DISCUSSION 411 



DISCUSSION 

 Kaplan : 



I should like to add a footnote to Evans' remarks on fractionation. I was 

 particularly interested in the reference to Sax's work on the effect of rest periods 

 and recovery on chromosome aberrations. We have had a rather large group of 

 mice under observation for two different biological end points, acute radiation 

 mortality and the incidence of induced leukemias, or lymphomas, in relation to 

 three dose variables: increments of total, number of treatments, and interval 

 between treatments. These experiments are not yet complete, but it appears 

 that the interposition of a rest period of 4 days or more results in a striking 

 reduction in mortality over a wide range of dosage, whereas the incidence of 

 leukemia is at least as high as, or perhaps higher than, that in mice subjected to 

 the same dose without a rest period. Thus the recovery factor may be of great 

 importance for some biological processes and not for others. 



Second, Evans has shown shdes giving LD50 data for mice, dogs, etc. I think 

 that we oversimphfy matters in referring to mice, for example, because such 

 factors as age, sex, and strain are all known to influence the response of mice to 

 irradiation, and a greater degree of specificity is required in dealing with such 

 data. 



Smith: 



The wide variability encountered in radiation dosage-mortality studies seems 

 to be due in large part to metabolic differences in stock animals. A total 

 of 739 mice in nine experiments, each divided into two treatment groups ap- 

 proximately balanced as to litter and irradiation animals, were irradiated at one 

 of four dosage levels between 325 and 470 r. To the diet of one treatment group 

 0.3 per cent of desiccated thyroid was added immediately after irradiation. The 

 X^ of the responses about the dosage-mortality hne was 14 for the thyroid-treated 

 animals and 44 for the controls, each showing evidence of significant variation. 

 The probability that the difference between the two x^ values is due to chance 

 alone is about 7.5 per cent. Thyroid administration seemed to reduce the hetero- 

 geneity of response, possibly by reducing the metabolic differences between the 

 experimental groups. 



Bond: 



I should like to add a footnote to this paper by relating certain observations 

 on the effects of partial body irradiation that Marguerite Swift and I have 

 made in collaboration with Tobias. 



We have worked with rats and have used clinical signs and the dosage neces- 

 sary to produce mortality as end points of biological damage. In addition, we 

 have, with each type of partial body irradiation, determined the volume of 

 tissue irradiated, as well as the particular organs exposed; hence dosages in gram 

 roentgens could be approximated. 



