Initiation and promotion in young and old animals. Implications for human tumour formation.

The significance of ageing in experimental skin carcinogenesis and on the manifestation of tumour formation in man was studied. Initiation with the carcinogenic hydrocarbon 7,12-dimethylbenz [a]anthracene and promotion with croton oil in mouse skin showed that tumour formation was unchanged when promotion was delayed 3, 10, 23 or 43 weeks in comparison with that in untreated animals of the same age. The sensitivity to carcinogen treatment of animals over 71 weeks of age was less than that of animals 11-51 weeks of age; this finding affected the interpretation of data on animal groups treated 63 weeks after the start of the experiment. In a human autopsy study, the tumour incidence increased with age to the eighth decade; benign tumours of types not related to malignant tumours occurred in older age groups. The percentage of epithelial tumours was higher in humans aged 60-80 and in organs exposed to environmental agents, but lower in humans aged 80 and more.