PHYSICAL AND BIOLOGICAL FACTORS 923 



tion in the mode of killing which depends upon the dosage of radiation 

 (Quastler, 1945a; Quastler et al., 1951; Henshaw, 1944). Death follow- 

 ing median lethal irradiation is the result of several factors, the most 

 prominent of which are leucopenia, anemia, hemorrhage, bacteremia, and 

 intestinal damage. The shocklike syndrome that appears with higher 

 dosages and results in death within several days is initiated primarily as 

 a consequence of intestinal injury. Finally, with still more massive 

 dosages of radiation killing occurs within a matter of minutes or hours 

 and is associated with disturbances of the nervous system. 



A number of factors influence the threshold for different radiobiological 

 effects. On the physical side these include the quality of the ionizing 

 radiation, its intensity or rate of delivery, and the manner of exposure, 

 i.e., local or total-body, external or internal, and single, continuous, or 

 fractionated. It is generally agreed that a, j8, y, and X radiations and 

 neutrons produce more or less similar physiological effects. However, 

 their efficiency varies considerably since the effects of ionizing radiations 

 depend not only upon the amount of energy absorbed per unit of volume 

 and of time but also upon its distribution along the individual ion tracks 

 (Zirkle, 1943; Gray, 1946). Specific ionization (ionization density) 

 increases progressively from fast /3 rays to y rays, hard X rays, soft X rays, 

 fast neutrons, and a rays, and for most systems the biological effective- 

 ness increases with increase in the ionization density. The time factor 

 of irradiation and the dosage distribution among different tissues may, of 

 course, contribute to the observed differences in efficiency and should be 

 considered in their evaluation. The y- to X-ray ratio of effectiveness has 

 been found to vary from 1.3 to 2.0 for a number of biological effects 

 (Sugiura, 1939; Lasnitzki and Lea, 1940; Mottram and Gay, 1940). 

 When the irradiation periods are similar, the efficiency ratio for acute 

 lethality in mice appears to be about 1.3. This is consistent with the 

 finding that 200-kv X rays are about 1.3 times as effective as 20-Mv 

 X rays, which have a specific ionization comparable with that of hard 7 

 radiation (Quastler and Clark, 1945; Quastler and Lanzl, 1950). The 

 200-kv X rays are also about 1.4 times as efficient as Na 24 /3 rays for acute 

 killing of mice (Snyder and Kisieleski, 1950) ; it is worth noting, however, 

 that the actual ratio may be somewhat lower, since the time factor of 

 irradiation varied in these experiments. For acute effects in mammals, 

 1 n of fast neutrons (2.5 rep) is equivalent to 3 to 32 r, depending on 

 the criterion (Lawrence and Lawrence, 1936; Aebersold and Lawrence, 

 1942; Lampe and Hodges, 1943; Mitchell, 1947; Evans, 1947; Henshaw 

 et al, 1947; Gray and Read, 1948; Dowdy, 1949). The effectiveness 

 ratios are generally somewhat larger for periodic or protracted exposures 

 than for acute, presumably because of a slower recovery from neutron 

 irradiation. However, for certain effects that are related presumably to 

 very small volumes (gene mutations or killing of small microorganisms) 



