On the Incidence of Leukemias in Hiroshima During. the Past Fifteen Years from 1946 to 1960* WATANABE Susumu **

Transcription

1 On the Incidence of Leukemias in Hiroshima During the Past Fifteen Years from 1946 to 1960* WATANABE Susumu ** (Received in September 1961) We have followed up the incidence of leukemias in Hiroshima, especially on the problem of relationship between the incidence of leukemias and exposed dose to the atomic bomb radiation within 5,000 meters from hypocenter, since The follow ing results were obtained by our investigations which have covered the past fifteen years from 1946 to According to the results of our national census taken Oct. 1, 1960 and a simulta neous investigation of the number of the atomic bomb survivors in Hiroshima, the gross population of Hiroshima City was 431,285 of whom the exposed in the city within 5,000 meters from hypocenter counted 95,684. At the time of the national census taken Oct. 1, 1950, the gross population of Hiroshima was 285,712, while the exposed numbered 98,102. Based on these four figures, an estimated yearly change in gross population of Hiroshima and population of the exposed in the same city during the period of was as shown in Table 1. A result of our observations in the incidence of leukemias in Hiroshima during the period of is indicated in Table 2, in which yearly number of the total cases of leukemia and its incidence per 100,000 people are given each in "Total Cases " and " Exposed Cases ". In 1960, the number of leukemias developed in Hiroshima in total was 14 and its incidence per 100,000 was While in that * Addition to former Report, GL 400** Research Institute for Nuclear Medicine and Biology Hiroshima University

2 Table 1. Change in Population of Hiroshima City These figures are depended on the results of census, Oct. 1, 1950 These figures are depended on the results of census, Oct. 1,1960 Table 2. Incidence of Leukemia in Hiroshima ( ) : Number of cases exposed at the distance of 5,000-10,000 meters from the hypocenter and followed by onset of leukemia in the city under the distance of 5,000 meters from hypocenter.

3 same year, only four cases of leukemia were developed among the atomic bomb exposed in Hiroshima and its incidence per 100,000 did not exceed the figure of It is noticiable that these figures mentioned above are evidently below those of the previous years. These values, however, should not be yet conclusive ones, because there is a possibility that those people who have not yet consulted, with doctors, despite the fact that they are affected by the disease or those who still are not definitely diagnosed as leukemia. So it is expected fairly accurate values will not be obtained until at the time of the survey next year. These are to be duly infered by the fact that according to the survey in 1960, the number of leukemia cases in 1959 was recorded 11, whereas the investigations in 1961 disclosed four more cases, putting the total cases to 15. Fig. 1 shows the incidence of leukemia in Hiroshima (Fig. 1) Incidence of Leukemia in Hiroshima During the Period of during the period of It must be noted, therefore, that although Table 2 and Fig. 1 give the impression that the incidence of leukemias among the exposed people in Hiroshima has remarkably dropped in 1960 as compared with the previous years, it should be still too early to conclude that way at this stage of the investiga tions. The number of deaths and the death-rates from leukemia are as shown in Table 3 and Fig. 2. The number of deaths from leukemia in Hiroshima in 1960 was 21 and the death-rate per 100,000 people On the other hand, the number of deaths from leukemia in the exposed in the same year was 8 and its death-rate The figured version of this is as shown in Fig. 2. On assuming an upward curve of the death-rate from leukemia in all Japan year by year, making the figure of 2.65 (per 100,000 people) in 1958, and as this tendency is supposed to be still continuing, it may well be said that the death-rates from leukemia in both the general people and the exposed people in Hiroshima have rather dropped relatively in recent years.

4 Table 3. Death-rate from Leukemia in Hiroshima ( ) : Number of cases exposed at the distance of 5,000-10,000 meters from the hypocenter and followed by development and death of leukemia within the city under the distance of 5,000 meters from the hypocenter. (Fig. 2) Death-Rate from Leukemia in Hiroshima During the Period of The summarized version of the above-mentioned results is shown in Table 4. In short, during the past tltteen years, there have been developed 249 cases of leukemia in Hiroshima, in which 164 cases of exposed to the atomic bomb within

