formulas for its calculation have been proposed and may be utilized in radia-~
tion experiments [U5!.
Upton, Kastenbaum and Conklin [U6], in an analysis of
the age-specific death rates in irradiated LAF1 and RF mice pointed out that
this parameter beyond a certain age tends to assume an exponential trend.
Also, death rate curves specific for certain diseases are found to vary in
shape and slope.
These changes emphasize the complexity of the relationships
between dose and disease incidence.
Therefore, the generalized notion that
irradiation may advance the onset of old-age diseases is to be regarded as
an oversimplification in the light of the variability observed among specific
injuries.
17.
Other refinements in the analysis of life-shortening data may be intro-
duced in order to account for the effect of competing diseases.
It has long
been known that the estimates of final incidence of diseases occurring late
in life may be affected by the rate of mortality at times preceding the on-
set of these diseases {[M4, F91.
Hoel and Walburg [H2} have compared various
interval techniques of analysis and a non-interval technique by Kaplan and
Meyer and have come to the conclusion that the latter one may be used with
advantage when the age at death of the animals is known.
This technique has
been employed for analysing the significance of the difference between treatment groups in respect to their cumulative mortality.
There are also tech-
niques to adjust the comparisons of mean ages at death, according to the presence of competing, lethal and non-lethal, diseases.
On their basis Walburg
(W1l has reanalysed some of the data on life-span-shortening in experimental
animals and has convincingly shown the usefulness of such methods in discriminating between specific, i.e. neoplastic, and non-specific life-shortening.
C.
18.
THEORETICAL FOUNDATIONS
Although it is the primary object of the present document to review and
discuss experimental data on life-span-shortening, it is impossible to do so
without some background information on the hypotheses of aging.
Such infor-
mation will be given in the next few paragraphs in a very simple form and will
be limited to those hypotheses that were proposed in the field of radiation
research.
More comprehensive discussions of the various theories of aging
will be found, for example, in Strehler |S7] and Walburg |W1].