108 Applied Biophysics 



The problem can be approached from another angle. Instead 

 of attributing the destruction of a tumor to a single radiation 

 effect, irradiated malignant tissue may be examined to see how 

 many types of action can be recognized, and an attempt can be 

 made to assess the relative importance of each in the eradication 

 of the growth. If serial biopsies are taken from tumors during 

 and after radiation treatment, it is possible to follow histologically 

 the changes in cellular activity in a quantitative manner for 

 each type of cell present.^*' ^^ Radiosensitivity measured by 

 rapidity of disappearance of the tumor soon after irradiation is 

 by no means synonymous with radiocurability, i.e., permanence 

 of radiation effect.*' Thus, while much emphasis is often placed 

 on the marked changes produced in anaplastic tumors by radia- 

 tion, several observers have pointed out that the differentiating 

 tumors, which seem clinically to respond to radiation more 

 slowly, give on the whole a more satisfactory ultimate re- 

 sponse.^' ^^' ^^^' -^^*^ These clinical results may be explained in 

 the following w^ay. It is obvious that, if sterilization of all poten- 

 tial dividing tumor cells could be achieved, their total destruction 

 by radiation would be unnecessary, since the altered cells would 

 gradually disappear in the normal course of events. In a differ- 

 entiating tumor, many of the daughter cells resulting from cell 

 division become sterile because they differentiate, although ab- 

 normally. In this connection, the fact that radiation can promote 

 differentiation as w^ell as injuring proliferating cells is of some 

 significance,'*^^' ^^' ^^"^ since, with suitable types of malignant 

 tumors, radiation may exert a curative action both by mitotic 

 inhibition and by sterilization. In the undifferentiated or ana- 

 plastic tumor, on the other hand, even a marked destruction 

 of cells following a heavy dosage may lead to a recrudescence 

 of the tumor from residual cells, incapable of sterilization by 

 differentiation, which have survived the radiation. 



It must be recognized, however, that a tumor, capable of 

 responding to radiation by an increase of differentiation, may 

 be adversely affected by excessive exposures which interfere 

 with, instead of promoting, this process. Over-irradiated normal 

 tissues show an increase in cell division and a decrease in cell 



