112 RADIOBIOLOGY 



roentgens) and a knowledge of the ionizations produced by the radia- 

 tions utilized, we can deduce the volume of the virus. 



There are important corrections to be made before the number of 

 ionizations can be used as in the illustrative calculation above. The 

 most important is the correction for the clustering of ionizations. It 

 turns out that the (secondary) ejected electrons produce enough ioniza- 

 tions near their own origin that the ionizations are clustered in threes, 

 on the average. This effect depends, however, on the size of the or- 

 ganism. For large organisms, we would simply divide the total number 

 of ionizations by three to find the effective number. For very tiny 

 organisms, it is possible that the clustering is on a scale larger than the 

 virus diameter, so that we would actually count all the ionizations. 

 Since this and other corrections require specialized knowledge of the 

 physics of radiations, we will not deal with them further. 



It is extremely important, however, to point out that we have tacitly 

 assumed that a single ionization anywhere in the virus (or whatever 

 biological organism is being used) will produce the effect being measured. 

 This is simply not generally true, any more than it would be true that a 

 single bullet passing through any part of your body would kill you. 

 There are parts of any organism which may be damaged without affect- 

 ing some of the properties of the organism. For viruses, for example, it 

 turns out that the volume deduced from irradiation experiments is defi- 

 nitely less than the volume of the virus; it turns out to be equal to the 

 volume of the nucleic acid of the virus. This is to be expected, since 

 radioactive tracer experiments have shown that the nucleic acids of 

 viruses can, by themselves, produce new viruses, complete with protein 

 coats. Accordingly, we should refine our thinking and our terminology 

 by talking about the radiosensitive volume — the volume within which 

 an ionization will produce the effect being studied. 



This definition includes a second refinement in our thinking. It is not 

 at all necessary to study only viability. There have been studies of the 

 radiosensitive volume associated with many other properties, such as the 

 ability of the virus to kill a cell, the ability to adsorb to a cell, the 

 ability of viruses to agglutinate red blood cells, the ability of cells to 

 make enzymatic adaptations, etc. 



We have discussed how sparsely ionizing radiations are used. If we 

 use densely ionizing radiations, we need a physical device to count the 

 number of incident particles per cm 2 . Then, as before, we give a series 

 of doses of radiation, interpolate to the 37% survival point, and find the 

 number of particles per cm 2 which give this survival. In a way entirely 

 similar to that used for volume calculations, we here calculate the area 

 of the target organism or, more accurately, the radiosensitive area. As 

 we mentioned above, use of the two kinds of radiation in parallel 



