Biomedical Engineering Reference
In-Depth Information
compensator and the patient there would be beam undershoot, and
hence reduced dose, in the target volume and beam overshoot in
downstream normal tissues, as shown in Figure 11.7c.
Figure 11.7. Schematic representation of the design of a compensator
(see text).
Figure 11.7d shows a possible solution - designed to avoid target
underdose at the expense of delivering dose to a larger volume of
downstream normal tissues. The modified part of the compensator is
widened so that, even if it is slightly mis-registered, the target volume
will nevertheless receive full dose. The full prescription for this
approach, termed
smearing
of the compensator, is given in Urie
et al
.
(1984). Compensator smearing allows not only for mis-registration
caused, for example, by patient or organ motion, but also for the
blurring effects of multiple scattering.
One useful technique, which is however little used, would be to
introduce additional smearing through degradation of the beam
directionality
. Usually, in order to achieve sharp penumbras, the
angular confusion of the beam is kept as small as possible. However,
as simple inspection of Figure 11.7 would suggest, a spread in beam
directions - for example, by delivering a few beams separated in
angle from one another by a few degrees - can smear out the dose
perturbations beyond an inhomogeneity. In general this strategy will
lead to the delivery of a smaller dose perturbation over a larger
volume.
