MOTION MANAGEMENT - Radiation Oncology: A Physicist's-Eye View - page 144

can be compared with the CT image set used for planning, and the
geometric differences between the bony (or other) anatomy in the two
studies can be used to compute a positioning correction, using the
techniques of image registration described in Chapter 3.

Localization relative to the immobilization device

Fiducial markers embedded in the immobilization device can be used
in the same fashion as bony landmarks. Because fiducial markers can

generally be located very accurately, localization in such cases can be

more accurate than when using bony landmarks provided that the

patient is securely and reproducibly held in the immobilization device

−

as, for example, when a stereotactic head holder is used. It is im-
portant that this proviso be satisfied; otherwise, there is a danger of

treating the immobilization device rather than the patient.

Localization based on identification of target markers or the tumor itself

In some circumstances, radiographically visible objects such as gold

seeds or surgical clips can be deliberately or fortuitously embedded

in or close to the tumor. These can be radiographically localized -

and even tracked during radiation delivery. For example, gold seeds

have been introduced into the prostate for the purpose of tumor

localization (Shipley et al ., 1979) and tantalum clips are sutured to the
sclera of the eye in treating uveal melanoma as described in Chapter
11. In some cases, the GTV itself may be visible, for example when
using ultrasound to locate the prostate just before treatments. Such

techniques provide accurate target volume localization. The locali-

zation process in the case of radioopaque markers follows that for

bony landmarks.

V ERIFICATION
Once the patient has been positioned for treatment, it is desirable to
verify the alignment of the beam relative to the target volume. It may
also be desirable to determine, after the treatment has taken place,
whether or by how much the patient has moved during treatment.
Comparison of “before” and “after” measurements can provide
valuable information on the efficacy of the immobilization techniques
(Verhey et al ., 1982; Verhey and Bentel, 1999).

Verification using portal radiographs

The most direct method of verification for photon treatments is to use
the therapy beam itself to make a radiograph of the anatomy through