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Dosimetric Evaluation of MLC and Couch Tracking On TrueBeam for Abdominal Cancer Treatments

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) 10:00 AM - 12:00 PM

Room: Davidson Ballroom A

Purpose: Real-time tumor tracking aims at mitigating motion effect through active correction by the MLC or couch during treatment delivery. It has the potential to improve dose delivery accuracy while maintaining a high clinical throughput when treating targets with significant intrafractional motion. This study investigates the dosimetric performance of MLC and couch tracking using iTools Tracking prototype in TrueBeam Developer Mode and its clinical feasibility for abdominal tumor treatments.

Methods: Dose measurements with GafChromic EBT3 films were performed using a motion simulation platform which drives an acrylic phantom following 3D tumor trajectories previously acquired from pancreatic cancer patients. Calypso transponders embedded in the phantom provided the 3D target position for tracking. Clinical SBRT VMAT plans for abdominal tumors were delivered under four conditions: 1) delivery without motion; 2) delivery with target motion but no tracking; 3) delivery with target motion and MLC tracking; and 4) delivery with target motion and couch tracking. Dose distributions were quantitatively compared among different delivery scenarios.

Results: Both MLC and couch tracking greatly reduced the motion-induced dose errors. Gamma passing rate (3%/3mm) was significantly better for couch tracking (98.4% ± 0.8%) or MLC tracking (76.8% ± 9.4%) compared to untracked deliveries (39.9% ± 8.9%). Substantial amount of overdosed and underdosed regions were still present with MLC tracking and found to be correlated with the MLC leaf travel direction and the major motion axis.

Conclusion: This study demonstrated the capability of MLC and couch tracking to achieve higher dosimetric accuracy compared to untracked delivery. MLC tracking may be insufficient due to its interplay with planned leaf motion and the leaf speed limitation. Collimator angle played an important role in the MLC tracking performance. Couch tracking outperformed MLC tracking in all plans indicating its clinical potential for treating abdominal cancers.

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


Organ Motion, Radiation Dosimetry, Image-guided Therapy


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

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