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CORNELIUS A. TOBIAS 



in well-localized regions of the cortex, thalamus, brain stem, spinal cord, or 

 cerebellum. Generally two relationships were observed: first, the higher the 

 single dose, the sooner the onset of degenerative changes. Secondly, the 

 larger the irradiated volume, the sooner the degenerative changes appear. 

 Combining these two observations, we found it possible to compute the 

 "integral dose,'' that is, dose times volume irradiated. As seen in Fig. 5, the 

 integral dose and the time of death due to necrosis correlate reasonably well. 

 Recently, Zeman ct al. (1959) produced radiation lesions in rats with very 

 small beams of alpha particles. It was found that many tens of thosuands of 

 rads are necessary to cause an eflfect as seen from Fig. 5. 



The effects of protracted dose schedules and of the size of the irradiated 

 volume make it appear that some other factors enter in brain radiosensitivity 

 besides direct interaction of radiation with neurons. 



One suspects the state of the capillary bed as being essential for the in- 

 tegrity of neurons, particularly knowing that proper oxygen supply or nutri- 

 tion is essential for the maintenance of the latter. Capillary walls are known 

 to regenerate more efTectively following protracted dose schedules than a 

 sino^le large dose. Certainly vasodilation following radiation and increased 

 permeability of the blood-brain barrier have been observed in the early post- 

 irradiation period (see communications by Van Dyke et al, Haymaker ct al., 

 this conference.) 



Fig. 5. Relation.ship of integral dose to time of death due to necrosis and hemor- 

 rhage in a number of experiments. 



