GENERAL DISCUSSION 



Percival Bailey (University of Illinois): I was happy to hear that the effect 

 on the nerve cells can be primary and not secondary to the disturbance of the 

 circulation. That I have always believed from long years of examining brains 

 after irradiation for brain tumors and from results of work in my laboratory by 

 Dr. Arnold and his associates with the effect of x-rays of high energy produced 

 by the betatron. 



Webb Haymaker (Armed Forces Institute of Pathology): Dr. Bailey, I believe 

 you mentioned that radiation "can" injure the nerve cell. I suppose you used 

 "can" intentionally, rather than "may"? 



Percival B.mley: Naturally. Nerve cells can be injured also by disturbance of 

 the circulation, and they are injured at times by disturbance of the circulation in 

 brains that have been radiated. I believe there is also a primary effect on the 

 ner\'e cells which is unrelated to the circulation. 



Carmine D. Clemente (University of California Medical Center): The problem 

 of direct or indirect effects of radiation on the various elements in the nervous 

 system is something that has plagued this field for a long time. I don't think we 

 can quite let Dr. Bailey get away with what he has said without replying with 

 some remarks. When we think about the radiosensitivity of the various elements 

 in the brain we must talk about dosage. If one considers radiating an entire brain 

 with high doses such as 5,000 r, 10,000 r, 50,000 r, or 100,000 r, that is one ex- 

 periment, but if one considers the effects of brain radiation in the same tissue 

 areas of maybe doses of 1,000 or 2,000 r, that is a completely different experiment. 

 Furthermore, I believe that when a small part of the brain is radiated in com- 

 parison to the entire brain it becomes a third experiment, and the dose-brain 

 volume variable must be considered. When effects of high dosage radiation are 

 described in the realms of 10,000 rad and 15,000 rad, it certainly is conceivable 

 that neurons are directly affected. On the other hand, not only are the neurons 

 affected, but the neuroglia and the blood vessels are equally affected. With these 

 high doses, it seems to me that there is an analogy of taking a piece of brain and 

 putting it in a frying pan, and thereby getting a lot of pathology in the tissue, and 

 then saying "look at what's happened to the neuron." Let us think, however, of 

 discussing the differential radiosensitivity of endothelial cells, glia and neurons, 

 and discuss low dose effects. Under these circumstances an endothelial cell in the 

 brain is a much more radiosensitive element than the neuron. 



Arthur Arnold (Chicago, Illinois): Dr. Clemente brought out the important 

 point of total dose, namely dealing with 1,000 r or 10,000 r. .'\nother fact or 

 equally important is dose rate. One can take a dose of 3,000 r and create an effect 

 at 50 r per minute, and increase that rate to 200 r per minute and have an 

 entirely different effect, not so much a qualitative effect, but a quantitative one. 

 For example, I can take a monkey and deliver 10,000 r from a betatron through 

 the frontal lobes in a single small beam, and it will lobotomize the animal, 



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