12 NEUTRON EFFECTS ON ANIMALS 



tempting mitosis or produce non-viable daughter cells. In cancer it is 

 obvious that, if sterilization of all potential dividing tumor cells could be 

 achieved, their total destruction by radiation would be unnecessary, since 

 the altered cells would gradually disappear due to their short life. Like 

 weeds in a field of wheat, if prevented from multiplying without injury to 

 the grain, the weeds will die out and it is unnecessary to kill both weeds and 

 grain together. 



Measurements of the biological action of X-rays and of 7-rays show that 

 living cells vary in their response to irradiation. Differentiated cells are, 

 in general, more radioresistant while dividing cells, immature cells, are 

 more generally radiosensitive, and these are more sensitive at certain stages 

 of their di^'ision cycle than at other stages. There is also variation in cells 

 of a homogeneous population and this has been put forward as an evidence 

 of individual variation and, on the contrary, as an expression of the relation- 

 ship between the close of radiation and the percentage of the cells which 

 have received by chance a number of quantum hits to produce death, the 

 "quantum hit" or target theory. 



According to the first theory, when a group of cells is irradiated the radi- 

 ant energy is equal in amount and is similarly distributed in each individual, 

 whereas the capacity to resist the absorbed energy is different in different 

 individuals. According to the second theory, the energy which is absorbed 

 by cells or by specially vulnerable parts of cells is different in different 

 individuals, whereas all the individuals have an equal capacity to resist a 

 given injury. 



The quantum hit theory assumes that the resistance of all individual 

 cells to a given amount of absorbed energy is the same. The energy is ab- 

 sorbed in quanta and presumably the quant al absorption will have a ran- 

 dom distribution amongst the different cells. Some ha^e calculated that 

 the number of quanta absorbed in the cell must be great if a lethal action 

 is to occur and the quantum hit theory has been modifted accordingly. 

 The present form of the quantum hit theory is that a lethal number of 

 quanta are absorbed in a particular "sensitive spot" in the cell and this 

 causes its effect. It is as if a duck Avere shot with a large number of small 

 shot and only one or a few struck the brain or another mortal part to pro- 

 duce death, while the other pellets were absorbed, but not to produce lethal 

 effects. 



Individual variation is a characteristic biological phenomenon and, no 

 matter what care is taken to achieve homogeneity, the occurrence of indi- 

 vidual variation can be proved in respect to every property of individual 

 <;ells that is capable of measurement. It is reasonable to assume that in all 

 living cells, the resistance to injury varies. The difficulty in this assump- 

 tion of variation is the absence of any plausible explanation as to how this 

 variation, susceptibility or resistance to radiation is brought about. 



