Biomedical Engineering Reference
In-Depth Information
Of course, this exercise is highly simplistic. For one thing, we are
lumping together all tissues outside the tumor and assuming they have
the same structure and radiation sensitivity. For another, the models
themselves are largely unproven. So, one can certainly not draw any
quantitative conclusions from our exercise. However, we did come
away with one very important qualitative conclusion, namely
that
there is probably no universal answer to the question of which of two
crossing DVHs is better
and, therefore, to the question of whether a
fixed-field or rotation plan is better
−
the answer depends on the
normal tissue architecture
. Biology matters.
My own inclination is toward the use of a fewer number of beams
covering less normal tissue, but to a higher dose. This is because of
three main considerations:
1.
In my experience, the particular geometry of the tumor and
normal tissues in a given case often allows the choice of beam
directions which can advantageously spare specific organs
which commonly limit the intensity of treatment which can be
given, and this may be better than just spreading the dose
around throughout the entire patient cross section.
2.
As discussed in Chapter 5, the bath of dose around a given
OAR may negatively impact its response to radiation. Thus,
−
constraining the dose bath to a small volume seems wise.
3.
Until the recently, common practice over several decades
favored the few-fixed-fields approach over arc therapy
and I
tend to give considerable weight to established experience.
The advent of IMRT has challenged this preference, since it
favors plans which cover a large fraction of the patient cross-
section. However, this arises from an algorithmic need, rather
than being motivated by biological considerations
−
and I tend
to distrust changes which arise from purely technological
limitations.
−
U
NCERTAINTY IN THE
D
OSE
D
ISTRIBUTION
In fairness to you, my readers, I should warn you that what I am about
to say in this section reflects what I think
ought
to be done. In current
practice, unfortunately, most practitioners lack the tools either to
make detailed analyses of dose uncertainties, or to display the results
of an uncertainty analysis. I write this in the hope that you will lobby
for change.
