110 RADIOBIOLOGY 



Fig. 51. Each shaded block represents a virus particle. The other areas are 

 volumes equal to the virus particle volume but within which there are no virus 

 particles. 



proportional to the thickness of the organism. In this way we can obtain 

 estimates of the size and shape of the organism. 



If we knew enough about the physics of the incident particles, we 

 could also deduce the actual volume of the organism. We now look at 

 what is involved in this deduction. Consider a solution containing some 

 viruses and imagine that the solution is broken up into subvolumes each 

 equal to the volume of the virus particle. As indicated in Fig. 51, most 

 of these hypothetical volumes will contain no viruses, for the virus con- 

 centration will never be more than, say, 10 12 per ml, which is approx- 

 imately 10 -8 molar. Suppose next that we shine enough sparsely ionizing 

 (low LET) radiation so that we have created an average of one ioniza- 

 tion in each of these small compartments. This situation is entirely 

 similar to the one we have previously dealt with in our discussion of 

 biometry, when we were expounding the Poisson distribution. When 

 there is an average of one ionization per compartment, there will be 

 e~ l = 0.37 of the compartments which have no ionizations; the others all 

 have one or more ionizations per compartment. 



If the viruses are randomly placed in the solution, as of course they 

 actually are, then 37% of the viruses, too, will escape ionizations. Thus 

 the dose which leaves 37% of the viruses in a viable condition will be 

 that dose which gives an average of one ionization per compartment the 

 size of the virus. The way in which the 37% dose is actually found is 

 by giving a series of doses, measuring the surviving fraction of viruses 

 at each dose, and then interpolating to the 37% survival values, as in 

 Fig. 52. 



If we know the physics of the radiation, we can actually calculate the 

 radiosensitive volume of the virus. Physicists have measured the ionizing 



