Some inferences about the origin of tumours derived from their age distribution

The common sites for tumour development are those where repeated demands are made for normal growth, either for the repair of damage done or for hypertrophy to meet some functional demand. The battle to maintain structural stability in the face of our environment sets the main pattern of rising tumour age incidence in many long-lived communities. While there may be an inherent reason for a rising tumour rate with age in some tissues associated with physiological decline, it is likely that a real reduction in tumour incidence might be achieved in a community such as ours by a determined attempt to soften the impact of our environment, particularly on our body surfaces. Clean air and no smoking, for example, would greatly reduce the risk of lung cancer—by far the worst of the rising tumour risks of modern society—and a better understanding of the underlying forces in the production of tumours of the gastro-intestinal tract, particularly the stomach, should also be rewarding. Some tissues show peak tumour incidences at different ages because their growth-promoting influences are not evenly spread over most of the life of the individual. Others produce tumours in childhood, some arising from cells which become isolated from normal growth control during development; here tumour incidence tends to decline with age as the comparatively few cells at risk either form tumours or differentiate. Still other tissues may follow some quite different form of imbalance in their tumour production, perhaps induced by an immunological mistake or a virus infection for example. The common tumours, however, are associated with normal growth demand so frequently that we should surely pay more attention to the ways in which the breakdown of normal growth control mechanisms occur instead of concentrating quite so firmly on a belief in something specific to cancer causation to be found perchance in such a great variety of carcinogenic substances. Evidence derived from a study of the age distributions of tumours suggests that breakdown in normal control mechanisms governing group cell behaviour, especially following repeated demands for normal growth, may be one of the most important factors in the development of many neoplastic diseases. Isolation from normal growth controls or a diminution in restraint of growth through the failure of tissue-specific control actions may play a far more important part in tumour development than we have yet been prepared to accept.