QUANTITATIVE ASPECTS OF EXPEKIMENTAL 

 LEUKAEMOGENESIS BY RADIATION 



E. H. MOLE 

 M.R.C. Radiobiological Research Unit, Harwell, England 



SUMMARY 



When considering possible mechanisms for leukaemogenesis by radiation the 

 following experimental observations need to be taken into account. Irradiation need not 

 increase leukaemia incidence, it may decrease it. Continuing irradiation need not lead to 

 a continuing increase in leukaemia. The effect of fractionation depends on the size of the 

 total dose. Dose-rate may be significant of itself. 



Because of the curvihnear relation between leukaemia incidence and dose, experi- 

 ments on physiological and other factors affecting the induction of leukaemia by radia- 

 tion cannot be easily interpreted unless dose-response curves in the different experimental 

 situations are compared. 



A good deal of information exists on tlie quantitative aspects of experimental 

 leukaemogenesis by radiation and again the awkward observations seem to be 

 the more interesting ones. 



When mice are exposed to single doses of whole-body irradiation the 

 leukaemia incidence need not necessarily increase as the dose increases even 

 when corrections are made for life- shortening by other processes and the 

 consequent reduction in the population at risk (Upton et at., 1958). With 

 increasing radiation-dose in fact one form of leukaemia may decrease in 

 incidence while another increases (Upton et al., 1960). The effect of a wide 

 range of dose-rate does not seem to have been examined. 



With multiple doses of irradiation, also, leukaemia incidence in mice need 

 not necessarily increase as the dose increases (Mole, 1959a, Fig. 1) and the 

 failure of the mcidence to rise with dose was not due to any differential loss of 

 population at risk. With a fixed total dose of radiation the details of the way 

 in which the irradiation is given may be of over-riding importance. With a 

 given kind of fractionation, daily exposure 5 days a week for 4 weeks, 

 leukaemia incidence varied from 5 to 40% depending on the dose-rate of the 

 individual exposures (Mole, 1959b). 



Fractionation was shown some years ago to be of similar quantitative 

 importance (Kaplan and Brown, 1952) but the experiment was defective in 

 one respect. The over-all exposure-time varied with the fractionation over the 

 range 4 days to 3 weeks so that the different groups of mice also varied in the 



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