Thet change in the slope at the age of 45y is known as Clemmessen’s hook
(Clemmesen J: Carcinoma of the breast. Br
J Radiol 21(252): 583–590, 1948). It is attributed to menopause. Yet there
is more to it. The curve indicates the capability of the organism to resist
cancer. Two processes determine the outcome of cancer. 1. From birth
and onward cancer is driven by an independent carcinogenic process. 2. Tumor
growth which is controlled by the organism. The age adjusted incidence depicts
the probability of tumor surfacing at a given age.
This probability is determined by the tumor growth rate. The faster tumor
growth the steeper the curve. Tumor growth rate depends further on host
resistance. Initially host resistance is low and the rate of tumor surfacing
rises. At the age of 45y the curve takes a turn and rises slower. Beyond
the age of 80y tumor surfacing stops.
The downward trend of the curve in old age has yet another explanation. Since
at this age death from myocardial infarction or stroke predominates, some
of these patients may carry into their grave a hidden
cancer
and contribute to the downward incidence rate.
However phenomena like Clemmessen's hook indicate that the decline of the
tumor surfacing rate starts relatively early in life when strokes are not
so common like in old age.
Since tumor surfacing depends on host resistance the curve indicates that
with age the organism resists cancer better and better attaining its greatest
potency in advanced age. Menopause is a manifestation of rising host resistance.
Since some tumors are estrogen dependent, menopause slows down tumor growth.
This also is the rationale of the adjuvant chemotherapy of an estrogen antagonist
called tamoxifen.
Since a similar curve flattening
appears in most chronic cancers, like lung and bronchus (see below). Host
resistance is mediated also by factors other than hormones.
Further reading: Late life mortality