248 



BIOLOGICAL EFFECTS OF IONIZING RADIATIONS 



The rule is generally obeyed, but there are exceptions. For instance, 

 leucocytes (white blood cells) are quite mature, don't divide in vitro, divide 

 only slowly in the body, and they have a low basal metabolic rate; but 

 in spite of these facts, they are among the most radiation-sensitive cells 

 known. 



The relation between the number of surviving cells and the dose, Z), ab- 

 sorbed, has had far better quantitative demonstration (Figure 9-7), es- 

 pecially for cells. If N is the number at any time, and N is the number be- 

 fore irradiation started, then 



N 



N e 



-aD 



or log JV/jV = -0.434 a D 



This is simply the integrated form of the natural law (see Chapter 1) 

 which says that the rate at which cells die from irradiation is proportional 

 to the number of living or nondamaged cells which are being irradiated. 

 This expression describes the case in which o is constant during the whole 

 irradiation. 



Dose (rods) 



Dose (rods) 



Figure 9-7. Radiation-Sensitivity, a-. The Slope of the Straight Line in the Logarithmic Plot 

 (b) for Haploid Cells. Low slope means low a. Broken curve is for multiploid cells: sensi- 

 tivity increases as irradiation proceeds. 



The radiation sensitivity constant, a, is small for radio-resistant cells (e.g., 

 nerve cells in adults), and large for radiosensitive cells (e.g., lymphocytes). 

 It increases with increasing oxygen concentration ("the oxygen effect"), or 

 increasing nitric oxide concentration. This is true also for whole animals. If 

 the dose rate is raised, the value of a increases, for the same reason it in- 

 creases as the relative biological effectiveness of the impinging radiation is 

 increased. It decreases with increasing concentration of certain protector 

 chemicals, P, as we would infer from the discussion on protection of mole- 

 cules earlier in this Chapter. Therefore we can incorporate all these effects 



