DAMAGE TO REPRODUCTIVE CAPACITY OF HUMAN CELLS 



189 



administered after intervals of :2, 4 and 12 lir. Fii the interval the cells 

 were maintained at 37 ^C-. 



With a-radiation no statistically significant differences were observed 

 between the effects of single doses of respectively 50, 100 and 200 rad 

 and of two doses of respectively 25, 50 and 100 rad administered at 

 intervals of 2, 4 and 12 hr. At doses up to 500 rad of 200 kV X-radiation 

 and intervals of 2 and 4 hr the effect of fractionation was very small. At 

 higher doses and with longer intervals the effect is significant however. 

 For example the surviving fraction after a dose of 900 rad was found to 

 be 1-3 ± 0-2 per cent, whereas after two times 450 rad, 12 hr apart, 

 4-0 + 0-05 per cent of the cells had retained the capacity for clone 

 formation (see Fig. 2). Thus more cells survive after fractionated 



5 6 7 8 9 10 



Dose (rod x 100) ^ 



Fig. 2. Effect of fractionation of doses of 200 kV X-radiation on the capacity for clone 



formation. 

 Curve 1, Surviving fraction after irradiation 4 hr after plating. 



Curve 2. Surviving fraction after 450 rad at 4 hr after plating +250 rad and 4;>0 rad 

 respectively, administered 12 hr after the first dose. 



doses i.e. part of the damage is repaired in the interval. The higher 

 survival was not due to cell multiplication in the interval, as cells 

 irradiated with 900 rad of 200 kV X-radiation 16 hr after plating 

 showed a surviving fraction of 1-5 ± 0-1 per cent. These results are in 

 agreement with the work of Elkind and Sutton (1959) except that in 

 our system repair appears to start somewhat later. 



Effects of combination of doses of a- cmd 200 kV X-radiation 

 In Fig. 3 survival curves are given from experiments in which the 

 same cells were irradiated with X- and a-radiation. No statistically 



