to use the mean energy density in the organ as the absocbed dose.
In other
words, in these high-exposure regions, the cell dose and the orgun dose
are, for all practical purposes,
identical.
Ome can then characterize and
predict the probability of a biological response
in the organ itself,
{in
in the cell population, or
terws of a single parameter,
the absorbed dose D to
the organ,
However, at the bend in the curve,
independent components,
cell, F,
large,
the exposure splits into
the mean cell dose z and the number of hits per
Note that the expectation value of Z, even tnough the variance is
remains constant,
so
that the only cellular parameter that can
increase with increasing expusure is F,
cell.
the number of hits per exposed
Thus, with LLE, neither the dose to the cells nor the mean dose
fnereases; it is only the nunber of cells dosed that can increase,
While LLE has
{ts counterpart in the macro accident situation,
ia
which only a small fraction of an exposed human population is hit with
increasing exposure,
acciécents,
there
is no analogue of HLE exposure with macro
The reason for this is that, for practical and ethical reasons,
if the accident rate in a gtven population Increases above a very small
fcactlon per year, even drastic action is likely to be taken to effect a
decrease.
With radiation, on the other hand,
the accident rate can be
increased at will, so that any given cell can readily be subjected to
dozens or more accidents,
is only because of
this
in the course of minutes,
seconds, or less.
It
fact, which may permit interactions between the
effects of the hits before repair can take place,
that the “quadratic”
term, seen only with high-level exposure of cells tu low-LET radiation,
exists.
The transition from low- to high-level radiation exposure is depicted
1a Figure 9,
for cell lethality only.
the LLE ragion,
Because of
in
hote the initial linear Increase Lu
the number of quantal ras ponders as a function of 7.
multiple hits and interactive processes,
steeply beginning in
the
transition zone,
the curve rises rather
ae
so that a large fraction of organ ak
cells have been killed as one enters the HLE reyion.
‘7B
In this region, some #4
a:
of the organs and therefore the organisms, at a given value of 0D, will fail %
functionally and dite,
Increases,
with HLE the
Again,
focus
and
the fractinn will
iucrease
to unity as D
the largest difference between the two regions is that
{ts on
the
indfviduzl,
-222-
and
the single parameter DP ts
:
“4
Th.
Fat
Var
t