II. Age Differences in Susceptibility to Carcinogenesis in Man

Variation in man's susceptibility to the induction of cancer at different ages is of considerable interest both theoretically and practically. In the past, very little information on the subject has been available and it has not been possible to use human data to test any particular hypothesis about the mechanism of carcinogenesis. Recently, however, the situation has begun to change and it may, perhaps, be of interest to review the data to see whether any useful conclusions can be drawn from them. The theoretical interest of the question lies principally in the fact that in most of the common types of human cancer the incidence increases very rapidly with age. This increase begins at about the age of 20 years and continues at least until the age of 80 years and probably beyond. It occurs characteristically in cancer of the skin, in most of the epithelial cancers of the respiratory and digestive tracts and in such different types as chronic lymphatic leukaemia and chronic myelomatosis. In these types of cancer a rough working rule is that the incidence is approximately proportional to the 5th or 6th power of the age—for example, when the age is doubled the incidence increases about 25 or 26 times. In some other types of cancer, however, the relationship with age is quite different.