154 GENERAL DISCUSSION 



jaw of the animal, damage to these particularly sensitive areas causing death by 

 means that are not clearly defined. It would be particularly interesting if repeated 

 doses of 1,000 r can avoid this syndrome and allow as much as 5,000 r to be given, 

 but I would suggest that Dr. Brownson has irradiated only the forebrain so that he 

 sees relatively little of the change in the cerebellum because of a slight rostral 

 advancement of the posterior margin of his x-irradiation. My own experience is 

 only with the adult animal. I am sure that in the adult guinea pig one has to 

 include the cerebellum and has to go well below the cerebellum to produce 

 neurologic signs and death of the guinea pig in a short time. This leads me to 

 Dr. Schjeide's paper, in which, unfortunately, the wet weights are not available. 

 I would predict that, when the wet weights become available, the most striking 

 chemical change will be in the degree of hydration with marked edema of the 

 cerebellum. Dr. Sauer, have you with tritiated thymidine been able to apply this 

 at certain times after the irradiation, with the idea of establishing whether these 

 DNA bodies are dead or still metabolically active? 



Wolfgang Zeman (Indiana University Medical School): I think we should 

 strive for a more accurate definition of cumulative or fractionated doses. In 1949, 

 I tried to arrive at an understanding as to the radiobiologic effectiveness of frac- 

 tionated doses as compared to a single dose. My data at that time were rather 

 scanty, but in the meantime Lindgren (Stockholm) arrived at a simple formula 

 for converting fractionated doses into single exposures. He determined the x-ray 

 dose which was necessary to produce radiation-induced brain damage in 50% 

 of rabbits. He used various single and cumulative dosages and found that in plot- 

 ting the morbidity dose (50%) in r logarithmically against the total amount of 

 days over which this dose was given, also logarithmically, a straight line results 

 which has a slope of about 0.26 for the adult rabbit brain. In other words, a dose 

 of say 2,000 r given in one day, compares to a dose of 2,000 r times lO^-^'^ given 

 over 10 days. For the human brain the slope has been shown to be about 0.34, and 

 it stands to reason that each different species does have a specific slope. I would 

 predict that within one species, the slope might be dependent on the develop- 

 mental stage. I wonder whether Dr. Brownson would be good enough to convert 

 his data into terms which would make for an easy comparison to the radiobiologic 

 effectiveness of cumulative doses with single dose exposures. 



L. J. Peacock (University of Georgia): I would like to ask Dr. Brownson 

 whether the decline in response rate in his rats was due to an error in their timing 

 behavior or to a decrease in motivation. That is, was there a decline in the over- 

 all food intake of these animals, or was it a matter of their not being able to 

 properly time the intervals and schedule? 



Robert H. Brownson: Dr. Alvord, the radiation instrumentation was conducted 

 by our physicists in the biophysics department. Each animal was prepared by 

 placing it in a cage in which the head was elevated out of the cage and held by 

 a clamp with the remainder of the animal shielded. A Victoreen Chamber R-Meter 

 was used to monitor the dosage. Each animal received 1,000 r delivered at the 

 rate of 237 r per minute. The animal was given 1,000 r per week, so that it 

 accumulated as scheduled per week the desired total roentgen dosage. Our ex- 

 perience with guinea pigs has indicated that they are more liable to radiation 



