The incidence of breast cancer: analysis of the age dependence

The incidence of female breast cancer was studied as a function of age in 71 different populations throughout the world. Incidence data was normalized for differences in frequency of occurrence. Median normalized incidence increased exponentially from age 20 to 45. After age 45 the rate of incidence decreased, and we suggest that menopause may be responsible for the rate reduction. Populations were not randomly distributed about the total population median normalized incidence at the various age intervals. Populations were classified according to their age-incidence patterns in 3 groups: Group 1 (with median % incidence after age 69=50): Group 2 (with median % incidence after age 69=47), and Group 3 (with median % incidence after age 69=31). At most age intervals, the normalized incidence in Group 1 populations was less than the total median, while the normalized incidence in Group 3 populations was greater than the total median. Although we cannot exclude the possibility that the variation in age-incidence patterns among the population groups is an artifact of tumor registration errors, evidence suggests that such an artifact is unlikely. Breast cancer occurs more frequently in Group I populations than it does in Group 3 populations. We suggest that Group 3 populations may share an element of protection from, or that Group I populations may share an element of risk for, breast cancer.