Biomedical Engineering Reference
In-Depth Information
immobilization,
this
device
suffers from the problem that it
is of questionable value in the
edentulous patient and may
place some strain on the patient.
These problems can be largely
overcome through the use of
vacuum suction on a bite block
made to conform to the patient's
palate.
Stereotactic head holder
Figure 7.4. Immobilization using a
dental bite block.
A stereotactic frame is a cube-
like frame which surrounds and is attached to the head by pins set into
burr-holes made in the skull. It serves both to immobilize the head
(when it itself is held firm) and to provide fiducial landmarks relative
to which the target volume can be located in imaging studies and for
treatment. This device was first used in external proton beam radio-
therapy for the irradiation of intracranial targets such as the pituitary
gland (Kjellberg
et al
., 1962). Similar but noninvasive devices, where
frame fixation is accomplished through the use of a bite block and
pressure points, are also used now.
When an immobilization device is used, one has to be aware that it
may affect the patient's reaction to the delivered dose. For example,
some skin-sparing is lost when a mask is used, and the use of a bite
block may exacerbate mucosal reactions to radiation.
L
OCALIZATION
Once the patient has been adequately immobilized, the next step is to
locate the target volume in space, relative to the treatment equipment.
This requires that: a) the patient be located reproducibly relative to
the treatment equipment; and b) the target volume be in a known
spatial relationship to the patient. The latter is generally based on
imaging studies, as discussed in Chapter 3. There are four general
approaches to localization that are described below.
Localization based on skin marks
In some not very common circumstances such as cancers of the skin,
lip, etc., when the target volume is relatively superficial, the target
