Biomedical Engineering Reference
In-Depth Information
would escape the patient without further interactions. However, just
as with the initial photon, there is a chance that it will itself suffer a
further Compton interaction. If so, the interaction will probably be
some distance away from the site of the initial interaction, since the
interaction probability of photons is low. The second interaction will
proceed very much like the first. There will be a scattered photon
which will probably escape the patient without doing further damage
and the ejected electron will cause 10's of thousands of ionizations as
it loses its energy and comes to rest. The only difference is that the
scattered photon will have
less energy than the initial
photon, and so the, on
average,
lower
energy
ejected electron will cause
somewhat fewer ionizations
over a slightly smaller
distance than in the case
of
the
first
Compton
interaction - that is, its
“splash” of dose will be
slightly smaller in extent.
And, of course, the
scattered photon from the
second Compton interaction
may
suffer yet another
Compton collision and so
on… Figure 4.10 recaps
this story. In this figure,
scenario (a) is most likely;
scenario (b) is the next most
likely; and scenario (c) is
the least likely to occur.
Figure 4.10. The biographies of 3
photons: (a) The photon passes through
the patient without interaction; (b) the
photon suffers one Coulomb inter-
action with the ejected electron causing
10's of thousands of ionizations (
red
splash
); (c) the scattered photon from
the first Compton interaction suffers a
second Compton interaction, and its
ejected electron causes 10's of thous-
ands of further ionizations (second
red
splash
);
The experience of many incident photons
Typically, for a beam of 4 MeV photons for example, several times
10
10
photons per cm
2
are needed to deliver a dose of 2 Gy. A beam
comprised of such a large number of photons will result in overlaying
an equally large number of histories such as those depicted in Figure
4.10. As a result, there will be a large number of electron-induced
dose “splashes” laid down all throughout the irradiated medium.
These will overlap one another and will merge into one big dose
“splash” extending throughout the volume of tissue which is within
