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DISCUSSION 



CURTIS: I agree with most of the pomts which Dr. Casarett has raised. There is one thing, 

 however, to which I should Uke to enter an objection and that is the concept that he 

 proposed in the first part where he mdicated that radiation may be used as a tool for 

 measuring the residual injury which is present in a group of animals. I thmk at several 

 times we've shown, and it has also been shown by a number of other people, that the 

 LD50 reaUy changes remarkably little with age. There are certain strains of animals 

 apparently m which it does go down, but there are many strains in which it's almost flat 

 right to the end and then goes down again. I think that perhaps a fallacy in reasoning 

 might be brought out by a much over-simplified concept. You could thmk for example 

 that the acute 30-day mortaUty of mice might be primarily due to the bone-marrow effect, 

 whereas perhaps in the long-term effects which we are discussmg here it might be due to 

 failure of completely different organ systems — the kidney has been mentioned as one 

 possibility. So that when you test the residual injury m these animals, what you really 

 want to do is test the residual injury in the organ in question, which might be the 

 kidney, whereas when you give an LD50 dose what you are doing is testing the residual 

 injury m the haematopoietic system which may have almost completely recovered from 

 the disease. 



