An example to illustrate the principle of real-time feedback tracking with DRRT. DRRT does not change the leaf sequence directly; rather, it changes the temporal schedule of the dose delivery. This, in turn, causes the leaf sequence to change in the same way, so as to catch up with any irregular organ motion.
Schematic flow diagram for real-time feedback tracking with DRRT.
An example of tracking. The beam dose rate has been adjusted to change the MLC speed to match the irregular breathing motion. The dose rate was controlled over a range from 0 to . The tracking signal reflects the MLC sequence prepared before treatment; the treatment signal represents the breathing signal on the treatment day. The modified tracking signal (mod track) is the sequence modified during treatment by adjusting the machine dose rate.
Tracking error (or residual motion) for the example shown in Fig. 3.
Films exposed to IMRT fields with and without DRRT for an irregularly moving phantom: (a) A film for a static IMRT with a stationary phantom; (b) a film on a moving phantom which keeps changing its frequency irregularly; and (c) a film on the irregularly moving phantom using DRRT. The result for (c) is essentially the same as that for (a). The dashed lines shown crossing the film images are along the profile axis shown in Fig. 6.
The dose profiles of IMRT fields with and without DRRT. The dose profile with a step-and-shoot field and no phantom motion is shown as the solid line [Fig. 5(a)]; that with no DRRT and with irregular motion is shown as the dotted line [Fig. 5(b)]; and the one using DRRT with the same pattern of irregular motion is shown as the circular markers [Fig. 5(c)]. We see that the curves representing no motion and the use of DRRT are virtually identical.
Integrated gamma histograms for moving phantom film dosimetry for DRRT delivery and for non-DRRT delivery (see text).
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