Biomedical Engineering Reference
In-Depth Information
Since the volume irradiated to high dose in each beam is convex, their
intersection must be convex and the part of the OAR within that
volume will inevitably receive full dose.
IMRT is, rather simplistically, illustrated in the right hand side panel.
Here, each beam can be blocked so as to avoid the invaginating OAR.
If this blocking is done, the target volume will not receive dose in the
shadow of the OAR, and so will have an inherently inhomogeneous
dose distribution from that beam. However, other beams, coming
from other directions, can “fill in” the dose that the first beam failed
to deliver. Consequently, through the use of several non-uniform
beams, a fairly uniform target volume dose can be achieved, while
largely sparing the OAR. This approach can thus create concave dose
distributions. The strategy of sparing selected normal tissues has
been given the name
conformal avoidance
, in analogy with the
traditional conformal coverage of target volumes.
It is not only invaginating OARs that can be spared radiation; other
selected neighboring or distant OARs can also be spared or partially
spared. On the other hand, it is not possible to spare
all
the OARs
−
the integral dose has to be deposited somewhere
one can only spare
some few selected ones. In general, the smaller the OAR, the easier it
is to spare it.
IMRT also makes it simple to deliver a non-uniform dose distribution
to the target volume. In general there are two situations in which a
non-uniform dose distribution may be desired. First, when there are
two target volumes, one nested inside the other, and one wishes to
deliver a higher dose to the inner volume than the outer, all in the
−
same fraction (the so-called “field-within-a-field” approach). This
might be the case when the inner volume encompasses only the GTV
and the outer includes sub-clinical disease. The second situation is in
the case of so-called dose-painting (Ling
et al
., 2000). Dose painting
may involve delivering additional dose to sub-regions of the target
volume due to the judgment that, based perhaps on functional
imaging studies, they contain more resistant cells, Or, dose painting
may be desired in order to deliver a reduced dose to sub-regions of
the target volume because a critical normal tissue runs through, or is
closely adjacent to, the target volume.
Figure 9.2 shows an example of an IMRT plan. A transverse section
of a patient with a nasopharyngeal carcinoma is shown, irradiated by
nine equally spaced photon beams. The dose distributions of the
individual beams are shown around the periphery of the central larger
