1072 RADIATION BIOLOGY 



Allen et al. (1948) have reported that 450 r of total-body X irradiation 

 is invariably lethal to dogs. With head shielding (Allen, 1951), however, 

 mortality following this dose is reduced to 75 per cent, and other aspects 

 of the usual postirradiation syndrome, such as hemorrhage and evidences 

 of infection, are greatly reduced or absent. Further work on the dog 

 comparing the relative effectiveness of shielding parts such as the head, 

 spleen, intestine, limbs, and liver after exposure to various dosages of 

 total-body X irradiation will be of interest if for no other reason than to 

 obtain baselines on the potential effectiveness of these tissues for com- 

 parison with mice, rats, and rabbits and to accumulate some facts on the 

 potential production of the factor on a tissue-weight basis, type of tissue 

 shielded, etc. The species differences that are apparent cannot be ade- 

 quately evaluated since, for example, considerable differences may exist 

 between rabbit and mouse spleen in terms of the potential production of 

 the factor (or factors) involved in survival or early regeneration of 

 hematopoietic tissue. In this connection the work of Bond et al. (1949), 

 in which the abdomens of rats were shielded with lead during total-body 

 X irradiation, is of interest. The LD 50 for these abdominal-shielded rats 

 was 1950 r compared with ca. 700 r for nonshielded control rats. For 

 very practical reasons it would be of interest to determine more precisely 

 the relative importance of the various abdominal tissues in enhancing 

 survival on a weight basis and to assess more adequately the volume-dose 

 factor. This has been done to some extent with rats by Gershon-Cohen, 

 Hermel, and Griffith (1951), Jacobson et al. (1949), Jacobson, Simmons, 

 Bethard, et al. (1950), Jacobson, Simmons, Marks, et al. (1950), and 

 Jacobson, Simmons, Marks, Gaston, et al. (1951) with mice, but further 

 data for all species are needed. As was pointed out by Bond et al. (1949), 

 the radiosensitivity of the part of the body irradiated may be more impor- 

 tant than the gram-roentgens sustained by the balance of the body. To 

 this must be added the fact that the actual or potential production of the 

 factor (under consideration in this section) by the shielded or nonirradi- 

 ated tissue may be more important in determining survival of the animal 

 than the radiosensitivity of the tissue in the radiation field and, within 

 certain limits, more important than the gram-roentgens sustained by the 

 balance of the body. 



The amount of tissue shielded in the intestine- and liver-shielding 

 experiments in mice was greater by factors of 25 and 8, respectively, than 

 that shielded in the mouse spleen-shielding experiments, yet survival was 

 considerably less. These findings suggest that the potential production 

 of a factor (or factors) by the intestine and the liver is not as great as that 

 by splenic tissue but yet is sufficient to institute recovery early enough in 

 a sufficient number of cells of the body to have a definite effect on survival. 



It is conceivable that practically all tissues of the body are capable of 

 producing the factor (or factors) concerned in recovery from radiation 



