Cancer 99 



that different types of cancer have distinct origins which do not have 

 a great deal in common. On the other hand, it may be that there is a 

 single pattern, the nature of which is still unknown, which underlies 

 the different manifestations; and that the different theories outlined 

 above, which emphasize one or other of the different features, all 

 express a certain aspect of whole truth. 



Cancer is not like an infectious disease in which, once the agent 

 has been identified, measures of control or cure can often be devised, 

 and the natural defence mechanisms of the body can also be stimu- 

 lated. It is essentially a disease of organization and as such it is 

 probably peculiar to the higher forms of life. To understand it we 

 obviously need to know much more about the way in which the 

 normal healthy organism works. We have seen that cancers can be 

 produced by extraneous agents acting on the body cells. The fact 

 that the incidence of different types of cancer varies very considerably 

 from one country to another shows that it is to a considerable extent 

 a disease produced by environmental conditions, and as such it ought 

 to be possible to reduce the incidence very considerably when the 

 causative agents have been recognized. It is well known that human 

 beings are subjected to ionizing radiation from cosmic rays, which 

 may be expected to produce a certain incidence of cancer. In thirty 

 years, at sea level, the dose from this source is about one roentgen. 

 In some localities there is also a quite appreciable amount of radio- 

 activity in the ground and in drinking water, which would have to 

 be taken into account. As we have seen radioactive substances like 

 radium and radiothorium (and also strontium from atomic bombs) 

 are concentrated in the bone, where they may remain for long periods 

 and eventually cause bone cancers. 



We might ask whether cancer would occur if these environmental 

 influences were eliminated. Do cancers arise spontaneously in an 

 apparently healthy organism? Since a fair number of chemical sub- 

 stances have been found to be carcinogenic, it may be that the 

 organism itself may produce carcinogenic substances, either in its 

 normal operations or through some errors of metabolism. The weak 

 carcinogenicity of one normal constituent of the body, cholesterol, 

 which is present in fats, has been demonstrated by Hieger. While 

 the function of this substance is unknown, it is rather curious that 

 it is related chemically to the steroid sex hormones and it might be 

 that it is a by-product of the formation of these substances. 



It can hardly be asserted that the body has developed any natural 

 defence mechanisms against cancer, but the fact that the incidence 

 of cancer increases rapidly with age, suggests either that as the body 

 ages, the controlling mechanisms become weaker, so that it is easier 



