192 



G. W. BARENDSEN 



capacity for clone formation decreases ^^■itll decreasing oxygen concen- 

 tration. It may be noted that there is a shght difference in radio- 

 sensitivity between cells over which no gas mixture is passed at all and 

 cells which are in equilibrium with air. An explanation of this effect 

 cannot as yet be given (see Fig. 4). 



Comparison of the curves given in Fig. 4 shows that with an oxygen 



6 8 10 12 ' 14 16 18 

 Dose (radXiOO) »- 



Fig. 4. — Effects of a- and 200 kV X-radiation on cells in equilibiiuui with air and nitro- 

 gen respectively. 



1 , Curve obtained for a-radiation under normal conditions with medium not re- 



moved (see Fig. 1). 



2 , Curve obtained for a-radiation witli cells in equilibrium with air (medium re- 



moved). 



3 . Ciu-ve obtained for a-radiation with cells in equilibrium with i)!)-7% N2 + 0-3% 



O2 (medimn removed). 



4 , Curve obtained for 200 kV X-radiation under normal conditions with medium 



not removed (see Fig. 1). 



5 , Curve obtained for 2(10 kV X-radiation with cells in eciuilibrium with air (medium 



removed). 



6 , Curve obtained for 200 kV X-radiation with cells in equilil)iium with 99-7 "0X2 



+ 0-3% O2 (medium removed). 



concentration of 0-3 per cent as used in most of the experiments, a re- 

 duction of the sensitivity was obtained by a factor of aVjout 2 as com- 

 pared with the sensitivity observed with oxygen concentrations be- 

 tween 10 and 100 per cent i.e. twice as high a dose is needed to produce 

 the same effect. With purified nitrogen a reduction of the sensitivity 

 by a factor of 2-6 was found. 



Expei'iments carried out with a-irradiation of cells in equilibrium 

 with 99-7 per cent N2 + 0'3 per cent O2 showed that with this radiation 

 of high LET only a very slight decrease in radiosensitivity by a factor 

 of about 1-10 ± 0-05 can be achieved as compared with aerated cells 

 (see Table I and Fig. 4). This result is in agreement with that found 



