Chapter IV 



SPECIAL PROBLEMS 



A, Localization of Radiation 



There are good reasons to believe that, when radiation is uniformly delivered 

 to tissues, the biological effects may differ from those of the radiation from focal 

 aggregations of radioactive material (point sources)13. in the latter case, dose rates 

 close to the point source are different from those near the end of the range of the 

 particles. An extremely high dose rate is found near the origin. These spatial dif- 

 ferences in dose may have considerable importance if the relationship between bio- 

 logical injury and energy absorbed is not linear. 



When the radioelement is diffusely deposited, the probability of injury is the 

 same for all cells in the tissue as a whole; in discrete deposition, the probability of 

 injury to the cells close to the focal aggregate has increased, but that of injury to the 

 cells far away from the focal aggregate has been reduced. 



Of course, spatial distribution will be of significance when particular tissue ele- 

 ments are selectively irradiated. It is also of significance insofar as the relation be- 

 tween dose and the degree or probability of any type of injury is not linear. Our present 

 information is not adequate to define differences in hazard between focal and diffuse 

 radiation, a question of special importance in the estimation of hazards from internal 

 emitters. 



B. Tr ansmu tation 



When radioelements are introduced into the internal structure of biological sys- 

 tems, certain of these may be incorporated into vital molecules. While the ensuing 

 biological effects result for the most part from ionization, some effects may arise 

 from disturbance of the molecule by the transmutation of the incorporated element. 

 The new daughter element has different and. in most instances, incompatible bonding 

 characteristics, and also receives recoil and excitational energy in the process of 

 transmutation. 



In general, the ionization and excitation resulting from the ionizing particle are 

 so large compared with the recoil energy that in most considerations of radiation injury 

 the former process outweighs the importance of the transmutation process. However, 

 certain radioelements which might be deposited preferentially in vital molecules could, 

 upon transmutation, produce unique biological effects not readily accomplished by 

 ionization or excitation from a charged particlel*. 15. There is no conclusive evidence 

 that transmutation is of critical importance when radioelements are incorporated into 

 biological molecules. However, some experimental data, together with theoretical 

 considerations, indicate that it should receive increased attention as a possible factor 

 in the toxicity of internally deposited radioelements, such as C ^4. The atomic number 

 of the radioelement, the type of decay, the valence change, and the energy are obviously 

 important in evaluating the significance of the transmutation process. 



