Biomedical Engineering Reference
In-Depth Information
I
MMOBILIZATION
It is common to employ some method of immobilization to better
relate the patient to the treatment equipment. In some special cases,
the immobilization device is built into the equipment. Usually, a
separate device is used and placed upon the couch top or, much less
commonly, treatment chair, often being indexed through the use of
locating pins.
An immobilization device is used to hold the patient as a whole in a
stable and near-motionless position during both imaging and treat-
ment. By doing so, the locations of internal organs and the tumor are
also constrained. The use of immobilization devices at the time of
acquiring the planning CT serves to minimize the problem of the
patient's position during treatment being different from the patient's
position during the imaging studies.
The two-joint rule
One might think that all that is needed is to immobilize the body part
within which the tumor lies. However, the adjacent body parts
usually have an influence on the part to be immobilized and
themselves need to be immobilized. For example, in treating prostate
cancer, the positions of the upper and lower legs, and their degree of
rotation are important in achieving reproducible positioning. A good
rule (attributed to Verhey) is that body parts that are at least two
”joints” away from the part within which the target volume lies need
to be immobilized. This rule is illustrated schematically in Figure 7.2.
Figure 7.2. Schematic illustration of the two-joint rule.
The target volume is within the cranium, but both the
neck and torso should be immobilized (the blue support
in this figure), as well as the head.
Immobilization techniques
A review of immobilization methods in radiation oncology can be
found in Verhey and Bentel (1999). Many types of immobilization
device are available, including bite-block/head-rest combinations for
stabilizing the head, partial body casts for stabilizing the thorax or
