Biomedical Engineering Reference
In-Depth Information
one would have to require that the lower bound 90% isodose surface
hug the “target volume.” If organ motion had been incorporated into
the CTV, then the relevant target volume would be the CTV.
However, this is usually not, and according to the ICRU definitions
should not be, the case. Then, the relevant target volume is the ITV
which is defined as enlarging the CTV to account for organ motion
within the patient (see Chapter 3).
Figure 11.6. Uncertainty analysis for a proton beam traversing a
water-filled human skull. The computed upper and lower-bound
90% isodose curves bracket the nominal dose curve - which is the
curve which would be estimated in the absence of an uncertainty
analysis. Reproduced with permission from Urie
et al
. (1986a).
T
HE
D
ESIGN OF
A
PERTURES AND
C
OMPENSATORS
In clinical applications, a proton beam needs to be “shaped” both
laterally and in depth. The former is achieved using one or more
apertures and/or blocks which intercept protons so that a negligible
dose is delivered in their shadow. What little dose there is comes
from neutrons produced in the aperture. The latter is achieved using a
so-called
compensator
which, in older terminology, was called a
“compensating bolus.”
Both apertures and compensators can either be physical objects, or
they can be virtual
- implemented in the latter case by restrictions on
the allowed pencil beams in a scanned beam. The principles of their
design are rather similar in either case, even though their
implementation is completely different. (This is why I have been
content to focus on scattered broad beams in much of the preceding
discussion; the same principles hold also for scanned beams.)
