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TomoTherapy MLC verification using exit detector data
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View: Figures


Image of FIG. 1.
FIG. 1.

An example of the TomoTherapy raw detector signal. The enlarged view shows six projections. Arrows points to 7 open leaves in one of the projections. Each leaf produces vertical streaks that start at leaf open events and stop at leaf close events. Scatter signal contributions produced from the opened leaf are visible across the closed leaves.

Image of FIG. 2.
FIG. 2.

Example of detector signals around one projection. Left graph shows the signal in log scale (dB scale). A snap-shot of the signal in the middle of the projection (indicated with a horizontal line) is plotted in the central graph. Ticks divide detector channels into small groups that corresponding to individual leaves in both graphs. The signal amplitude vs time for leaf i and i + 1 is plotted in the right graph. For leaf i + 1, which is closed at all times, the scattered signal from the adjacent open leaf shows a signal pattern that can be misinterpreted as a leaf opening and closing.

Image of FIG. 3.
FIG. 3.

The modeled point spread function used in the R–L deconvolution.

Image of FIG. 4.
FIG. 4.

Detector signal shown in Fig. 2 deconvolved. The same section of signal is graphed in the same log scale (dB scale) in the left graph. In the central graph, a snap-shot of the deconvolved signal in the middle of the projection is shown as a solid curve with the raw signal shown as a dotted curve for reference. The deconvolved signal amplitude vs time for leaf i and i + 1 is plotted in the right graph. The signal from the scattering contamination for closed leaf i + 1 is greatly reduced.

Image of FIG. 5.
FIG. 5.

Binning of detector signal into projections. The top graph shows the detector signal with the preliminary projection boundary with dashed vertical lines. The projection boundary is set after the removal of the mean negative latency. The shaded region is the ±6 pulse region used to refine the projection boundary and is magnified at the bottom graph. Dashed lines in the bottom graph indicate the corrected projection boundaries.

Image of FIG. 6.
FIG. 6.

Schematics of the operation of an optical MLC sensor. As shown in the figure, both optical sensors are un-blocked when the leaf is in a closed position.

Image of FIG. 7.
FIG. 7.

Example of raw detector signal when there is no attenuation in the beam versus when an anthropomorphic torso phantom (Supertech CIRS 600) is in the beam. The attenuation variation in the beam causes the variation in the detector signal amplitude.

Image of FIG. 8.
FIG. 8.

Plot of leaf open times reconstructed from temporal changes in the detector signal versus commanded leaf open times.

Image of FIG. 9.
FIG. 9.

Plot of leaf open times reconstructed from temporal changes in the detector signal versus optical sensor-measured leaf open times.

Image of FIG. 10.
FIG. 10.

Error histograms for leaf open times estimated with and without attenuation present.

Image of FIG. 11.
FIG. 11.

Example of dose difference computed between the exit detector-reconstructed dose and the planned dose for one clinical case that failed DQA. The prescription dose for this case was 70 Gy. The detector-reconstructed dose agrees with ion chamber measurements.

Image of FIG. 12.
FIG. 12.

Comparison between planned dose, ion chamber measured dose and DV reconstructed dose before and after pitch change. Pitch values are shown at the bottom of the bar graphs. The reconstructed dose agrees with measured dose to within 1% for all 3 patients both before and after pitch change.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: TomoTherapy MLC verification using exit detector data