10 



AN EXPERIMENTAL STUDY OF THE INFLUENCE 



OF OXYGEN ON THE RADIO-SENSITIVITY OF THE 



EHRLICH ASCITES TUMOUR CELL 



L. H. Gray 



British Empire Cancer Campaign Research Unit in Radiobiology, 

 Mount Vernon Hospital, Xorthwood, England 



I PROPOSE in this paper to give an account of an experimental investigation 

 which Dr. Eleanor Deschncr and I have carried out^ with the object of 

 comparing the radio-sensitivity of mammalian tumour cells in vivo and iri 

 vitro under controlled conditions of oxygen tension. It was known that the 

 radio-sensitivity of many mammalian tissues was positively correlated with 

 the availability of oxygen to the tissues, but the form of the relation between 

 radio-sensitivity and the concentration of dissolved molecular oxygen in the 

 immediate environment of the cell had not been investigated in the living 

 animal. The control and measurement of the oxygen tension in an organized 

 tissue seemed to present insuperable difficulties, and we considered that a 

 better chance of success lay in an attempt to measure the concentration of 

 dissolved oxygen in the peritoneal fluid at the time of irradiation in an animal 

 bearing an Ehrlich ascites tumour. This tumour grows as a single cell 

 suspension in the peritoneal cavity which contains nutrients, and generally a 

 certain amount of l)lood. An inoculum of 10' cells gives rise in the course of 

 5 days to about 2 ml. of fluid containing ~ 10^ cells/ml. This fluid, diluted 

 with plasma obtained by centrifuging the peritoneal fluid from other tumour- 

 bearing animals, with blood added to a mean concentration of 5 x 10® 

 red blood cells (r.b.c.)/ml., was the fluid used for in vitro irradiations. After 

 irradiation in the manner described below, alequots were inoculated into 

 fresh host mice for a period of about 15 hours, during which tumour cells 

 begin to enter their first division. The mice were killed, the peritoneal fluid 

 withdrawn, and the cells fixed and stained. Anaphase figures which showed 

 either chromosome bridge or a fragment were scored as abnormal. In 

 control material, the proportion of abnormal cells was arovmd 5 per cent. 

 In irradiated material the log percentage normal cells was linearly related to 

 dose provided the irradiation was not too heavy. The coefficient of aberration 

 production a, is: t r- t r- 



D 



where F^- is the fraction of normal cells in control material, F^ that in irrad- 

 iated material, and D is the dose in rads. This coefficient was used as a 

 measure of radio-sensitivity. In the interests of accuracy, the dose D, was 

 adjusted so as to give rise to about 65 per cent abnormal cells in all irradiated 



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