to assess the contribution of each specific cause to life-shortening, since
there is no reason to presume that each of them may be equally accelerated
by radiation.
Serial sacrifice experiments could, in principle, provide such
information, but they require considerable time and effort and are therefore not common in the literature [K1, C3, A3].
11.
Apart from the difficulties in defining and describing a complex effect
like life-shortening, its quantification could also create problems by giving
implicit support to one or another possible interpretation.
Interesting ob-
servations have been made by Mole in this respect
He points out that
[M2, M3].
it is not immaterial to think of the effect as a differential between lifespan of the control population and the life-span of the irradiated animals,
to - tp:
If it is postulated that survival after a given dose is a function
of that dose, it is implicitly ignored that animals may die of some unrelated
pathology, and there is no implication that the effect of radiation may persist up to the end of life.
If, on the other hand, the postulate is that the
differential life time is a function of the dose, then the implication is that
the effect of radiation may be equivalent to natural aging.
No problem arises
when single acute exposures are given at young ages or in duration-of-life experiments, because under these conditions the two formulations are identical.
But when experiments to various
to
are involved, that is, when groups of
animals are started on a course of irradiation at variable ages, one could
eome to quite different conclusions from the same experimental data simply by
following one or the other approach.
12.
By definition, life-shortening is an effect that must be estimated stati-
stically by comparing irradiated and non-irradiated animal populations.
There
are different ways of describing and expressing quantitatively the effect.
They are the mean or median life-span, the percent cumulative mortality or the
age-specific mortality rate.
All may be regarded as compounded expressions
over the whole population of animals of specific and non-specific causes, acting
and interacting within each individual to decrease its fitness and to take him
to death.
13.
Life-shortening is expressed in days of life lost and since the time to
death is a random variable, life-shortening may be represented by one of the
following statistics:
shortening of mean or median age at death;
shortening