Page 32 - 20dynamics of cancer

2 Age of Cancer

Incidence

Perturbations of the genetic and environmental causes of cancer shift
the age-specific curves of cancer incidence. We understand cancer to
the extent that we can explain those shifts in incidence curves. In this
chapter, I describe the observed age-specific incidence patterns. The
following chapters discuss what we can learn about process from these
patterns of cancer incidence.
The first section introduces the main quantitative measures of cancer
incidence at different ages. The standard measure is the incidence of
a cancer at each age, plotted as the logarithm of incidence versus the
logarithm of age. Many cancers show an approximately linear relation
between incidence and age on log-log scales. I also plot the derivative
(slope) of the incidence curves, which gives the acceleration of cancer
incidence at different ages. The patterns of acceleration provide partic-
ularly good visual displays of how cancer incidence changes with age,
giving clues about the underlying processes of cancer progression in
different tissues.
The second section presents the incidence and acceleration plots for
21 different adulthood cancers. I compare the patterns of incidence and
acceleration for 1993–1997 in the USA, England, Sweden, and Japan, and
for 1973–1977 in the USA. Comparisons between locations and time pe-
riods highlight those aspects of cancer incidence that tend to be stable
over space and time and those aspects that tend to vary. For exam-
ple, many of the common cancers show declining acceleration with age:
cancer incidence rises with age, but the rise occurs more slowly in later
years.
The third section describes the different patterns of incidence in the
common childhood cancers. The incidence of several childhood cancers
does not accelerate or decelerate during the ages of highest incidence.
Zero acceleration may be associated with a genetically susceptible group
of individuals, each requiring only a single additional key event to lead
to cancer. That single event may happen anytime during early life when
the developing tissues divide rapidly, causing incidence to be equally
likely over the vulnerable period.