Biomedical Engineering Reference
In-Depth Information
patient with a more significant dose deficit. As a result, systematic
positioning errors have a greater impact on TCP and hence require
larger margins, than random errors of the same magnitude.
Detailed models of the required safety margin
Much more sophisticated ways of selecting margins to compensate
for motion have been developed and a good review of many of
these can be found in van Herk (2004). One example of a margin
prescription is the following (van Herk
et al
., 2000):
1.
estimate the total random uncertainty,
, by adding the estimates
of random uncertainty from all sources together in quadrature;
estimate the total systematic uncertainty,
σ
2.
, by adding the
estimates of systematic uncertainty from all sources together in
Σ
quadrature;
3.
estimate the necessary safety margin that needs to be added as
.
This recipe was designed to ensure that 90% of patients have a
2.5
Σ
+ 0.7
σ
minimum dose to the CTV of 95% of the prescription dose. A weak-
ness of this approach is that the margin prescription is based purely
on target volume considerations, without reference to normal tissue
complications.
S
UMMARY
Motion and mis-registration of the target volume with respect to the
radiation beams is, at some level, inevitable. If the target volumes are
to be adequately irradiated, and adjacent OARs are to be protected, it
is essential that:
1.
the causes and possible magnitudes of motion and misregistration
(in ones own institution) are understood;
2.
their possible consequences are understood;
3.
measures be taken to minimize motion and mis-registration to
the extent possible and clinically warranted; and that
4.
steps are taken (primarily through the provision of judiciously
chosen field margins) to allow for the remaining degrees of
and mis-registration.
