Volumetric modulated arc therapy: IMRT in a single gantry arc
Continuous source and MLC motion is initially modeled as a series of static source positions in (a). As the optimization progresses new samples are introduced with the first new sample placed between the first and second existing samples in (b). Further samples are added in (c) to span the full range of gantry motion. The next sample is added at the beginning of the gantry range in (d). Samples are continuously added in this way until a desired sampling frequency is reached.
The percentage volume (target and critical structures) exceeding , , and dose error is shown as a function of the gantry and maximum MLC leaf sample spacing.
Optimization CPU time is plotted as a function of single arc gantry angle sample spacing and maximum MLC sample spacing for desired final cost values of 400 and 1200.
Final cost values are plotted as a function of gantry angle sample spacing and maximum MLC sample spacing for plans optimized with fixed sampling. A plan with the VMAT progressive sampling technique (final gantry and MLC maximum sample spacing of 1 deg and 0.5 cm, respectively) generated for the same patient is shown for comparison.
VMAT and Eclipse isodoses ( of PTV70, PTV56, and 31.5 Gy) are shown on a representative CT slice for the nasopharynx example. Dose distributions have substantially different shapes due to the use of 360 deg of gantry rotation with VMAT and seven fixed beam directions with Eclipse.
DVHs for a nasopharynx cancer test patient (VMAT and Eclipse). Overlapping PTV DVHs show that almost identical target coverage is attained. VMAT critical structure DVHs show a substantial improvement over fixed gantry IMRT.
The posterior beam’s eye view of the VMAT instantaneous MLC aperture shape visualized with target and healthy tissue structures at that beam direction. In (a) the MLC is shown transparent to the targets (light gray or green), spinal cord, brainstem, and parotid glands (dark gray or red). In (b) the MLC leaves are opaque, showing that the critical structures are effectively blocked while the targets are receiving dose from the open beam. Note that the collimator is rotated to 45 deg in this case.
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