Biomedical Engineering Reference
In-Depth Information
beam. An example is given in Figure 8.4, where the beam direction
has been decided upon and the beam aperture is being drawn, making
use of a circular cursor that helps the planner to leave a specified
boundary around the CTV.
Figure 8.4 is an old and even historic image (Goitein
et al
., 1983).
Modern graphics engines give much more attractive surface-rendered
images, but they do not really make the process any more accurate or
easy. Note, also, the equipment settings, displayed in white below the
BEV, and the three orthogonal views of, in this case, the proton
treatment apparatus
which includes a
treatment couch with
six degrees of free-
dom. In this system,
as in later planning
systems, the planner
could adjust the beam
settings using those
variables that are
intrinsic to the equip-
ment, such as couch
height, gantry angle
and so forth, provid-
ing what has since
Figure 8.4. Beam's-eye view of patient
anatomy. A beam aperture is being drawn
been termed “virtual
with the aid of a cursor whose radius equals
the desired margin. Reproduced with per-
mission from Goitein
et al.
(1983).
simulation.”
Computer tools exist to design the aperture automatically, given the
desired field margins (that need not be the same all around the PTV).
Decision as to how many beams to use
Generally, the choice of the number of beams depends critically on
the patient's individual geometry. One central decision, further
discussed below, is whether to use arc therapy, which involves
rotating the beam around the patient over a range of angles up to the
full 360
possible, o
r to employ a few fixed beams. Rarely, as has
already been discussed, would a single beam be a good choice except
°
in the case of superficial tumors. Generally, too, a pair of parallel-
opposed photon beams gives too high a dose outside the target
volume to be useful. Typically, then, a few beams
−
say between 3
and 7
−
are chosen. Plans with fewer beams are sometimes preferred
