J. F. LOUTIT 



background of radiation. A still greater source of emotional speculation is 

 the practice of the major powers to explode nuclear weapons for trial 

 purposes. Inevitably this releases radio-active fission-products and causes 

 the induction of radio-activity by neutrons in normally stable atoms. Thus 

 we can expect that there will be a gradual rise in the levels of external 

 radiation to which we are exposed. The released radio-active materials will 

 gain access to the body in various degrees depending on the chemical 

 nature of the materials and there will be an increased exposure to internal 

 radiation. 



One can predict the future state of affairs only if there are reliable laws 

 on which to base the predictions. Frankly there are not. But there is a 

 certain amount of information concerning dose and effect. For instance, in 

 their analysis of cases of leukaemia following irradiation for ankylosing 

 spondylitis, Court Brown and Doll found that if they plotted incidence of 

 leukaemia against the estimated mean dose to the marrow, the plotted 

 points fell roughly on a straight line. On the other hand, while the incidence 

 of leukaemia plotted against integral dose of radiation was linear at the 

 lower doses, there was a marked departure from the linear relation at the 

 higher doses. It is still arguable of course which is the better form in which 

 to assess dose for this purpose. 



In a theoretical appreciation of the problem Lewis'^ used the data of 

 Court Brown and Doll together with such data as are available of the 

 American radiologists and the population of Hiroshima and Nagasaki. 

 Given a few assumptions, he arrived at the conclusion that in each case the 

 information is compatible with an incidence of leukaemia of about one in 

 a million per year per roentgen received. This is tantamount to accepting a 

 linear relationship between the incidence of leukaemia and the integrated 

 dose of radiation irrespective of dose rate. In fact this presumes that the 

 situation is analogous with that which is generally supposed to hold for 

 gene-mutation. True-point mutation of genes, as distinct from the chromo- 

 somal structural mutations, is held to be strictly proportional to total doSe. 

 Thus the analogy leads to the conclusion that the induction of leukaemia 

 is a similar process to the induction of gene-mutation, or going further, 

 induction of leukaemia is due to somatic mutation. The concept of somatic 

 mutation has been raised a number of times in the past in attempts to explain 

 carcinogenesis. Hitherto the theory has always been found wanting as far 

 as the explanation of the facts is concerned. Today it still seems to me naive 

 to hope that such a simple relationship can be expected to hold even for one 

 type of malignant disease and it does not help me to understand the natural 

 history of leukaemia. 



According to certain published statistics, when all classes of leukaemia are 

 lumped together, there is an uneven distribution of incidence with age. An 

 early mode occurs in childhood, from 20 to 40 years incidence is minimal; 

 after which it increases with each decade. This sort of relationship holds 

 also for the acute leukaemias. Chronic myelocytic leukaemia increases pro- 

 gressively with age. The incidence of chronic lymphatic leukaemia also rises 

 progressively with age but not in linear fashion. On the hypothesis of somatic 

 mutation induced by natural background radiation one would expect a 

 linear increase, perhaps after some sort of incubation period to allow for 



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