MOTION MANAGEMENT - Radiation Oncology: A Physicist's-Eye View - page 140

pelvis, and whole body casts. Casts may be made, inter alia : from

plaster of Paris, using conventional moulage techniques; from ther-
moplastic sheets that are draped over the patient while warm and

become firm upon cooling; and from bags of foam pellets, that are

made rigid by being placed under vacuum once the bag is made to
conform to the patient's surface.

Some of the more common immobilization techniques are the

following.
Perforated thermoplastic masks

A perforated thermoplastic sheet

is formed to the patient's head
while warm and allowed to set

by cooling. The perforations keep

the head cool, are less claus-

trophobic for the patient, and

preserve some degree of skin

sparing. The sheet is captured
in a tennis racket-shaped frame
that is attached to the couch

Figure 7.3. The first use of per-
forated thermoplastic masks was

top or chair with the use of

for proton therapy. Since then more
elegant commercial versions are
now widely available. Reproduced

indexing pins (Verhey et al. ,

1982) - as illustrated in

with permission from Verhey et al.

Figure 7.3.
Whole body support
An individualized whole body support can be made from a plaster
cast. This is an effective approach, but is somewhat labor-intensive

(1982).

and has largely been replaced by the use of a plastic bag, filled with

foam pellets, upon which the patient lies. Once the patient is in the

desired treatment position, the bag is made stiff by pulling a near-

vacuum on it. There is a danger that the will be punctured, in which

case it will lose its vacuum and, hence, shape, and the whole scanning

and planning process must be repeated. Loss of vacuum, however,

seems to be a rare event. These bags are bulky and require a lot of

storage room, but otherwise are very convenient. They automatically
satisfy the two-joint rule.
Bite block

The patient's head can be well immobilized through the use of

a bite block fixed to the treatment equipment (e.g., to the couch

top as illustrated in Figure 7.4. While usually providing good