5 Table 4. Rate of Development and Death from Leukemia in Hiroshima During the Period of This section was inserted for reference. the distance of 5,000 meters from the hypocenter are included, the incidence of leukemia per 100,000 per year during this period was 4.99, while that of the exposed cases was On the other hand the total cases of death from leukemia in Hiroshima numbered 223, of which 146 belonged to the exposed cases, the death-rate of leukemia per 100,000 per year during this period was 4.32 and that of the exposed cases As the mean value of death-rate in leukemia in all over Japan within the same period is 1.85, the death-rate of leukemia in Hiroshima is about 2.3 times as high as that of all over Japan, while the death-rate of leukemia in the exposed people is about 5.4 times. As there are no accurate data available at present about the incidence of leukemia in all over Japan, the death-rate substituted the incidence in making these comparisons. The yearly distribution of the types of leukemias among the atomic bomb survivors in Hiroshima is shown in Table 5. There were 88 cases of acute leukemia, most of whom were diagnosed as myeloid type, except only one case of definite acute lymphoid leukemia. In addition to these cases there were 6 cases of subacute myeloid leukemia, one case of subacute lymphoid leukemia and 67 cases of chronic myeloid leukemia with only 2 cases of chronic lymphoid leukemia, so the most of the chronic form of leukemias were also myeloid type. The type of leukemias among the survivors in Hiroshima, in relation to the distances of the exposure, is as shown in Table 6. And the summarized version of the results is given in Table 7. There were observed many cases of chronic leukemia comparatively among those exposed within the distance of 2,000 meters from the hypocenter, and the ratio of acute to the chronic was 1: 0.8. On the other hand, in those cases exposed in the distance over 2,000 meters away from the hypocenter, the value of the ratio rises as the distance increases, with the number of the chronic type of leukemias falling off gradually. In the cases of leukemia in the non-exposed people in Hiroshima, number of chronic leukemia is much less than that of acute one and this ratio is 1 : There is still no data available to set the accurate

6 Table 5. Type of Leukemia in the Exposed in Hiroshima, According to the Year of Onset * One of the eight cases is regarded as the acute lymphoid type. Table 6. Type of Leukemia Developed in Hiroshima During the Period of ( ) Definitly diagnosed case as lymphoid type. This section was inserted for reference.

7 Table 7. Incidence of Acute and Chronic Cases of Leukemia in Hiroshima During the Period of Subacute form is included in acute form. '~ This section was inserted for reference. ratio of these two types of leukemia in all over Japan. However, judging from the statistics of many large clinics in Japan there seems to appear one case of chronic leukemia against every five or six cases of acute leukemia. The ratio of the two, therefore, is to be set somewhere between 1 : 0.2 and 1 : From above results, it may be said that the exposure to radiation from the atomic bomb in Hiroshima has played an important part in giving rise to both acute and chronic leukemias, and especially, the incidence of chronic leukemias, which takes place less frequently than acute leukemia in Japan, was remarkably raised. In view of the fact that most of these chronic leukemias were myeloid, it might be concluded significantly that chronic myeloid leukemia in these cases are the kind of leukemia which was induced by the radiation of atomic bomb. According to Dr. Moloney's investigation (1959), there were seven cases of chronic leukemia and five cases of acute one among those people who were exposed to radiation during their occupational works which are mostly regarded as whole body exposure. On the other hand, among those cases who were exposed to radiation in the course of their treatments, there were seen 18 cases of chronic leukemia and 62 cases of acute one which is partial body exposure of the body. Considering these facts and the findings obtained from the cases of atomic bomb survivors, Dr. Miyake (1960) explained that the whole-body exposure to radiation should have great in fluence in causing chronic leukemia, while the local exposure in causing acute leukemia. His theory might be agreeable in principle. Of the 35 cases of leukemia which took place after the radiation treatment of Anklyosing Spondylitis by Dr. Court-Brown et al. (1957), only 6 cases were those of chronic form one of these six was lymphoid type and the others myeloid type, the rest being those of acute form of leukemia. Although it is difficult to affirm that the whole-body exposure should determine the induction of chronic leukemias, it is interesting to note the fact that the intensive irradiation of body including a certain

