STUDY OF 11 11. IXFLUEXCK OF OXYGEN ON I< ADIO-SEXSITIVITY 



In the case of each animal irradiated while breathing air, we have calcu- 

 lated from the observed aberration coeflicieni the percentage of aerobic 

 cells. Indi\idual values range from zero to 36 per cent with a mean of 13 per 

 cent. Allowing that the couN'olutions of the viscera may double the en'cctive 

 area of a given \'olumc of lluid, this figure is ai:)out that which might be 

 expected in the absence of stirring. Since (jur animals were immobilized 

 during irradiation, there may well have been very little stirring action at this 

 period — though, as explained above, the abdomen was deliberately massaged 

 while the bubble of gas was being brought into eciuilibrium before irradia- 

 tion. Stirring would have to be rather rapid to be elTective. If the oxygen 

 storage capacity of the blood, which was present in variable amounts, is 

 excluded, an element of fluid containing 10* tumour cells/ml. which had been 

 raised to a />o, of 40 mm mercury by contact with a membrane, would, 

 through tunioiu" cell respiration, become anoxic in 7 sec. Thus, aerobic 

 sensitivity could only be maintained if every element of fluid were brought 

 into contact with a membrane at least once every 7 sec. 



Except when an animal was being turned over, the gas bubble would 

 always have Iain against a membrane. In the light of the figures given above, 

 it is evident that the dimensions of a • 1 ml. bubble are such that some parts 

 of the bubble surface are likely to have been in contact with virtually anoxic 

 fluid. The i"ecordcd/^o„ i'^ ^^^^ bubble is thus probably slightly lower than that 

 of the membrane. The more uniform the oxygen tension throughout the fluid, 

 the closer would the bubble indicate the correct value. This accords with the 

 fact that the observed aberration coefficients were occasionally up to the 

 level of the broken line in Figure 4, but never above it. Thus, in all respects, 

 our in vivo observations accord with what might be expected from a know- 

 ledge of the structure, size, cell concentration, and (^o, of a five-day tumour. 



Increasing the concentration of oxygen in the gas breathed by the animal 

 increases the mean sensitivity of the tumour cells; injection of hydrogen 

 peroxide increases it still more. Only an exceptionally vigorous massage of 

 the abdomen, continuing throughout the period of irradiation, would be 

 expected to increase substantially the mean sensitivity of the tumour cells in 

 an animal breathing any of the gas mixtures tested. 



REFERENCES 



^Deschner, E. E. and Gray, L. H. Radiation Res. (1959) — in the press. 



*KiRK, P. L. Quantitative Ultramicroanalysis : John Wiley, Inc., New York, 1950, 



p. 224, figure 90 

 ^Hersch, p. Inslrum. Pract. (1957) 937 

 *Gray, L. H. Progress in Radiobiology (Ed. J. S. Mitchell, B. E. Holmes and C. L. 



Smith): Oliver and Boyd, Edinburgh, 1955, p. 267 



DISCUSSION 



Dr. Duncan: Perhaps I might start the ball rolling on a chemical question. It seems 

 to me that this problem is very similar to the tuberculous problem which Professor 

 Rubold studied, and in this, one of the things which of course is interesting, is the 

 cfl'ccl of X-rays on the fluid. Now, we know perfectly well that irradiation can pro- 

 duce hydroxy-radicals and other oxidizing agents, and it therefore would appear 



84 



