326 RADIATION BIOLOGY 



Cases in Which Dose-Effect Curves Change Shape. It is a remarkable 

 fact that in most actions LET has no effect on the shape of the dose-effect 

 curve. However, in a few cases changes in shape clearly occur; most 

 striking are changes from a "one-hit" dose-effect relation at high LET to 

 a "multiple-hit" relation at low LET, as observed for certain types of 

 chromosome aberrations (e.g., item 84) and for lethal action on Aspergillus 

 spores (items 27, 28). The chromosome aberrations which behave this 

 way are of types which involve original breaks of two chromosomes (or 

 chromatids); accordingly it is concluded that a particle of high LET 

 breaks both chromosomes involved in an individual aberration whereas 

 one of low LET can break only one. It is not certain that an analogous 

 explanation can be given for the change in shape of Aspergillus survival 



curves. 



Influence of Linear Energy Transfer and the Nature of the Irradiated 

 Object. In some investigations small differences in the nature or condi- 

 tion of the biological object have been found to change the influence of 

 LET to a measurable and sometimes a remarkable degree. With the 

 Drosophila egg (item 38) the extent of influence varies by as much as a 

 factor of 1.5 depending upon the stage of development at the time of 

 irradiation. With wheat seedlings, measurable differences in the ratios 

 can be caused by difference in the variety of plant (items 75, 76) or by 

 the nature of the tissue studied in the same individual plants (item 76). 

 The extent of influence on different types of chromosome aberrations in 

 mouse tumors varies as much as a factor of 1.5 (item 67), while in plant 

 cells it varies by a factor of 2.7 (items 85, 86). For effects on mouse skin 

 the influence of LET depends slightly upon the criterion of injury (item 

 61), even when LET varies less than a factor of 10. In yeast also 

 (item 19) the criterion of injury is influential, while in the broad bean 

 root it completely changes the shape of the effectiveness curves (Fig. 6-2). 

 In this same type of root, oxygen tension markedly modifies the effec- 

 tiveness ratio (item 77). Different strains of the same virus (item 10) 

 have effectiveness ratios differing by as much as a factor of 2.7. 



Additivity of Radiations of Different Linear Energy Transfer. The 

 striking dependence of some radiobiological effects on LET raises the 

 question as to whether the mechanism of action is changed in some 

 qualitative feature or is simply changed quantitatively with no quali- 

 tative alteration. In some cases pertinent evidence can be adduced from 

 studies of radiobiological additivity. Such studies can only be made 

 when, for a given biological action, the dose-effect curves obtained with 

 all the radiations being tested are of the same shape and are not expo- 

 nential. Then, if doses are expressed not in physical units but in terms 



the other radiations, since a different strain (A) of mice was used and irradiation was 

 not wholly uniform throughout the body. However, the LD50 of X rays for strain-A 

 mice was almost identical to that for the CF-1. 



