ght
to
It then becomes additionally clear that each point on the linear curve
dees not represent a single value of cell dose, with all dosed individuals
having received nominally the same value, as is implied in the term
3,
“dose-response” curve.
Rather, each point equates
to an entire
distribution representing groups of cells with different doses.
Such
distributions are implied in Eq. (1) showing that D = ZF, in that
obviously,
to have a 2,
there must exist a corresponding distribution.
The
number of dosed cells at each value of z represents a yzraded series of cell
doses, identical in concept to such a series used in Md to determine the
probabiiity of an organ response curve as a function of dose.
A Cell Risk Meter:
Microdosimetry
“Microdosimetry”, although originally applied only in the context of
the
techniques devised by Rossi et al.
(4-6) to measure
the number of hits
per cell and their magnitude, has now been extended to include both
instrumental and calculational approaches to determining the same
quantities.
It is perhaps more {illuminating to describe the ifustrument
approach.
A microdosimeter can be regarded as simply a proportional counter
containing tissue equivalent gas.
Even though the counter may be
centimeters in diameter, partial evacuation and suitable scaling permits
teady simulation of subcellular volumes of several microns
Each time a particle impinges on or traverses
in diameter.
the instrument, a single
"hit" is registered, aud the size of the resulting “event”, measured in
eee
The idea of discrete, stochastic high-density energy depositions
resulting
from radiation exposure probably originated early with Dessauer's “point
heat" theory (7) and was certainly well appreciated by Lea (8).
agponse Se
8
However,
these {deas were not formally developed until the "microdosimeter" was
invented by Rossi (4-6).
Its use has been more in the context of a
substitute for the quantity LET, to describe energy depositions within a
non~anatomically defined “gross sensitive volume” within the cell. The
idea of a “cell dose” was probably first applied practically by Bord and
Feitnendegen (9), and developed in NCRP Report No. 63 (10). The practical
application of the microdosimeter as a cell phantom with which
stochastically delivered cell doses could be determined is relatively
recent (Bond et al., Feinendegen et al., Refs. 11-14).
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