Biomedical Engineering Reference
In-Depth Information
P
LANNING
A
IMS
The process of planning a treatment is almost never one of taking a
prescription and translating it directly into a deliverable plan. Rather,
it tends to be an iterative process, in two ways. First, the planner
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may be dissatisfied with the first plan he or she designs and try to
improve it
perhaps by using different or more beam directions, or
by using a different beam energy, and so forth. Second, when despite
the planner's best efforts, no satisfactory plan has been arrived at, the
clinician may decide to alter the requirements
−
−
e.g., allow a higher
dose to some OAR(s)
and then re-plan. This second form of
iteration is embodied in Figure 6.2, where the distinction is made
between the
planning aims
and the
prescription
for the patient's
treatment. Planning aims are the instructions to the planner, without
which he or she cannot proceed. The planning aims identify what one
would like to accomplish; the prescription bows to the reality of what
can practically be delivered. Of course, ideally the two will be the
same, but sometimes that is not the case.
The planning aims and, subsequently, the prescription establish a
number of goals. The nature of these has changed as the importance
of dose
−
volume effects has become increasingly apparent, so that
many requirements are now stated in terms of dose
−
−
volume
constraints.
Requirements on the overall treatment
The clinician must specify the
prescription dose
(e.g., 72 Gy) and the
fractionation scheme
(e.g., 1.8 Gy per day, 5 days a week). As ideas
have changed regarding how to prescribe treatments, the definition of
the prescription dose has also changed. It used to be the desired dose
at a specified point. Nowadays it is usually considered as a reference
value to which the tumor dose requirement can be pegged, as
discussed immediately below.
The clinician may have in mind a specific technique. For example,
“our class-solution for a prostate treatment featuring a 5-beam
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Of course, there is virtually always more than one person involved in
designing and evaluating a plan (physician and physicist or dosimetrist, at
least) and they must reach a consensus regarding it. However, to avoid
constantly repeating this caveat, I refer always to the planner, in the
singular.
