MODIFICATION OF LATE EFFECTS OF IONIZING RADIATION 307 



be expected, again if you observed a higher incidence without knowing this you would 

 have inferred that the "protection" had actually worsened the effects of irradiation and 

 so on. It is a pity I think that this kind of experiment hasn't been more adequately 

 extended, since the inital work of Kaplan and Brown. Dr. Mole has referred to similar 

 complexities under conditions of fractionated or long-term irradiation and until the 

 kinetics of repair is elucidated by the sort of work that Dr. Lamerton mentioned, until 

 we know more about repair rates, trying to assess the influence of protective agents 

 without the necessary control information may be quite misleading. 

 cottiee: I should like to mention that the importance of the duration of the interval may 

 even be different for different cell lines. But it also depends on what you actually look for. 

 If you look for thymic lymphomas you may choose different intervals than if you look 

 for myelogenic leukaemias. 



lamerton: I thuilc it is probably true to say that the effect of fractionation has only been 

 sho^vn for thj^mic lymphomas. I would like to ask in fact if any fractionation work has 

 been done with regard to other tumours, or whether there is any evidence that fractiona- 

 tion does affect considerably the incidence of other types of tumours and other types of 

 leukaemia too. 



UPTON: I have the impression that data were presented here on bone tumours. Was there 

 a fractionation effect there? (Loud laughter.) 



lamerton: Well, of course, our work on the effects of fractionation with external radiation, 

 which is the cleanest case didn't show very much, you see. That is what I meant. An 

 effect of fractionation has been shown very markedly with thymic lymphoma. I would like 

 to ask Dr. Upton whether he has looked at the effect of fractionation on myeloid leuk- 

 aemia, and, if so, whether there is any similar effect? 



UPTON: We have not seen any similar effects in connection with myeloid leukaemia, but I 

 regret to say that we haven't done such a nice systematic study. I think this kind of work 

 badly needs to be done. There are reaUy very few adequate experiments on the influence 

 of fractionation. I don't think one could say categorically that similar phenomena 

 won't turn up for other tumours. 



