Biomedical Engineering Reference
In-Depth Information
Planning IMRT
There are two main aspects of planning IMRT:
1. establishing a method for computing a numerical
score
,
expressing how well the goals were achieved; and
2. conducting a
search
through the space of treatment variables to
locate the set of values of those variables that gives the best
score.
The score is the value taken on by a
score function
for a particular set
of the variables upon which the value of the function depends.
2
What
one is attempting to do is to pick values for all of the variables that
together maximize the score.
In planning IMRT, the processes of establishing a score function and
searching for the optimum score are set within a broader range of
activities, namely:
1.
Evaluate the patient using all relevant diagnostic tools, and
decide whether to employ radiation therapy as at least a part of
the patient's treatment.
2.
Obtain
and inter-register appropriate imaging studies. The planning
CT study, which is taken with the patient lying in the position
and, usually, held in the immobilization device that will be used
for treatment, is almost always one of these studies.
Delineate on the planning CT the target volumes (GTV, CTV,
and PTV) and all OARs (and, perhaps, PRVs) whose proximity
to the target volume or sensitivity makes them of particular
3.
interest.
4.
Establish the planning aims for the treatment.
5.
Set or change values for the treatment variables. This defines a
plan
namely, a set of beams with, in general, non-uniform
fluence maps together with the beam weights.
Evaluate the plan (i.e., compute its score) and either select it for
use in treatment or continue the search by adjusting the values of
the treatment variables and returning to step 5.
−
6.
7.
Finalize the prescription.
2
The more formal term for this function is the
objective function.
However,
I use the synonymous term
score function
in what follows as being more
evocative.
