Biomedical Engineering Reference
In-Depth Information
field, one has a set of pencil beams of varying penetration. Typically,
some 20 beam energies might be involved, irradiating discreet
“layers” spaced
circa
0.5 g
-2
cm apart. This could potentially make
the task of computation one and a half orders of magnitude larger and,
hence, longer. However, there are some tricks available to speed
things up, such as ways to reduce the number of pencil beams which
are tried out.
As previously mentioned, in proton beam therapy the choice of beam
angles is more important than for photons and, therefore, unlike
IMXT, the use of equally spaced angles is unusual. Generally
speaking, for both IMPT and for uniform beam proton therapy, the
number of beams needed to produce a satisfactory plan is typically
less than the number needed for X-rays (Rutz and Lomax, 2005). The
use in IMXT of several (e.g., 5, 7, or 9) beams, equally spaced in
angle and non-collinear, tends to result in spreading the dose outside
⋅
the target volume over pretty much the full 360
°
available - and
favors coplanar beam arrangements. IMPT, on the other hand, can
use rather few beams and, thus, can distribute dose over a smaller
volume of the normal tissues. This brings up the question, already
raised in Chapters 8 and 9, of whether it is better to spread out the
energy outside the target volume, or to concentrate it in a smaller
volume at a higher dose level. I have emphasized that we don't really
know the answer to this question - but, at least, one has a choice with
IMPT, whereas IMXT is likely to result in near-360
°
dose spreading.
Immobilization, localization, and verification
Proton beam therapy has always emphasized the need for spatial
as well as dosimetric accuracy in treatment delivery, and hence
planning. This has arisen from the need to reduce compensator
registration errors, and to have the irradiated volume be as close as
possible to the target volume. As a consequence, traditionally greater
attention has been paid to patient immobilization, localization and
verification than in the case of photon therapy. However, this
situation has changed in recent years and the advent of first 3DCRT
and then IMXT has brought photon therapy much more in line with
proton therapy practice.
Uncertainty analysis
I fear that here I am beating a dead horse, but for completeness I have
to reiterate that, while uncertainty analysis is essential for all forms of
