NEOPLASTIC DISEASE 



Immunological considerations are paramount in the trans- 

 fer of tumours to new hosts and may have importance for 

 primary tumours, but they do not seem to merit the central 

 position in cancer theory that Green upholds. 



4. Mutagenic and carcinogenic agents 



If the somatic mutation theory of cancer is correct, the words 

 mutagen and carcinogen should be synonymous. Experi- 

 mentally this is not quite the case, but there is a sufficient 

 number of agents with both types of action to allow us to 

 retain the general hypothesis, with the reasonable qualifica- 

 tion that access to the nuclear mechanisms concerned will 

 vary according to many circumstances and that the mani- 

 festation of malignancy may require conditions beyond simple 

 mutation. If the point of view that we have adopted from 

 consideration of the age incidence of different forms of 

 cancer is correct, then a single exposure to a carcinogenic 

 (mutagenic) agent in a population of young individuals may 

 be expected to produce a large crop of what we called muta- 

 tion I and an occasional mutation 2. The overall result could 

 well be no more than a lower average age of cancer than in 

 an untreated population. 



In human experience, the only mutagen-carcinogen which 

 is available for discussion is ionizing radiation which, for all 

 practical purposes, means X-rays as applied in the course of 

 diagnosis and therapy. In the early days of X-rays, it was 

 a commonplace to find that radiologists suffered from 

 multiple skin carcinomata of the hands. A hundred radio- 

 logists were said to have died of malignant disease induced 

 by radiation by the year 1922. For obvious reasons, there is 

 little possibility of obtaining significant information in regard 

 to dose, latent period, etc., from such data. In recent years 

 much more attention has been paid to the rising incidence of 

 leukaemia and the impressive evidence that some portion 

 of this increase is attributable to ionizing radiation from the 

 medical uses of X-rays. This may be summarized as follows : 



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