these changes could at least in part be interpreted as being due to an effect
of wasted radiation.
According to Grahn and Sacher [G1] this concept would al-
ready be contradicted for rather high doses and short survival times (the concept was actually derived from experiments at 200 to 25 R/day and on the 1D 9/30
end-point); but for long protraction periods the idea would be injustified
since no radiation may be considered as truly wasted.
100.
The difficulty with the notion of wasted radiation is that in chronic
exposure to death time and dose have a bi-univocal correspondence and cannot
be experimentally separated from each other.
Thus, it becomes difficult to
isolate from an end-point like the life-span-shortening (which is measured in
units of time) the dose that would have been given in excess of the minimum required to kill the animal within a given time.
An additional difficulty lies
in the nature of the biological event of death which is in itself a final endpoint in survival experiments, whereas in experiments on tumour induction, for
example, it is possible to account in part for the wasted radiation by computing the dose absorbed at the tissue of interest up to the time of the first
appearance of the tumour or to some such extrapolated time
101.
[F3, M11, M12].
It is natural therefore that, in spite of its wide acceptance, specific
work to test experimentally the concept of wasted radiation has not been very
extensive.
In fact, this concept has its shortcoming in the difficulty of its
testable implications and in the precise evaluation of its importance under
each specified experimental condition.
There appears to be little hope that
these problems might be settled in the near future and any conclusion concerning the relevance of this notion in the interpretation of radiobiological experiments in animals or, even more, in human radiation biology must for the
time being remain open.
102.
The available evidence on the biological effects of chronic radiation
exposure for the whole life of the animals will be reviewed in the next few
paragraphs (paragraphs 103-126).
This field has been reassessed at various
times, among others, by the UNSCEAR Committee [U1], by Sacher and Grahn [SH],
Grahn and Sacher [G1], Grahn [G6], Sacher [S14].
The reader is referred to
those contributions for more extensive coverage of the subject.
The life-
shortening effects of incorporated radioisotopes will be considered for con-
venience in separate paragraphs (127-139).