INFLUENCE OF PROTRACTION 397 



volume of tissue irradiated in a given exposure is reduced if the radiation 

 is not capable of penetrating through to the deeper parts of the body, 

 and higher dosages are required to produce the lethal effect. For ex- 

 ample, Raper (55) has concluded from total beta/total gamma ratios for 

 killing in mammalian species that beta radiation brings about lethal ac- 

 tion through a total mass or volume effect, whereas gamma radiation 

 (which irradiates all elements of the body more or less uniformly) does 

 not. Using various portions of the total dose as beta radiation, and the 

 remainder as gamma radiation, gives only partial additivity. Where 

 absorption is equivalent (in small organisms such as Drosophila eggs), 

 complete additivity of the beta and gamma radiation has been reported 

 (72). 



In animal experiments, Jolles (35) found that the severity of skin re- 

 action to x-irradiation was related to the size of the field. Evidence 

 was found in addition, through use of a grid or sieve of lead strips, that 

 the local effect became more pronounced the greater the total energy 

 absorbed in the entire field. Two fields far apart were injured less (ery- 

 thema of human skin) than if the irradiated areas were close together 

 (36). The enhancement of the reaction in each field, if two were ir- 

 radiated close together, has been interpreted as indicating the formation 

 of a diffusible toxic substance with its concentration proportional to 

 (Dose X cm^) /Distance (37). It should be borne in mind that at least 

 a part of the above findings might be expected on physical grounds, such 

 as overlapping of scattered radiation at the edge of fields, and this has 

 possibly not been evaluated completely. 



Influence of Protraction 



As the intensity of the radiation (roentgens per minute) is reduced, 

 the biologic effect (that is, lethal action) for the same total dose becomes 

 less pronounced. There is abundant evidence from studies of organisms 

 other than mammals and from erythema studies in man (9, 18, 39, 47) 

 that protraction decreases the effect. There are exceptions such as ef- 

 fect on mutation rate, inhibition of growth in tissue culture, and lethal 

 effects in amphibia. The published data regarding this factor in whole- 

 body irradiation of the mammal are woefully incomplete. No doubt 

 one of the reasons for the variation in LD^q found by different investi- 

 gators is the difference in exposure rate employed. Henshaw, Riley, and 

 Stapleton (31) reported that increasing the period of exposure 10 times 

 reduced the lethal action (in whole-body irradiation of mice) for a given 

 dose to approximately 70 per cent. In a discussion by Glasser et al. (18), 

 it is pointed out that the effect of protraction varies for different organ- 



