Biomedical Engineering Reference
In-Depth Information
which it passes. In a short exposure, the transmitted beam will make
an imprint on a film or digital imaging plate of the anatomy included
within the beam. This radiograph can be compared with a DRR
designed by the treatment planning system in the beam's-eye view,
and any needed adjustments made before treatment proceeds.
Sometimes a double exposure is made consisting of one exposure of
the treatment field and a second with the field opened up
−
for
example by opening up the multi-leaf collimator jaws. A double-
exposure film provides visualization of anatomic landmarks close to
but outside the beam, but has the disadvantage that radiation is
delivered to tissues outside the target volume which do not require
it. Unfortunately, the quality of radiographs made by photons of
therapeutic energy is very inferior to those made with diagnostic
X-rays, as alluded to in Chapter 4.
Verification using X-radiography
It is feasible to employ a pair of X-ray tubes mounted in the treatment
room, in a known relationship to the treatment equipment, and
directed toward the isocenter. These X-ray tubes need not necessarily
be directed orthogonally to one another (Schweikard
et al
., 2004).
The radiographs thus obtained can be compared with DRRs from the
same radiographic viewpoints, computed by the treatment planning
program. It is also possible to provide fluoroscopic imaging and
hence real-time localization during treatment to adjust the position of
the patient relative to the beam in real-time.
O
RGAN
M
OTION
Organs and tissues both move within the body and change size and
shape, both during the delivery of a single fraction (intra-fraction
motion), and over the course of the entire therapy (inter-fraction
motion). Motion poses a number of problems, chief among which
are: (1) the imaging study or studies upon which the treatment
plan is based (that can either be at a single moment in time, or an
average/distortion over time) are inaccurate and hence give a false
picture of the anatomy; (2) larger fields are needed than the size
of the CTV would seem to require, and hence more normal tissue is
irradiated than would otherwise be necessary; and (3) if the extent of
the motion is not fully appreciated, fields may be designed too small
with the danger that parts of the tumor my be underdosed.
Inter-fraction movement, including size and shape changes, of the
tumor and/or organs may take place on a day-to-day or week-to-week
