LIX'KAKMIA I.NULCKD HV RADIAIION 



manifestation of the condition. Broadly speaking the pattern for chronic 

 niveloc\tic leukaemia is of this form, but that of acute leukaemia is not 

 and both these types are represented in the radiation-induced cases. 



Thus to me it is disappointing that the United Nations Committee, which 

 in general produced an admirable scientific report, gave so much weight to 

 the hypothesis of a possible linear relationship between the incidence of 

 leukaemia and total accumulated dose of radiation. Certainly in the present 

 state of knowledge it cannot be ruled out, but the accredited cases of radi- 

 ation-induced leukaemia are so far limited to those induced by radiation 

 given at high dose rates. 



It behoves us now to see what help we can derive from animal experiments. 

 As I have mentioned ionizing radiation has been recognized as an inducing 

 agent of leukaemia and other malignant diseases in experimental animals. 

 As far as leukaemia is concerned the mouse has been used to a far greater 

 extent than any of the other routinely kept animals of the laboratory. Mice 

 and fowls in particular, but other experimental animals also, suffer naturally 

 from varieties of leukaemia. In birds these have long been attributed to 

 viruses. There seems to be good evidence of separate viruses for avian 

 lymphomatosis, myeloblastomatosis and erythroblastomatosis. In mice 

 leukaemia is not uncommon and certain strains have been selectively bred 

 for this disease. Strains such as AK and C58 have an incidence up to 80 per 

 cent. Most of these selected natural leukaemias are apparently lymphoid in 

 type. Myeloid leukaemia is found much more rarely in the mouse. True 

 leukaemia, with an increased number of primitive cells in the peripheral 

 blood, is not common except in the terminal stages. However, a widespread 

 infiltration of the tissues is a necessary feature. The thymus, being part of 

 the lymphatic system and being late to regress in the mouse compared with 

 man, is commonly involved. In fact in some of the high-leukaemia strains 

 and frequently in the varieties induced by radiation or by chemical agents, 

 the thymus is involved earlier and to a greater extent than the other lymphoid 

 tissues. Some workers therefore speak of thymic and non-thymic leukaemias. 

 On the other hand, in many aged mice, a more localized type of malignant 

 disease occurs which may be called reticulosarcoma or local lymphosarcoma. 

 Thus, as in man so in the mouse there is a fair spread of clinical and patho- 

 logical types of disease all of which can be fitted into a classification of 

 malignant reticuloendotheliosis. The types are not identical with those 

 recognized in human disease but there is a sufficient closeness for there to 

 be a general recognition of an analogy between the diseases of mouse and 

 man. Given therefore that the mouse is a fair model of man, it is fair for us 

 to look at the data available from mice. 



The work with which I am most familiar is that of my colleague, R. H. 

 Mole*. He has chosen to use for most of this work the inbred strain of mouse, 

 CBA. This is one of the standard strains introduced over 30 years ago by 

 Strong who originally selected for longevity. The mice of our sub-strains 

 still possess this characteristic. This means that they do not normally die 

 prematurely of malignant disease as do most inbred strains which were 

 selected for susceptibility to one or other form of malignant disease. 



The control mice thus have a very low natural incidence of all forms of 

 leukaemia certainly for the first two years of their life span of approximately 



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