HISTOPATHOLOGY OF CNS RADIATION 169 



High Energy versus Low Energy Radiation 



In pre\ious experiments, the tolerance of the spinal cord of Macaca 

 mulatta for gamma radiation was approximately 135 r per hour and 

 about 125 kv r for roentgen radiation ( McLaurin et al., 1955). Radiation 

 from activated Ta has a much higher energy than the roentgen radiation 

 used in our studies. These results suggest that, under the experimental condi- 

 tions used, low energy radiation is slightly more effective in producing para- 

 plegia than high energy radiation. This agrees with the findings of Arnold 

 et al. (1954a). However, other investigators (Hicks it al., 1956) ha\e found 

 that central nervous system tissues are more sensiti\e to high energy radiation. 



It seems difficult to reconcile these dixergent results. Among the studies, 

 there are data from difTerent animals, and sources of radiation also \ary 

 somewhat. E\en so. when both types of radiation ha\e been carried out in the 

 same laboratory under conditions as nearly controlled as possible, contradic- 

 tions in results remain. The situation is no clearer in regard to the sensiti\ity 

 of tissues outside the central nervous system. The cjuestion remains a signifi- 

 cant one ior finthei' inxestigation. 



Effect of Intensity of Radiation 



The intensity of radiation has emertied as an important factor in the tissue 

 response in the nervous system (Hicks et al., 1956; McLaurin et al., 1955). 

 Intensity as a factor in other organs has produced more equixocal results 

 (Brunschwig and Perry, 1936; Pack and Quimby, 1932). 



The effect of the rate at which a given dose of radiation is administered is 

 strikingly demonstrated by 2 monkeys in personal material. Each received the 

 equivalent of 55,000 r of gamma radiation to the spinal cord, at the rate of 

 4.000 r per day in one and at 1,870 r per day in the other. The first animal 

 developed a flaccid paralysis in the 2nd week after radiation, which pro- 

 gressed steadily in severity until death at 2 months. The second monkey 

 showed no neurologic deficit until its death irom an independent cause 4 

 months after completion of radiation. 



In the series in general, it was found that 7.500 r as a single dose were 

 required to produce paraplegia, but two doses of 5.000 r were necessary 

 (McLaurin (/ al.. 1955). These results are somewhat diflferent from those of 

 Davidoflf <7 al. 1938). who found 5.000 r sufficient to cause paraplegia. 

 However, they used onlv 1 animal at that dosage. 



Differences in Tissue Reaction with Age 



There is e\idence that changes in young animals are different Irom those in 

 adults of the same species. In young animals, smaller doses of radiation are 

 required to produce behavioral and histologic changes in the brain than in 



