Biomedical Engineering Reference
In-Depth Information
diaphragmatic movement. In the absence of special measures, the
only way to deal with situations in which large excursions can occur
is to allow generous margins in delineating both the PTV(s) and
PRVs. It is quite possible that, where the extent of motion or of the
artifacts that it produces has been underappreciated, the probability of
local control has been compromised (Ling
et al
., 2004).
Organ motion with respiration gating
The most obvious and simplest way to handle respiratory motion is to
track the respiratory cycle, identify the phase(s), usually during
expiration or quiet breathing, where motion is least, and turn the beam
off (gate the treatment) during the other phases.
Respiratory gating (Ohara
et al
., 1989) uses an external breathing
monitor to gate the radiation beam on and off at a well-defined phase
of the breathing cycle. An example of such a monitor is a light
emitting diode or other optical target, placed on the patient's
abdomen, whose position is monitored by video cameras, while the
patient breathes. The diode position may be used to gate CT or
fluoroscopic data acquisition, and may be used during treatment to
gate the accelerator beam, thus reducing the effect of respiratory
motion by synchronizing the dose delivery to the patient's breathing
cycle. A wide variety of position monitoring devices have been used,
including a strain gauge or linear transducer attached to the abdomen
or thorax, or a temperature sensitive device inserted in the nostril.
Stereo-photogrammetric cameras, mentioned above, can also be used
to monitor respiration.
There are also approaches that seek to actively control the flow of air
to the patient, and hence tumor motion. These include: deep breath
hold at inspiration controlled by the patient, viewing a signal from a
spirometer; and active breathing control, an approach in which the
patient breathes through a mouthpiece connected to a pair of flow
monitors and valves which are closed at a preselected phase in the
respiratory cycle.
Organ motion with tumor tracking
A problem with the above techniques is that they reduce efficiency, as
radiation can only be delivered during a portion of the patient's
breathing cycle, or the irradiation must be interrupted between breath
holds. In addition, they rely on measurements of the relative posi-
tions of the tumor and the normal anatomy which are made well in
advance of treatment. These measurements are assumed to apply at
