PHYSICAL AND BIOLOGICAL FACTORS 



931 



Species sensitivity to penetrating radiation is not well correlated with 

 body size or with metabolic rate, although these factors, as will be dis- 

 cussed later, may be important in individual animals. There is little 

 difference in the basal heat production of the guinea pig and rat although 

 the LD 5 o for the guinea pig is lower than that for the rat by a factor of 

 about 2. Since a number of factors are involved in radiation death, 

 species sensitivity may reflect, in part at least, the particular suscepti- 

 bility of the different animals to the diverse mechanisms leading to 

 morbidity, e.g., toxins, leukopenia, bacteremia, hemorrhage, impaired 

 nutrition, and shock. There is a suggestion that species sensitivity may 

 be related to differences in the rates of recovery from radiation injury 

 since the mean survival time after median lethal irradiation is greater for 

 the more sensitive species (Sacher, unpublished observation, 1951). It 

 is of interest that a number of physiological and histological changes 

 reflect the amount of radiation and not the lethal effect; i.e., they are 

 more nearly independent of species (Bloom, 1947; Brues and Rietz, 

 1948; De Bruyn, 1948). 



In contrast to the well-established differences in species sensitivity to 

 penetrating radiations are the apparently negligible differences in the 

 acute lethal effects of total-surface /3 irradiation when a correction is 

 made for body size, i.e., for the proportion of the body mass irradiated. 

 The median lethal dose of external irradiation varies from 4700 rep for 

 the mouse to 17,000 rep for the rabbit; the total integrated dose, however, 

 is directly proportional to the body mass (Raper, 1947). That the 

 lethal action of surface rays is dependent upon a total mass or volume 

 effect is assumed from the fact that all the energy is absorbed in a super- 

 ficial layer of tissue whose mass is small relative to the mass of the animal. 

 The possibility that some compensating situation, also varying with 

 body mass, may be active is not ruled out. The lethal mechanisms are 

 undoubtedly different for the superficial and penetrating types of radia- 

 tions (Table 14-2); for example, the blood picture is unaffected by a 



Table 14-2. Dosage of External Beta (P 32 ) and Gamma (Ta 182 ) Radiation 

 Required to Kill 50 Per Cent of Animals within 45 Days 



(After Raper, 1947) 



lethal dose of external (3 irradiation. With internally deposited (3 

 emitters, or a emitters, however, the picture more or less resembles that 



