PHYSICAL AND BIOLOGICAL FACTORS 925 



about 30 times, e.g., from 435 r per minute to 15 r per minute (MacKee 

 et al., 1943). 



When a particular dose is given in several fractions, it is generally 

 less effective than when it is delivered at one time. Fractionation has 

 been studied by exposing animals at different daily doses over a specific 

 interval and comparing the effect produced by the same total dose given 

 in a single exposure or by continuing the exposures until death. Such 

 studies provide valuable information concerning lethal mechanisms and 

 recovery constants and bear directly on the problem of radiation tolerance 

 (MacComb and Quimby, 1936; Quimby and MacComb, 1937; Ellinger, 

 1943; Hagen and Simmons, 1947; Sacher et al., 1949; Ellinger and 

 Barnett, 1950; Sacher, 1950). Differences in rates of recovery may 

 account for the lower relative efficiency of 20-Mv roentgen rays with 

 fractionation as compared with that of 200-kv X rays (Quastler and 

 Lanzl, 1950) or neutrons (Henshaw et al., 1947; Evans, 1947). Although 

 recovery constants can be calculated for certain radiation effects, they 

 apparently cannot be used to predict quantitatively the recovery pattern 

 following various modes of irradiation (Sacher, 1950). 



Another aspect of the intensity-duration factor is the production of 

 different effects with the same total dose. Acute and delayed toxic 

 effects have been described in chick embryos, chicks, and ducklings 

 (Karnofsky et al., 1950; Jacquez and Karnofsky, 1950; Steamer, 1951). 

 Early deaths, within 24 hours, occur with dose rates above 5 to 10 r per 

 ' minute and are relatively independent of a total dose above 800 to 1000 r. 

 There is evidence that early lethality is associated with renal failure since 

 uricemia precedes death by several hours (Steamer et al., 1950). Delayed 

 deaths take place within one to two weeks following irradiation, and the 

 pathological physiology parallels that seen in mammals. From dose 

 fractionation studies it appears that the over-all time of exposure is more 

 important than the dose rate in eliciting the early killing in chicks and 

 ducklings (Steamer and Christian, 1951). The significance of the over-all 

 time of exposure has been indicated in other experiments. Dose rate as 

 such is of less importance than the dose per fraction or the intervals 

 between fractions in determining the lethal effect of X radiation on chick 

 fibroblasts (Paterson and Thompson, 1948). 



The response to radiation depends upon the portion of the cell or 

 organism that is irradiated. It has been shown by fractional irradiation 

 of parts of cells that ions formed in the cell nucleus are more effective in 

 the production of certain biological effects than those formed elsewhere 

 (Henshaw, 1938; Zirkle, 1932). Several hundred roentgens applied 

 locally may be relatively innocuous, whereas a similar dose administered 

 to all or most of the body leads to widespread effects. Irradiation of the 

 abdomen is more efficient than irradiation of the thorax, and penetrating 

 radiations are more effective than the superficial in producing acute 



