108 



R. H. MOLE 



ages at whicli they were irradiated. It had already been shown that the 

 leukaemic response of mice of the strain involved varied fairly sharply with 

 age (Kaplan, 1948). Therefore fractionation experiments have been repeated 

 covering a much wider range of fractionation of dose and keeping constant 

 the over-all exposure-tune and therefore the age of the mice duruig irradiation. 



lOOr 



o 



a 





E 

 o 



• SOr daily X 



25r daily X 

 .jO O 



' .. 50r daily y 



"""■X 



1000 2000 



Total dose (r) 



3000 



Fig. 1. Leukaemia incidence in female CBA mice given daily irradiation by X- or y-rays 

 for limited periods. X-Rays 5 days a week at about 1 r/sec for 1-24 weeks (Mole, 1959a). "^"Co 

 y-rays 5 nights a week at about 3 r/hr for 4-12 weeks. (Unpublished data.) 



With a total dose of 1,000 r the optimum fractionation was found to be 

 very similar to that reported earher by Kaplan and Brown, the effect of dose- 

 rate being evident as well (Fig. 2). However with a total dose of 2,000 r given 

 in just the same over-all exposure- time and in just the same way by the same 

 radiation source, the optimum fractionation was clearly quite different (Fig. 

 3). It is worth emphasizing that when a dose was given every hour for 672 

 hours (28 days) 1,000 r produced very little leukaemia. However when each 

 individual fraction was doubled in length from 5 minutes to 10 minutes, there 

 was roughly a 50% incidence of leukaemia. Thus the first half of each fraction 

 had little effect, whereas the second half of each fraction did a great deal. 

 Conversely when the dose was given once a day, 5 days a week, for 4 weeks, 

 the incidence with 2,000 r was about the same as with 1,000 r, so that with 

 this system of fractionation the second half of each fraction had no additional 

 effect at all. Clearly the leukaemogenic effect of the y-rays depended far more 

 on the circumstances of the irradiation than on the actual exposure dose. 



Although it is interesting to note the similarity of the optimal fractiona- 

 tion of 1,000 r in mice (Fig. 2) to the standard radiotherapeutic practice of 



