Page 45 - 20dynamics of cancer

30 CHAPTER 2

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Figure 2.8 Fatal lung cancer in males for groups that quit smoking at different
ages. The six curves defined in the legend show individuals who never smoked
(quit at age 0), individuals who quit at ages 30, 40, 50, and 60, and individu-
als who never quit (shown as age 99). (a) Age-specific mortality per 100,000
population on a log 10 scale versus age scaled logarithmically. Data extracted
from Figure 2 of Cairns (2002), originally based on the analysis in Peto et al.
(2000). Most cases of lung cancer are fatal, so these mortality data provide a
good guide to incidence, advanced slightly in age because of the lag between
the origin of the cancer and death. Curves fit to the observations (circles) by
the smooth.spline function (R Development Core Team 2004), with a smooth-
ing parameter of 0.3. (b) Age-specific acceleration calculated as the derivative
(slope) of the smoothed curves fit in (a). Some of the curves in (a) are based
on only four observed points, causing the fitted curves to be sensitive to the
level of smoothing; the plotted accelerations in (b) for those curves should be
regarded only as qualitative guides to the general trends in the data.

exposure until tumor onset, and n determines the scaling of incidence