Biomedical Engineering Reference
In-Depth Information
sensitive organ lying close to the target volume, rather than the distal
edge.
It goes without saying that the algorithm for dose computation is quite
different for protons than for photons. However, this is largely
transparent to the planner. Given the complexities of the effects of
inhomogeneities, however, the urge to use a Monte Carlo algorithm is
rather greater with protons.
The RBE of protons, as discussed above, is not a problem in practice,
given the fortuitously simple recommendation that a constant value of
1.10 be used everywhere. It is, of course, essential that, wherever
absolute doses are shown, it is made absolutely clear as to whether
physical or RBE-weighted doses are at stake. In general, ICRU78
(2007) recommends using RBE-weighted doses - identified by
including the qualifier “(RBE)” after the dose statement - pretty
much everywhere.
Nevertheless, a planner should be aware that, when bringing the distal
end of a proton beam up close to a sensitive organ: (a) the effective
dose may extend from 1 to 2 mm beyond the physical dose: and
(b) that there may be an elevation of the effective dose in the last
several millimeters of range - that is, the “blip” featured in
Figure 11.9.
Design of plan(s)
Once the differences discussed so far are taken into account, the
design of a plan of treatment (a set of beams and beam weights)
proceeds very much as for photons. One has manual and automatic
approaches in both cases. For manual planning, the issue is largely
the choice of the number, direction, and shape - including for protons
the shape in depth - of the beams and, of course, of their weights. As
with photon beams, as discussed in Chapter 8, the use of non-coplanar
beams (beams whose central axes do not lie in a single plane) can be
very advantageous. All the tools needed to create a set of beams, to
assess the resulting dose distribution, and to compare rival plans are
the same.
IMPT is, in principle, just as easy (or hard) to design and implement
as
intensity-modulated X-ray therapy
(IMXT). The computational
task is made greater, but not really more complicated, by the greater
number of degrees of freedom involved in IMPT. In essence, one has
a set of pencil beams which cover the area of the projected target
volume, as is also the case in IMXT, but then, for each location in the
