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AGE OF CANCER INCIDENCE 35

as a depression in the male:female incidence ratio in the left column
of Figure 2.12. Prostate and lung cancers also influence the aggregate
male:female ratio—these cancers rise strongly in later years and occur
only (prostate) or mostly (lung) in males.
Figures A.13–A.18 in the Appendix show the male:female ratios for
the major adult cancers. The plots highlight two kinds of information.
First, the values on the y axis measure the male:female ratio. Second,
the trend in each plot shows the relative acceleration of male and female
incidence with age. For example, in Figure 2.12, the positive trend for
lung cancer shows that male incidence accelerates with age more rapidly
than does female incidence, probably because males have smoked more
than females, at least in the past.
Figures A.13–A.18 show that positive trends in the male:female inci-
dence ratio also occur consistently for colon, bladder, melanoma, leu-
kemia, and thyroid cancers. Negative trends may occur for the pan-
creas, esophagus, and liver, but the results for those tissues are mixed
among samples taken from different countries. Simple nonlinear curves
seem to explain the patterns for the stomach and Hodgkin’s cancers, and
maybe also for oral-pharyngeal cancers.
The patterns of relative male:female incidence probably arise from
differences between males and females in exposure to carcinogens, in
hormone profiles, or in patterns of tissue growth, damage, or repair. At
present, the observed patterns serve mainly to guide the development
of hypotheses along these lines.

2.7 Summary

This chapter summarized patterns of cancer incidence. The best the-
oretical framework to explain those patterns arises from the assumption
that cancer progresses through multiple stages. Before turning to multi-
stage theory and its connections to the data on incidence, it is useful to
consider the observations on how cancer develops within individuals
with regard to stages of progression. The next chapter summarizes ob-
servations of multistage progression.