Biological Effects of Penetrating Radiations 107 



Radiation and Malignancy 



Much of the experimental work on the biological effects of 

 radiations has some relation to the radiotherapy of maHgnant 

 disease. The demonstration that radiation can cure a cancerous 

 tumor raises the question of how this effect is brought about. 

 There is a tendency for the results of experiments in any one of 

 the fields of experimental radiology which we have considered 

 to be applied too exclusively to the cancer problem. For ex- 

 ample, the effect of radiation up on a prohferating tissue is so 

 striking that it has been suggested that maHgnant cells die 

 mainly by degenerative mitosis. '^^' ^^ Although this has been 

 disputed/^' -^ the idea has been revived by recent genetical work 

 which has attributed the death of the cancer cell to the effects of 

 radiations on chromosomes. 



There can be no doubt that very many irradiated cells die 

 when mitosis is attempted after irradiation. That this action of 

 radiation is frequently due to direct hits on chromosomes seems 

 also beyond dispute. In the light of Dale's work, however, there 

 is now the further possibility that radiation may act also on 

 dissolved enzymes via the solvent molecules, and where dosage is 

 high enough to affect blood supply, the destructive effect on 

 malignant cells of damage to the circulation is obviously an- 

 other important factor. Objections can be raised against accept- 

 ing any one of these explanations as the principal means by which 

 radiotherapy achieves its success. Thus, as regards the mitotic 

 effect, the low percentage of dividing cells present at the time 

 of any one irradiation leaves the majority of cancer cells in a 

 tumor unaccounted for, and a high proportion of mitotic cells in 

 a tumor is not in itself an indication of marked radiosensitivity. 

 A direct lethal action upon all tumor cells seems to be excluded 

 (except where radiation is used as a cautery) in view of the 

 high dosage required to produce such an effect under experi- 

 mental conditions, while the suggestion that all therapeutic effects 

 are the result of an indirect effect of radiation on the blood 

 circulation is against clear experimental evidence ^^ and has 

 never received any substantial support. 



