Biomedical Engineering Reference
In-Depth Information
Feathering in depth
When two proton beams abut at
the end of range of one or both beams, one should employ depth
feathering - that is the use of a few beams, each closely separated in
range by several millimeters, instead of a single beam. This is done
in beam-patching which is described below.
Beam delivery techniques
A treatment planning program needs to simulate the full variety of
beam delivery techniques available for each treatment machine. For
protons, these are quite varied - and quite different from those of X-
ray beams. Proton beams can be delivered by scattering, wobbling,
or scanning techniques. Then, too, proton beams can be shaped both
laterally and distally. The former by apertures and blocks, the latter
by distal shaping using a compensator or, when using IMPT without
patient-specific hardware, by 3D shaping achieved by adjusting the
intensities of the scanned pencil beams.
One particular point of difference, already discussed in Chapter 10, is
that the dose distribution is sensitive to whatever upstream material is
in the beam. As a result, the planner must pay greater attention to
locating patient-specific devices at an optimal distance from the
patient. One wants them close, to minimize the effects of scattering
in the compensator and to have as small a penumbra as possible, but
not too close, because of edge-scattering in the aperture if there is
one.
The planning target volume (PTV)
Being an alert reader, you have probably noticed that I often use the
term “target volume” without being specific as to whether I am
referring to the CTV or the PTV. In part, this is because the PTV is
difficult to define in the case of protons and it may be of less use in
designing a beam than it is in the case of photon treatments.
In the case of photon beams, the PTV is primarily used to set the
lateral margins of the field in order to compensate for motion and
setup uncertainties - and, even then, its use has to be supplemented by
knowledge of the characteristics of the beam penumbra. A single
PTV can set the margin no matter the direction of the beam.
In the case of proton beams,
both lateral and distal margins are
needed
- the former to set the lateral margins of the field in order to
compensate for motion and setup uncertainties, the latter to set the
proximal and especially distal margins of the treated volume in order
