Page 198 - 20dynamics of cancer

CARCINOGENS 183

16
(a) (b)
continuing cigarette smokers
stopped age 60
14 stopped age 50
stopped age 40
stopped age 30
lifelong nonsmokers
12
Cumulative risk (%) 8 6
10

4

2

0
45 55 65 75 45 55 65 75
Age

Figure 9.9 Reduction in relative risk of lung cancer between men who contin-
ued to smoke and those who quit at different ages. (a) Redrawn from Figure 3 of
Peto et al. (2000). Samples for this case-control design include 1465 case-control
pairs in a 1950 study combined with 982 cases plus 3185 controls in a 1990
study. (b) Model fit to the data in which smoke carcinogens affect equally all
stages in progression. The subsection All Stages Affected describes the details
of the model.

a comprehensive explanation to cover all of the available data. In my
opinion, the existing studies do not provide enough evidence to decide
between competing hypotheses. Instead, the smoking data define the
challenge for future studies.

ALTERNATIVE EXPLANATIONS
All theories must account for two observations. First, the relative risk
of lung cancer decreases in those who quit compared with those who
continue to smoke (Figures 9.8 and 9.9). Second, the rise in incidence
with smoking fits an increase in incidence with roughly the second power
of number of cigarettes smoked per day (dose).