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Simultaneous MLC and Couch Tracking: A Clinical Feasibility Study

L Zhang*, T LoSasso , P Zhang , G Mageras , G Tang , Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, NY


(Thursday, 8/2/2018) 7:30 AM - 9:30 AM

Room: Karl Dean Ballroom A1

Purpose: MLC and couch compensation are two promising strategies for real-time tumor tracking. However, MLC alone may be inefficient in tracking motion perpendicular to leaf travel direction whereas rapid couch correction poses concern in patient comfort and reaction. This study investigates the feasibility of simultaneous MLC and couch tracking and its dosimetric impact for GI diseases using iTools Tracking prototype in TrueBeam Developer Mode.

Methods: Currently, TrueBeam does not allow simultaneous MLC and couch tracking. This study was based on sequential measurements using a motion phantom combined with simulations. Two pancreas VMAT plans were used, each with a pair of arcs (collimator at 0° and 90°). A 3D tumor trajectory was decomposed into 2 sets of motion axes - motion parallel to MLC leaf travel (L-R or S-I direction) and motion in the remaining directions. Each arc was delivered twice: 1) with MLC tracking the motion along the leaf travel direction, and 2) with couch tracking the remaining motion. Trajectory logs were collected and converted into DICOM plans. The target motion during each delivery was retrospectively synchronized to the trajectory logs and integrated to the DICOM plans as isocenter shift at each control point. The dose distribution with simultaneous MLC and couch tracking was calculated and compared to other tracking strategies.

Results: Simultaneous tracking significantly decreased the local dose errors compared to MLC tracking. The dosimetric accuracy is similar to couch tracking, but with greatly reduced rapid couch motion especially for the arcs with 90° collimator angle. The gamma passing rate (3%/3mm) was 100.0% ± 0% for both simultaneous tracking and couch tracking, and 76.7% ± 17.1% with MLC tracking.

Conclusion: This study illustrated the clinical feasibility of employing MLC and complementary couch tracking simultaneously to ensure accurate dose delivery with reduced couch motion and minimum perturbation in patient comfort.

Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by Varian Medical Systems.


Image-guided Therapy, Radiation Dosimetry, Organ Motion


TH- External beam- photons: Motion management (intrafraction)

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