84 Applied Biophysics 



was noted that proliferating tissues showed a more marked re- 

 action to radiation than those without dividing cells and that 

 a latent period, which varied for different types of response, 

 elapsed between exposure and the appearance of radiation effects. 



From about 1920, biological response was, in the laboratory 

 at least, much more frequently measured quantitatively, though 

 all tissues were not equally convenient for experiments of this 

 kind. Some observers chose what was alreadv familiar to them, 

 and others what was most conveniently available. Meanwhile, 

 work on the physical measurement of dose made progress, cul- 

 minating in the international unit of measurement for X-rays, 

 now applicable to gamma radiation as w^ell. 



Experimental radiobiology has thus grown to a science in 

 which physical dose and biological response can be measured with 

 reasonable accuracy. Its development has been greatly influenced 

 by its relation to medicine and, while attempts are sometimes 

 made to distinguish those investigations which have obvious 

 application to medical practice ("applied radiology") from those 

 which have not ("pure research"), opinion would often be 

 divided as to which category any particular investigation should 

 be assigned. At least one major effort has been made to review 

 the literature not immediately concerned with practical radio- 

 therapy.^*^ The vast mass of literature which has accumu- 

 lated on the other side has been the subject of many re- 

 views.-^' ^^' -^' ^^' '^^' ^^-' ^^^- ^-^ The purpose of this paper will, 

 however, best be served by ignoring this somewhat arbitrary 

 division and giving a brief summary of each of the main branches 

 into which the subject has, through circumstance or convenience, 

 become divided. 



Background Theory 



The most conspicuous advances in experimental radiobiology 

 have been made when physicist and biologist have worked in 

 harmonious collaboration, an achievement which in practice is 

 too seldom realized. This is mainly due, perhaps, to a difference 

 in training and outlook which needs to be remedied by reeduca- 

 tion on both sides.*^ 



