranging from 4 to 7 (UNSCEAR,1977). The present estimate for leukaemia incidence is thus in line with these observatfons.. Unless it can be shown that the reported figure for leukaemia among the early entrants is wrong by a,Iarge factor, the following conclusions are inescapable: 1. Whatever the reason for the lower incidence of cancer amongthe survivors of the explosions and the absence of other radiation effects (Rotblat, 197'?), it is dear that the resuIts obtained from the A-bomb survivors cannot be used to establish risk 2. The risk factors recommended by ICRP are too low, probably by an order of magnitude for leukaemia, and by a somewhat smaller factor for other cancers. Brown, d. M. (1977) Rad. Res. 71, 34. Hirose, F. (1968) Acta Haem. Jaem. Jap. 31, 765. Rotblat, J. (1977) New Scientist 75: c2; August), 475. UNSCEAR (1977), Sources and Effects of Ionizing Radiation, U. N., pp. 361-424.