398 INFLUENCE OF EXPOSURE FACTORS 



isms, and that the total duration of the irradiation is significant. An 

 example of the latter factor was found to be that an exposure of 660 r 

 produced a threshold erythema whether delivered at 75 r per min or 

 20 r per min. However, when given at 4 r per min, 750 r were required 

 to produce the same reaction. Similar findings for whole-body irradi- 

 ation of mice have been noted in a progress report.*. Gamma-ray ex- 

 posures were given at intensities of 2300, 896, 240, and 67 r per hr. 

 The LD^o values were found to be 817, 805, 796, and 1006 r. Thus, 

 when the overall time was less than 3 hr, intensity was not a factor. 

 However, when the exposure was extended to 11 hr, the amount of ex- 

 posure required to kill 50 per cent was definitely increased. 



Influence of Fractionation 



In general, if a certain dose is given in several fractions the effect will 

 be less than if it were all delivered at one time. In other words, some 

 recovery takes place between the different exposures. However, in cer- 

 tain tissue-culture, amphibia, and in vitro tumor studies, fractionation 

 has not been found to reduce the effect of a certain total dose. There 

 are many variables and factors involved. These have been discussed 

 by Glasser et al. (18) and by Paterson.f Two important factors are 

 (1) the time between exposures, and (2) the amount of each exposure. 

 Another point noted by Quimby (18) is that recovery from the first expo- 

 sure may be considerabl}^ greater than that from succeeding exposures. 

 Many attempts have been made to generalize regarding recovery rates, 

 but it is apparent that the most reliable information comes from experi- 

 mental data for each tj^pe of biologic material and condition of exposure. 



It has been reported recently that little difference in mean accumu- 

 lated doses occurs among groups of rats exposed to daily doses varying 

 from 50 to 200 r per day. J At 25 r per day the accumulated dose rises 

 to twice that at 50 r per day. 



Thomson et aL§ find a greater accumulated dose in rats at 60 r per 

 day than at 120 r per day of gamma radiation. Earlier data indicate 

 an equivalence of 120 r per day of gamma irradiation (continuous) 

 and 50 r per day of 200-kvp x-radiation delivered in a few minutes. It 



♦Thomson, John F., W. W. Tourtellotte, M. S. Carttar, and John Neff, The 

 toxicity of gamma radiation to mice exposed at varying dose rates, UCTL, Quart. 

 Progress Rept. 4, Jan. 15, 1950, pp. 9-12. 



t Chapter by Edith Paterson in The Treatment of Malignant Disease by Radium 

 and X-rays, Ralston Paterson, Edward Arnold Co., London, 1948. 



X Hagen, C. W., and E. L. Simmons, CH-3815, Jmie 17, 1947. 



§ Thomson, J. F., W. W. Tourtellotte, and M. S. Carttar, Continuous exposure 

 of animals to gamma radiation, UCTL, Quart. Progress Rept. 4, Jan. 15, 1950, pp. 1-8. 



