Biomedical Engineering Reference
In-Depth Information
the planning process just the same as for photons, except that proton
beams have a different depth
−
dose distribution?
In a certain sense, there is some truth to that last claim. However, the
devil is - as always - in the details. Proton beam therapy has
acquired a reputation of being harder to plan than photon therapy, and
I want here to describe what the differences seem to be. Let us start
with the list of planning tasks laid out in Chapter 6 in the context of
photon beam therapy. Which tasks are different when protons are
used? Table 11.1 reproduces these steps and identifies those which
differ.
5
One can see at once that the steps that are the same far outweigh those
which are different; There is only one, admittedly large, step for
which the planning of proton therapy is substantially different from
planning photon therapy.
One can say, in a nutshell, that the differences arise from three causes,
the first two of which are related:
the finite penetration of protons in matter;
the sensitivity of protons to the traversed materials both in terms
of their penetration and their lateral characteristics;
the different beam formation techniques used in proton therapy.
The differences manifest themselves in a number of ways. The
following sections offer a brief description of the differences - many
of which, as I have touched on them already, are mentioned without
additional elaboration.
6
D
IFFERENCES IN
P
LANNING
:
S
TEP
1
-
C
HOICE OF
M
ODALITY
Step 1 of Table 11.1 involves the decision as to whether radiation
therapy is appropriate for the patient. One aspect of this question
relates to the choice of radiation modality. It may even be that a
5
I would like to acknowledge the input, which was given in the context
of preparing ICRU78 (2007), of a number of colleagues in addressing
this question, namely: J. Adams, M.Moyers, P. Petti, S. Rosenthal,
B. Schaffner, N. Schreuder, and L. Verhey.
6
The issue of the interpretation of the PTV, mentioned in step 3, is addressed
in the discussion of step 5 issues.
