INFLUENCE OF OXYGEN ON R ADlO-SENSniVITY OF CELLS AND TLSSUES 



In the five-day old ascites tumour, the cell density is about 10** ml., which 

 may be much higher than cell concentration in human ascites. We believe, 

 on the other hand, that the condition of low oxygen tension inferred for the 

 solid mouse tumours is typical of many human tumours. This belief is based 

 partly on indirect evidence derived from observed features of the histological 

 structure and ])lood supply of some hiunan tumours^**, and partly on the 

 fact that in seven test cases in which different portions of the same human 

 tumour were exposed to the same dose, with or without oxygen adminis- 

 tration, considerably greater histological damage was evident in the regions 

 of the tumour which had been irradiated when the patient was breathing 

 oxygen ^^. 



When suspensions of small numbers of tumour cells are inoculated into the 

 mouse intradermally-'', or are allowed to invade the liver (Hewitt — personal 

 communication), so that the tumour cells take up the oxygen tension of the 

 normal tissues, their radio-sensitivity is not increased to any significant degree 

 by the administration of oxygen to the mouse at the time of irradiation. 

 We conclude that in such cases the oxygen tension of the tumour cells is 

 probably at least 30 mm Hg. 



It is evident that if a tumour contains even a very small proportion of 

 nearly anoxic cells, these rather than the bulk of the aerobic tumour cells 

 wall determine the success or otherwise of the treatment. This is illustrated in 

 Table 3, which supposes that a total dose of 4300 r is delivered to a mass of 



Table 3. The influence of oxygen tension on the proportion of cells which 

 survive 4500 r calculated for cells having Dj f. (aerobic) of 150 r 



Number of cells and f 

 \olume effectively J 

 treated (90°o chance | 

 that no cell sur\i\es) |^ 



tumour cells at different oxygen tensions. In line with the observations of 

 Puck^^ for HeLa cells growing in culture, and the observations of Hewitt^^ 

 for mouse leukaemia cells growing in the living animal, it has been assumed 

 that the proportion of cells which retain reproductive integrity is exponen- 

 tially related to dose, and that under aerobic conditions 37 per cent of cells 

 survive 150 r. In this hypothetical situation there would be a 90 per cent 

 chance of sterilizing all of a population of 10^^ aerobic cells, but only 2x10^ 

 anaerobic cells. Table 3 shows that while a minority of completely anaerobic 

 cells could jeopardize success in therapy, a comparatively small increase in 

 oxygen tension could have a very favourable influence on the chances of 

 successful sterilization. 



WAYS IN WHICH OXYGEN MAY AFFECT RADIO-SENSITIVITY 



The oxygen molecule has an embarrassingly wide range of properties which 

 might be relevant to a discussion of the manner in which it influences radio- 

 sensitivity. 



160 



