610 SCIENCE PROGRESS 



for two hours. The probable effect of this would be a slowing 

 up in the rate of growth of the tumour, and it is quite clear that 

 only a very small percentage of the cells would be irradiated 

 while they are in the specially vulnerable dividing stage. If 

 we take it that the dividing cell is eight times as vulnerable 

 as the growing cell, then ^ milligrams will have as large an 

 effect upon the dividing cell as ioo milligrams acting for the 

 same time upon the growing cell. 



Further, ioo milligrams acting for two hours is equivalent 

 (as far as the product quantity and time is concerned) to ^ 

 acting for sixteen hours. The essential difference is that in 

 the latter case eight times as many cells in the dividing stage 

 are being irradiated compared with what occurred in the first 

 irradiation. 



It is clear that they are not receiving so much radiation, 

 but the inference is that in the first case the few dividing 

 cells receiving radiation had far more than was necessary to 

 ensure subsequent hindrance of growth. 



It is obvious that some limit is imposed upon the replace- 

 ment of a large quantity of radiation acting for a short time 

 by a small quantity acting for a long time. Experiment has 

 shown that as regards the human skin comparable effects are 

 obtained by reducing the quantity (gamma rays from ioo 

 milligrams of radium bromide) to one-tenth and increasing 

 the length of exposure by ten times, but if the quantity be 

 reduced to T ^ with appropriate extension of exposure the 

 reaction is not of a similar nature ; it appears that the 

 processes of repair can more easily cope with feeble radiation 

 acting for a long time than with intense radiation acting for a 

 short time. Too little is known of what may be called " flash- 

 light " radiotherapy to say at what stage the " equivalence of 

 effects " again breaks down when the intensity of radiation 

 is increased. 



Another limitation arises from the fact that the radiation 

 must not be reduced in intensity beyond what is warranted 

 by the factor of vulnerability (vide above — factor taken as 

 one-eighth), otherwise the dividing cells will receive an intensity 

 of radiation too small to prevent their subsequent division. 



Such an adjustment of the quantity of radiation and the 

 length of exposure can only be made with certainty when both 

 the ' vulnerability factor " and the intensity of radiation 



