Biomedical Engineering Reference
In-Depth Information
the time of treatment, but there is evidence that this may not always
be the case. A more elegant solution would be to track target motion
during the treatment (i.e., by imaging implanted seeds or surgical
clips) and adjust the position of the beam relative to the patient
appropriately, while the accelerator runs continuously. The adjust-
ment could be achieved by moving the patient couch or by moving
the radiation beam
for example, by adjusting the settings of a
multi-leaf collimator to track the tumor as it moves (that has the
−
added advantage that it could compensate for shape changes, if they
are known).
Correlation of tumor position with phase of respiration
The various methods of gating or breath control all have the
advantage that the extent of motion of tumors and organs due to
respiration can be substantially reduced. The only fly in the ointment
is the extent to which tumor position correlates with the respiration
monitor or method being used. This is a matter of intense investi-
gation at the time of writing. Observations have been made that call
the exactness of the correlation into question. However, I believe that,
in the majority of instances, breath gating can significantly reduce the
amount of motion and permit tighter field margins to be used, even if
they are not as tight as might be possible with more complex appro-
aches such as tumor tracking.
C
OMPENSATION FOR
P
ATIENT AND
O
RGAN
M
OTION
For any given set of patient immobilization and patient and organ
localization techniques, there always remains some degree of residual
motion and some uncertainties about where the patient, target
volume(s), and OARs are located relative to the treatment couch top.
These uncertainties must be taken into account in planning the
treatment.
Adding lateral margins to the beam
When a uniform beam is directed at a target volume with the intention
of irradiating the entire target volume, there are two factors that affect
the size that the beam must have, namely:
1.
Motion, in a broad sense, of the CTV with respect to the beam
that has two components: intra-fractional patient and organ
motion that gives rise to the ITV; and equipment set-up errors
that, added to the ITV, constitute the PTV (see Chapter 3).
