Biological Effects of Penetrating Radiations 103 



The table shows that there is no single type of response 

 which can with any justification be called tJie biological effect 

 of radiation, but that at various dose levels a change in behavior 

 occurs in the irradiated cells. At the highest dose level the result 

 is "immediate" death, presumably caused by a breakdown of the 

 physicochemical structure of cell protoplasm ; at lower dose levels, 

 however, death of cells results from different kinds of initial 

 injury ; at the threshold dose for any observable change, com- 

 plete recovery of the cell from the effect of radiation occurs. 

 These dose levels are altered if the physical conditions of irradia- 

 tion are changed. Thus, there is a minimum amount of radiation 

 energy required to produce any given type of biological response 

 in organic tissue, which can only be determined by the method 

 of trial and error. 



Siiminary of Effects on Normal Tissue 



The biological effects of radiation upon normal tissue may be 

 summed up as follows : 



Radiations are always injurious to the cells which absorb 

 them ; the changes produced may be transitory ( reversible 

 effects) or permanent ( irreversible effects), with an intermediate 

 class of effect where the radiation changes disappear completely 

 but leave the tissue in a state of lowered resistance to further 

 radiation (conditioned reversible effect). There is a latent 

 period between irradiation and the recognition of the biological 

 effect it produces. ^^ 



There is a tenthousandfold difference between the extremes 

 of sensitivity among different types of living cells when measured 

 by the lethal effect.^-^ 



Radiation has a marked effect in interfering with cell prolifera- 

 tion, and the dose which produces the first recognizable changes 

 in cell proliferation is always small relative to the direct lethal 

 dose for the same tissue. 



During development, radiosensitivity decreases as the age of 

 the individual increases, but the decrease is not necessarily pro- 

 gressive throughout development. Sensitivity to radiation is 



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