PHYSIOLOGICAL EFFECTS 247 



eral terms there are two methods by which the rate can be increased: 

 through increase in local temperature (thermal energy of vibrations, etc.) in 

 the vicinity of the ionized track, and through excited electronic states of re- 

 actant molecules (photochemical processes). The mechanisms have been 

 discussed in Chapters 4 to 8. The synthesis of new isomers and of entirely 

 new molecules was considered also in Chapter 6, as well as the nature of 

 toxins, catalysts, and useful and destructive mutants. 



PHYSIOLOGICAL EFFECTS 



Outlined in this section are the effects of ionizing radiation on cells, 

 organs and tissues. 



Sensitivity of Cells 



The sensitivity, a (sigma), is the rate at which cells die because of irre- 

 versible damage suffered during irradiation. Since the unit of absorbed 

 dose, D, is the rad, the fraction of cells lost per rad is the sensitivity. Thus 



dNIdD 

 a = 



N 



cells killed per unit dose per unit number of cells irradiated. If the dose rate, 

 dD/dt rads/sec, is a constant, p, then the sensitivity can be expressed 



dN/dt 



a = 



p-N 



cells killed per sec per unit number of cells irradiated. Based on what is now 

 known about factors affecting the radiosensitivity of cells, the early (1905) 

 "law" of Bergonie and Tribondeau can be extended and rewritten: 



l [dN/dt] g e W,a,dD/dt \ 



where / denotes a functional relationship between a and the quantities in 

 parentheses; [dN/dt] g , the rate at which the cells reproduce themselves (i.e., 

 the growth rate, or number produced per unit time); 'W , the metabolic rate 

 — energy used up per unit time; a, a number less than 1 which varies with 

 the state of cell division — unity at the prophase of mitosis, much less at any 

 other time; m, the degree of maturity — unity for old, well-developed, spe- 

 cialized cells, less for those newly formed; and dD/dt, the dose rate. In sum- 

 mary, the sensitivity increases with increasing rate of cell division, metabolic 

 rate, and dose rate; increases sharply at prophase; and decreases as the cell 

 becomes more mature. The exact functional relationships are not known. 



