NEOPLASTIC DISEASE 



modified by the specific histological features of the different 

 situations, takes place. Leblond and Walker make the 

 interesting statement that if human intestinal epithelium 

 turns over at the same rate as that of the rat, each of us sheds 

 more than half a pound of used cells into the intestinal 

 lumen each day. 



The function of this rapid turnover in cells exposed to the 

 environment is fairly clearly to forestall the effect of environ- 

 mental injury by discarding cells before inevitable damage 

 renders them ineffective. It is a necessary feature of any such 

 mechanism that it can also function rapidly and effectively 

 to repair any traumatic damage that does occur. Irritated 

 regions subject to multiple minor traumata will always show 

 an increased production of expendable cells. 



Malignant disease can occur in any part of the body but 

 in some regions it is much more frequent than in others. If 

 any generalization is permissible, it is that cancer arises most 

 frequently from cells with a high normal turnover and at 

 those sites where the turnover is increased by the existence 

 of chronic trauma of one sort or another. 



2. The age incidence of cancer 



Of all the objective characteristics of cancer, the relation to 

 age is the most clearly evident. It is part of universal human 

 knowledge that cancer is predominantly a disease of the old, 

 that it is variable in its site and its rate of progression and 

 that it is almost always lethal. Any attempt to understand the 

 malignant process will always have to be primarily concerned 

 with those three aspects. Of the three, the characteristic age 

 incidence probably provides the best starting point for any 

 general discussion. 



A close study of the numerical relationship between age 

 and the incidence of various forms of cancer shows an impor- 

 tant regularity which is best demonstrated graphically. If 

 one plots the incidence of death from cancer of the stomach 

 at each quinquennial age interval as the number per 100,000 



188 



