Page 48 - 20dynamics of cancer

AGE OF CANCER INCIDENCE 33

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Figure 2.11 Dose-response lines from a variety of animal experiments. For
each experiment, I list the slope of the line, −r/n =−1/s, from Eq. (2.4): (×)
methylcholanthrene applied to mouse skin three times per week, skin tumors
with slope of −1/2.1; (+) 4-dimethylaminoazobenzene fed to rats in daily diet
(dosage multiplied by 1000), liver tumors with slope of −1/1.1; (filled circle)
3,4-benzopyrene applied to mouse skin three times per week, skin tumors with
slope of −1/4.0; (open triangle) methylcholanthrene given as a single subcuta-
neous injection to mice, duration measured as time after exposure, sarcomas
with slope of −1/4.0; (open circle) 1,2,5,6-dibenzanthracene given as a single
subcutaneous injection to mice, sarcomas with slope of −1/4.7; (filled triangle)
3,4-benzopyrene, single subcutaneous injection to mice, sarcomas with slope of
−1/4.7; (open square) diethylnitrosamine fed to rats in daily diet, liver tumors
with slope of −1/2.3; (filled square) dimethylaminostilbene fed to rats in daily
diet, ear duct tumors with slope of −1/3.0. Redrawn from Figure 9 of Druckrey
(1967).

Apart from the reproductive tissues, other distinctive patterns occur
in the incidence of cancer in males and females. The left column of
Figure 2.12 shows that, over all cancers, the relative age-specific inci-
dences follow the same curve in different time periods and in different
geographic areas. The curves show the ratio of male to female incidence
rate at each age. Early in life, males have a slight excess of cancers. From
roughly age 20 to 60, females have an excess of cancers, with a distinc-
tive valley in the male:female ratio at about 40 years of age. After age
60, during which most cancers occur, males have a significant excess of
cancers, rising to about twice the rate of female cancers.
Part of the aggregate pattern over all cancers can be explained by
breast cancer, which occurs at a relatively high rate earlier in life than
the other common cancers. The relatively high rate of breast cancer
in midlife causes a female excess in the middle years, which appears