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Comparison of MLC and Treatment-Couch Tracking for Motion Mitigation During Prostate SBRT

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S Ehrbar

S Ehrbar1*, S Schmid1 , A Joehl1,2 , S Kloeck1 , M Guckenberger1 , O Riesterer1 , S Tanadini-Lang1 , (1) Department of Radiation Oncology, University Hospital Zurich and University of Zurich, Switzerland (2) Product Development Group Zurich, Department of Mechanical and Process Engineering, ETH Zurich, Switzerland


SU-E-605-7 (Sunday, July 30, 2017) 1:00 PM - 1:55 PM Room: 605

Purpose: Intra-fractional prostate motion is more relevant for SBRT treatments and might lead to target miss or overdosage of surrounding organs at risk. Motion mitigation can be performed at a conventional linear accelerator by following the motion with the multileaf-collimator (MLC tracking) or by counter-motion with the treatment couch (Couch tracking). The dosimetrical and geometrical performance of both motion mitigation techniques was compared.

Methods: For ten prostate cancer patients SBRT treatment plans with integrated boosts (prostate: 5x7 Gy, index lesion: 5x8 Gy) were prepared and applied to a dosimetric phantom (Delta4, Scandidos, Sweden). The phantom was moved according to five prostate motion curves (i) without motion compensation, (ii) with real-time compensation using Couch tracking or (iii) MLC tracking. The rectum gamma agreement (γ1%/1mm) and the prostate D₉₅ were evaluated and compared to a static measurement. For the geometrical evaluation, a lead ball was placed in the beam isocenter and moved under the above conditions while MV images were taken. These MV images show the field edges in respect to the lead ball. The over- and underexposed areas were evaluated by comparison with static reference images.

Results: Over all patients and trajectories, the median (quartiles) over-/underexposed area was reduced significantly from 2.02 cm² (1.55, 2.51) without motion compensation to 0.45 cm² (0.40, 0.54) with Couch tracking and to 1.00 cm² (0.77, 1.21) with MLC tracking. The median rectum γ1%/1mm was improved significantly from 64.2% (47.3, 88.9) without compensation to 100% (100, 100) with Couch tracking and to 99.9% (98.8, 100) with MLC tracking. The median prostate D₉₅ was significantly improved with Couch and MLC tracking.

Conclusion: Couch tracking and MLC tracking were both able to improve the accuracy of prostate SBRT in the presence of motion, whereby Couch tracking showed slightly better agreement with the static references than MLC tracking.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by the grant CR32I3_153491 of the Swiss National Science Foundation.

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