Biomedical Engineering Reference
In-Depth Information
give rise to very small artifacts that are purely calculational and have
no clinical implications.)
As just alluded to, one great advantage of cumulative DVHs is that
many dose statistics
such as the minimum, near-minimum, median,
near-maximum, and maximum dose for the VOI represented by the
DVH
−
can be directly read off them, as Figure 6.8 indicates. This is
not the case with the mean dose, D
mean
, which has to be calculated.
−
0D measures of biological effect
Another group of scalar quantities that can be used to characterize a
dose distribution are estimates from biophysical models, as described
in Chapter 5. These would include: TCP and EUD for the tumor, and
NTCP and EUD for specified OARs.
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P
LAN
A
SSESSMENT
T
HE
B
ALANCING
A
CT
There are two approaches to plan assessment: (1) inspection of the
dose distribution and quantities derived from it by an expert; or
(2) the computation of a “score” for the plan. The latter approach is
generally restricted to a computational search for the “optimal” IMRT
plan and this aspect of plan evaluation is deferred until Chapter 9.
However, even when a computer has arrived at a plan with the best
computed score, the radiation oncologist will need to review it, and
will use the techniques of
expert inspection
for that purpose.
It has already been mentioned, but it bears repeating, that the choice
of a good treatment plan involves a balancing act between, on the one
hand, the likely effect of the proposed irradiation on the tumor and, on
the other hand, its likely effect on the normal tissues. Whether one
judges a plan using quantitative biological models explicitly or by
inspection from the dose distribution, the ultimate evaluation relates
to the need to achieve a balance between local control and morbidity.
The planner will also have in mind the feasibility of safely delivering
a particular plan in practice. This judgment is an important part of the
planning task and it is one that requires a good deal of experience to
make.
Organ by organ inspection
When judging plans “manually”
clinicians tend to look at the dose
distributions within the tumor and within individual organs and