8 portion of bone marrow effects more for the development of acute leukemia, which is regarded as that of higher character of malignancy of the tumor compared with that of chronic one. It is noteworthy that the incidence of leukemias were also observed among the people who entered into Hiroshima shortly after the atomic bomb explosion, it is as shown in Table 8. According to the investigations up to date, there had been seen no cases of leukemia until 1950 in these cases. Ever since then, however, the disease has begun to appear among them, marking a slightly upward curve every year. There occured 5 such cases in 1956, 7 cases in 1957, 5 cases in 1958 and 5 cases in (Table 8 does not include 5 more cases whose entry dates are not confirmed. Namely, there were 2 such cases in 1956, and one case in 1957 and 2 cases in 1958). Table 8. Type of Leukemia Seen in Those Cases who Entered Hiroshima City just after A-Bomb Explosion ( )

9 In other words, these cases were detected somewhat later than those directly exposed, and the peak of the incidence in these people is supposed to reach maximum a few years later than that of directly exposed. Those cases shown in Table 8 are classified in Table 9 by the type of the Table 9. Case and Type of Leukemia Seen in those People who had Entered Hiroshima Immediately After A-Bomb Explosion ( ) Total number of people who had entered the city immediately after explosion is estimated about 30,000. disease. It is noteworthy that 23 of these 28 cases of early entrants had entered the city within three days after the atomic bomb explosion. The estimation of total number of people who entered the city after the explosion is based upon the number of registrants under category 2 of the Atomic Bomb Survivors Medical Treatment Law set up by the Japanese Government. Category 1 of the same law involves those people who were within the distance under 2,000 meters from hypocenter at the time of atomic bomb explosion. And this category 2 involves those who entered the city area less than 2,000 meters away from the hypocenter within two weeks after the explosion. In this latter category are registered 24,876 people. It is supposed, however, that there should have been those entrants who had not been registered, so. the total number of these entrants may be roughly estimated at 30,000. Under this assumption, the incidence of leukemia per 100,000 per year among these people comes to 9.33, as there were 28 cases of leukemia during the period of This figure is nearly as high as that of those exposed within 5,000 meters from the hypocenter (11.24). As shown in Table 9, there were in all 15 cases of acute form of leukemia (including subacute), against 18 cases of chronic leukemia, it is noteworthy that there were more cases of chronic leukemia than those of acute one, although we

10 know little about the explanation of higher incidence of chronic leukemia in these cases whether or not it depends upon the difference in mode and type of exposure to radiation. SUMMARY 1) In this report, all leukemia incidences in Hiroshima area are included so that one can study the difference between the leukemia of non-exposed people and that of exposed to the lowest dose of atomic bomb radiation. It may be beyond the scope of studies made by ABCC on the inductions of leukemias in Hiroshima, especially if one concerned with the minimum effective radiation doses, for in their studies they use for the sample of the exposed people only those who were exposed to the atomic bomb within 2,000 meters from the hypocenter, where the estimated air dose by American physicists is around 20 rads. In our investigation of this time, in addition to these cases, we covered also leukemia incidence of the people who have entered Hiroshima after the atomic bomb explosion for the study of other category of exposure. 2) The incidence of leukemia in those who were exposed to the atomic bomb within the distance under 5,000 meters from hypocenter is higher than that of non exposed. It begins to rise from 1947 and keeps its high level during the period to form a plateau ranging from the maximum incidence of 174 per 100,000 in 1951 to the minimum 92 in It is not until 1960 that the incidence begins to decline rather sharply. 3) The ratio of acute form of leukemia to chronic is remarkably higher in exposed people in near distance from hypocenter than that of non-exposed. This ratio decreases as the distance increases. It must be noted, however, that the high value of this ratio is significantly different from that of non-exposed until the distance reaches 5,000 meters from hypocenter. 4) The first case of onset of leukemia to the people who entered in Hiroshima, within 2,000 meters from hypocenter, immediately after the explosion was first found in Its incidence is about three times as high as that of all over Japan, in addition, the pattern of the form of leukemia is similar to that of directly exposed people in the near distance from hypocenter and also that of occupationally exposed cases. Most of these leukemia cases were found only to those people who entered the city within 3 days after the explosion. These facts suggest that we should not overlook the effects from induced radioactivity causing from atomic explosion, although the estimated value of induced activity has been said to be negligible in this Hiroshima bomb case from the dosimetric study of ABCC Report.