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BEST IN PHYSICS (THERAPY): MLC Tracking for Lung SABR Reduces the Dose to Organs-At-Risk and Improves the Geometric Targeting of the Tumour

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V Caillet

V Caillet1,2*, B Zwan3,6 , N Hardcastle1 , Ricky O Brien1, P Poulsen4 , P Greer5,6 , P Keall2 , J Booth1,2 (1) Northern Sydney Cancer Centre, Sydney, Australia, (2) School of Medicine, University of Sydney, Australia (3) Central Coast Cancer Centre, Gosford, NSW, (4) Aarhus University Hospital, Aarhus C (5) Calvary Mater Newcastle, Newcastle (6) Faculty of Science and IT, School of Mathematical and Physical Sciences, The University of Newcastle, Newcastle, Australia


MO-DE-FS1-7 (Monday, July 31, 2017) 1:45 PM - 3:45 PM Room: Four Seasons 1

Purpose: This study assesses the performances of real-time MLC tracking of lung tumors in terms of reconstructed delivered dose distributions and tracking errors. Two hypotheses were tested. (1) The delivered MLC tracking plan reduces dose to the healthy tissue. (2) 2D beam-target error is improved with tracking.

Methods: The performance of MLC tracking delivery and ITV-based treatment were compared by determining the patient delivered dose and calculating the beam-target alignment, and compared against standard. (1) For the target and lung, an isocentre shift algorithm reconstructed the delivered dose. Spine, heart, esophagus, and trachea reconstructed plans were not motion encoded and were simulated with static motion. (2) In the presence of motion, tracking error gives rise to misalignment between the actual tumour motion and delivered MLC field, which was compared for each control point with the error that would have been without tracking between the actual tumour motion and the planned stationary field.

Results: Seven patients treated with MLC tracking on an ethics approved clinical trial (LIGHT SABR NCT02514512) were analyzed for this study. (1) Target dose was achieved for both plan types for each patient. Estimated delivered doses for MLC tracking showed lower OAR doses than estimated for ITV-based treatment plans. Organ-at-risk dose metric reductions were: mean lung dose reduction of 0.4 Gy; heart (D1cc) of 0.4 Gy; spine (D0.03cc) 0.9 Gy; trachea (D4cc) 0.6 Gy and esophagus (D3cc) 0.9 Gy. MLC tracking dose metrics were all significantly smaller than ITV-based plans (Wilcoxon test, p<0.05, one sided). (2) Geometric targeting error were smaller than the would-be error without tracking, the latter being equal to the actual tumour motion.

Conclusion: Real-time adaptive radiotherapy with MLC tracking of lung tumors can minimize the dose delivered to the OAR and improve the geometric alignment between the target and the MLC aperture.

Funding Support, Disclosures, and Conflict of Interest: Author Keall is an inventor on issued patents related to the MLC tracking technology that is licensed from Stanford University to Varian Medical Systems. Authors Keall and O Brien are inventors on an issued patent related to the KIM technology that is unlicensed.

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