LEUKAEMOGENESIS: QUANTITATIVE ASPECTS AND CO-FACTOKS 9 



In other human populations which have been studied, exposure dosage is 

 either not known, as in radiological specialists in U.S.A. and in Great Britain 

 (cf. Court Brown, 1958), or is clearly not the only determining factor, as in 

 infants and children given radiotherapy (Murray and Hempelmann, 1961). 

 Thymic enlargement in infants is not a necessary precursor of radiation- 

 induced leukaemia but whether it predisposes to radiation-induced leukaemia 

 is not yet known. The size of the radiation port or field, i.e. the amount of 

 tissue irradiated (which in infants can vary from a small fraction to a large 

 fraction of the whole body) is quite probably important but under given 

 conditions of irradiation there did not seem to be much dependence of 

 leukaemia incidence on dose. Firm conclusions are difficult to draw since in 

 one thorough foUow-uj) of 1,600 infants and children given radiotherapy 

 sufficient to cause four cases of thyroid cancer (approximately a 100-fold 



Table II. Leukaemia and cancer in children irradiated in utero 



Selected data from Tables 4, 8 and 9 of MacMahon (1962). 



increase over the normal) there was unaccountably no appreciable increase in 

 leukaemia. 



The vexed question of the leukaemogenic effect of whole-body irradiation 

 of the human foetus in utero seems to have been largely clarified by MacMahon 

 (1962) who followed up three-quarters of a million children for at least 6 years 

 each. Diagnostic radiology of the mother was followed by a statistically real 

 risk of leukaemia (Table II) but at 40% above the unirradiated level the risk 

 was smaller by half than that suggested by the original observations of 

 Stewart and co-workers. The satisfactory conclusion was reached that the re- 

 sults of aU previous studies, both positive and negative, are statistically com- 

 patible wdth those of MacMahon. The actual dose received by the foetus is not 

 known and will certamly have varied considerably from time to time and from 

 hospital to hospital for technical reasons. The leukaemia incidence was twice 

 as high in children exposed to pelvimetry (three or more films) than in children 



