Biological Effects of Penetrating Radiations 87 



taken place between the contending parties. ^-^ The idea of a 

 compromise has come late, but the results of at least one in- 

 vestigation ^•'- were shown by the author to be equally well 

 explained either by the quantum hit theory or by that of variation 

 in individual sensitivity. 



That the target theory holds for particular cases now seems 

 indisputable. It is true in certain instances where the criterion 

 of effect is a lethal action, or a type of injury is produced from 

 which there is no recovery.*^^' ^^ But it cannot be made to fit 

 all types of biological response to radiation, since by definition 

 it makes no allowance for adaptability in living organisms to 

 changes of environment, including those brought about by radia- 

 tion. The cell is not inert until it is dead, and so long as it is 

 alive it is capable of a change of behavior, and with that change, 

 an alteration in its susceptibility to radiation, which cannot be 

 predicted. The types of response must be learned from observa- 

 tion under different biological conditions. For example, the same 

 cell differs in its susceptibility to radiation, among other things, 

 according to its state of dryness, its metabolic activity, its stage 

 of growth, and its age.'*^^' ^^' ^'^' ^^^' ^^^ There is a danger in at- 

 tempting too much simplification by physical explanations when 

 dealing with such complex biological material. 



Physical Dose and Biological Response 



The need for a quantitative measure for radiations was ap- 

 parent as soon as their biological effects had been recognized, 

 and one equally suited to experimental and clinical use was 

 desirable. The question of a biological or a physical basis for 

 radiation dosimetry has been debated for many years. As early 

 as 1918 it was suggested by Russ ^^^ that the amount of radiation 

 necessary to kill mouse cancer cells might be used as a standard 

 for which he suggested the name "rad." Since then, many similar 

 methods of dosage have been devised and will be considered 

 under Biological Indicators. 



The most practical and useful method of dosimetry, however, 

 is that based upon the ionization produced in air by radiation, 



