LF.UKAI.MIA INDrCI.I) WY RADIAIIOX 



Table /*. I'-fTerts of dosc-iiitcnsity on (lie iiuiciciicc ol icukacniia in f<'nialc mice 

 c;i\(Mi wliole-hod\' t;aiiiiii:i irradiation ()\cr a four-week exposure period 



Leukaemia has occurred in substantial or significant numbers in both groups 

 of mice — CBA and C57BL — when the dose rates during exposure were 81, 

 21 or 3-3 r per hour. There has been no leukaemia in the two varieties 

 of mice exposed at 1 -3 r per hour or in the unirradiated controls. It might 

 at this stage be worth while to quote from this paper of Mole* : 



'If leukaemia in the mouse can be taken as a model for leukaemia in man, 

 it seems clear (/) that there is no evidence to support the hypothesis that the 

 chief determinant of the incidence of radiation-induced leukaemia is dose, 

 and (//) that there is some evidence which denies the validity of extrapolation 

 from existing human data on the relation between dose and leukaemia to 

 situations where the dose intensity is very much less. It must be conceded 

 however that these conclusions are demonstrably true only of leukaemia 

 incidences of 10 per cent or higher. It is not possible to refute experimentally 

 a claim that the relation of dose to leukaemia incidence has a different form 

 at lower incidences of leukaemia, i.e., that the experimentally observed 

 i^elation cannot be extrapolated beyond the observed limits. Observations 

 can never bridge this logical gap which can only be filled by a generally 

 acceptable hypothesis of the mechanism of leukaemogenesis by radiation. 

 Even though no such hypothesis exists at present, one firm conclusion at least 

 can be reached. The experimental facts cannot be accounted for by the 

 theory of somatic mutation without assuming the existence of an additional 

 factor of overriding importance for the clinical expression of leukaemia. 

 And if this factor is so important of itself, it seems logically unnecessary to 

 postulate an additional mechanism, such as somatic mutation, to account 

 for the facts.' 



But this is a negative sort of conclusion. Let us keep the word mutation, 

 but instead of somatic 'gene-mutation' let us consider somatic 'chromo- 

 somal-mutation'. Ford in our laboratory has been studying cytologically 

 the cases of murine leukaemia provided by Mole. Most of these are radi- 

 ation-induced leukaemias but a few of them have been spontaneous. Their 

 results were also reported at Geneva^^ and again I quote directly* : 

 ' (?) The large majority of the leukaemias examined consist of mixed cell 

 populations in which some, many, or even all of the cells contain chromo- 

 somes which differ from the basic diploid set of the species either in number 



* Reproduced from the Proceedings of the United Nations Organization Second 

 Conference on the Peaceful Uses of Atomic Energy. 



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