444 RADIATION INJURY AND LETHALITY 



can say only that the dosage range for acute mammalian death is one 

 in which the frequency of chromosome breaks and dicentrics becomes 

 noteworthy (18); that there is good reason to believe that such lesions 

 result in cell death; and that the cells susceptible to irradiation are, gen- 

 erally speaking, those which show rapid division for replacement and 

 which do not have a large nucleic acid reserve. Marshak's observation 



(19) that the lymphocyte is susceptible to chromosome damage over a 

 remarkably large part of its mitotic cycle may be pertinent here. 



Although it is not the primary purpose of this conference to discuss 

 practical matters, it is not out of order to mention therapy briefly, since 

 our present position in therapy points up the discussion. Those forms 

 of therapy which appear to act primarily on a chemical basis, anoxia 



(20) and cysteine (21), are prophylactic and must be present with ir- 

 radiation. Other prophylaxes are on the cytological level, like spleen 

 protection (1) or preirradiation stimulation of the blood-forming ele- 

 ments by induced anemias (22) or by estrogens (23, 24). Our position 

 in therapy after the fact is in the realm of supportive treatment: anti- 

 biotic treatment of bacterial invasion, replacement of lost blood, etc. 



Thus, most of the sequence in acute irradiation sickness can be ex- 

 plained on a cellular hypothesis with, of course, antecedents in biochem- 

 istry, chemistry, and physics. The striking low-level irradiation effects 

 on enzymes as described by Barron, however, make it clear that supple- 

 mentary information may lead in quite a different direction; at the pres- 

 ent time, our thinking should be pointed both ways. The entire realm 

 of basic information discussed here will be very pertinent to this ques- 

 tion, and it is likewise important to note that the quest for practical 

 knowledge is being pursued by a group of individuals alive to the basic 

 aspects of the sciences here represented. 



When we turn to the chronic, lifetime effects of irradiation, we find the 

 situation more obscure. The main elements here are cancer and short- 

 ening of the life span. Here we see an enhancement of natural processes 

 about which we know altogether too little. The remaining discussion 

 will deal with some efforts in the direction of developing means of han- 

 dling this problem. Although we may appear to be drawing conclusions 

 regarding lifetime human effects, this is not the case, and we are merely 

 trying to define the parameters of the problem as a guide to future in- 

 vestigation. 



Let us first mention cancer. We have a wealth of information on 

 local cancer development following local irradiation. It would appear 

 that most tissues are susceptible to spontaneous and to radiation-induced 

 cancer. Total-body irradiation appears to enhance the normal inci- 

 dence (25, 26). One is tempted to suggest that cancer is the consequence 



