Biomedical Engineering Reference
In-Depth Information
are “wasteful” in that they cannot do much more than a single beam
in terms of conformal avoidance. There is some suggestion that fewer
beams are needed when heavy charged particles such as protons are
employed.
Lateral extent of beams
There is little reason, at first glance, to have any beam intensity for
those pencils which are not directed toward the target volume since
they appear only to intersect normal tissues and not the target. For
this reason, such pencils are usually set to zero intensity and excluded
from further optimization. Doing so, however, can lead to problems
with dose at the edge of the target volume. The reason is that the
pencils directed toward the target volume, but close to its edge, lose
dose through electron transport and angular beam divergence to
tissues outside the target periphery. In a uniform intensity beam,
designed to cover the target volume
with some margins
, this loss of
dose is exactly compensated for by pencils directed just outside the
target volume which contribute dose to the parts of the target just
inside its periphery. When the outer pencils are lacking, there is then
a dose reduction at the target edge.
Many algorithms use some kind of trick to prevent such a dose
reduction. For example, the intensities of those pencils that are
directed towards points some small distance outside the target volume
can be included in the optimization. That distance then becomes a
parameter of the algorithm. A user needs to be aware of such often-
hidden algorithmic features.
T
HE
S
CORE
The state of our knowledge of the impact of radiation on normal
tissues is so inadequate that it casts considerable doubt on the realism
of present techniques for assigning a score to a plan. As discussed in
Chapter 8, we still do not know the answer to the very basic question:
is a 4-field box in better or worse than a 360
rotation in a given
situation? If one cannot answer even this question, then one can
hardly expect to be able to answer many of the other questions of
concern in planning
°
−
and optimizing
−
treatments.
