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VMAT Performance Assessment for Novel Straight-Through Linac with Prototype Fast Jawless MLC Collimation and Rapid Gantry Rotation

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C Kennedy

C Kennedy*, R Scheuermann , M Constantin, S Anamalayil , D Mihailidis , L Dong , J Metz , University of Pennsylvania, Philadelphia, PA


MO-F-FS1-9 (Monday, July 31, 2017) 4:30 PM - 6:00 PM Room: Four Seasons 1

Purpose: Precise control of dose rate, gantry speed, and MLC speed are critical for accurate delivery of Volume Modulated Arc Therapy (VMAT). We report testing results for a prototype jawless, straight-through linear accelerator capable of gantry speeds up to 4 RPM, and MLC speeds up to 5 cm/s during VMAT delivery.

Methods: Two methodologies previously described in the literature were used for evaluation: 1) Patterns of equivalent dose delivered under varying levels of dose rate, gantry speed, and MLC speed were captured by the portal imager, and normalized to an open-field image. Three VMAT test plans provided by the vendor delivered dose rate levels from 128-800 MU/min, gantry speeds from 3.7-23 deg/s, and MLC speeds from 1-5 cm/s respectively. 2) Imaging of a couch-mounted fiducial marker at a known off-isocenter location by a narrow field defined by the MLC (“Snooker Cue”) to assess angular accuracy of VMAT delivery. Portal images were acquired for dose delivered around four discrete gantry angles for 180 degree VMAT fields, delivered in both clockwise and counter-clockwise directions. Accurate delivery was indicated by the fiducial appearing in the center of the MLC aperture. In-house generated treatment plans were developed with varying degrees of MLC delay between control points to test system performance. Induced errors of one and two degrees were performed to demonstrate test sensitivity. Log files acquired at 20ms frequency were analyzed as an additional check of VMAT accuracy.

Results: The maximum deviation for any pattern was 1.9% when normalized to open field. All “Snooker Cue” images show the fiducial located near the center of the MLC aperture. Log file analysis revealed a maximum MLC error of 0.2 mm, 0.21 MU, and 0.09 degrees for any sample during delivery.

Conclusion: Accurate control of VMAT delivery was demonstrated for this prototype fast-MLC/fast-gantry system.

Funding Support, Disclosures, and Conflict of Interest: This work was performed as part of a research agreement with Varian Medical Systems.

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