The age distribution in female breast and genital cancers

Abstract

When the age-specific incidence rates for cancers of the female breast, cervix, and body of the uterus were plotted on semi-logarithmic paper they could be resolved into two linear components. One component consisted of rates prior to 40-60 years of age and the other of those rates after this age period, the age separating the two components varying with the specific types of cancer. The slope of the 2d linear component was less than that of the 1st, indicating that the rates comprising the 2d component increased at a slower rate than those in the 1st component. To explain this change in the slopes an hypothesis was developed which assumed that the female population consisted of two sub-groups, one group being susceptible to the development of these tumors while the other was not susceptible, and that prior to the age when the slope changed, all women were susceptible. The slope change was therefore considered to be due to a decrease in the proportion of susceptible women in the population occurring at the time of the change. A distribution representing the proportion of females remaining susceptible at various ages was derived from the age-specific incidence rates. This distribution is similar in shape for breast, cervical, and uterine body cancers but differed with regard to the age distributions. These distributions were compared with those representing the percentage of women still menstruating and a general similarity was noted. Cancers of the ovary, vulva, and vagina were studied in a similar manner and no linear relationship was seen, possibly due to the existence of heterogeneity in types of tumors and etiological factors. The hypothesis was considered to be consistent with the idea that estrogen and/or other hormones may form a necessary substrate upon which other factors may act in order to produce the specific cancer. This hypothesis was discussed from the viewpoint of providing a conceptual framework for more definitive investigations.