DEPENDENCE OF RADIATION-INDUCED 

 LIFE-SHORTENING ON DOSE-RATE AND ANAESTHETIC 



P. J. LINDOP AND J. EOTBLAT 



St. Bartholomew's Hospital Medical College, Charterhouse Square, London, England 



The amount of life-shortening produced by a given dose of radiation depends 

 on many factors, mckiding the dose-rate. Thus, it is known that less life- 

 shortening per unit dose is produced when an animal is exposed to chronic 

 irradiation than when given a single dose. Some of this difference may be 

 attributed to an age factor (Lindop and Rotblat, 1962) rather than to a true 

 dose-rate effect, but even with single exposures life-shortening appears to 

 depend on dose-rate. 



In connection with investigations on the protective action of hypoxia 

 (Lindop and Rotblat, 1960) it became necessary to expose mice at very high 

 dose-rates, so that a dose of over 1,000 r could be delivered in less than a 

 second. These mice had also to be anaesthetized before making them anoxic in 

 order to avoid convidsions. It became, therefore, necessary to investigate the 

 possible effects of these two factors, high dose-rate and anaesthesia, on the 

 sensitivity of animals to radiation. 



The primary criterion was the acute effect of radiation, i.e. the LDgg for 

 30 days mortality. For this purpose SAS/4 mice, male and female, all 4 weeks 

 of age, were exposed to single whole-body doses of radiation in the range of 

 600-1,000 rads. The radiations used were 15 MeV electrons or X-rays from 

 a linear accelerator. The dose-rate was varied over a very wide range, from 

 77 to 160,000 rads/min. From these data the LDgQ were determined with an 

 accuracy of about 3%. 



Many of the mice, particularly in the lower dose groups, survived the first 

 30 days and were kept in the cages imtil the end of their lives. The ages at 

 death were recorded and life tables plotted; from these the median life-spans 

 were determined. By comparing with the life-span of the control mice, and 

 assuming a linear relationship between life-shortening and dose, which had 

 been previously established for these mice at this age group (Lindop and 

 Rotblat, 1961), the amount of life-shortening per 100 rads was calculated 

 with an accuracy of about 10%. 



The results obtained for both the LD50 values and the life-shortening are 

 given in Table I. It is noted that over the whole range of dose-rates studied, 

 covering several orders of magnitude, there is very little variation of the LDjq. 

 In fact, the data are compatible with the assumption that the LDgg remains 



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