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Motion Representation in 4DCBCT: Impact of Reconstruction Algorithms and Surrogates

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E Steiner

E Steiner1*, C Shieh1 , V Caillet2 , N Hardcastle2 , C Haddad2 , T Eade2 , J Booth2 , P Keall1 , (1) University of Sydney, Camperdown, NSW, (2) Royal North Shore Hospital, Saint Leonards, NSW


SU-K-FS4-4 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: Four Seasons 4

Purpose: Lung tumour motion exceeding the motion from planning 4D computed tomography (4DCT) is of concern in stereotactic lung radiation therapy. 4D cone-beam CT (4DCBCT) facilitates the verification of tumour trajectories before each treatment fraction. This work aims to assess the impact of the reconstruction algorithms and surrogates for binning on the motion representation in the 4DCBCTs utilizing lung-implanted Calypso beacons.

Methods: 4DCBCTs were reconstructed from 60s-treatment-setup CBCTs for 1-2 fractions of 6 patients using the prior image constrained compressed sensing (PICCS) method and the FDK (Feldkamp-Davis-Kress) method. The internal Calypso motion trajectories (three beacons per patient) or an external respiratory signal (Philips Bellows belt) served as the motion surrogates for binning. The Calypso beacons were segmented for all 10 bins of the 4DCBCTs. The Calypso-measured beacon centroid motion was compared to the motion range from the volumetric images. In order to identify a reconstruction method optimally capturing the true motion, paired t-tests were performed on the extent and time of beacon motion exceeding the motion ranges from the reconstructed 4DCBCTs.

Results: All methods for 4DCBCT reconstruction failed to capture sudden motion peaks during scanning and underestimated the actual beacon centroid motion, especially in LR and AP direction. For the SI direction in general, reconstructions using the belt signal, led to a representation of a larger motion range (PICCS: 4.9±3.3mm, FDK: 4.8±3.4mm) than the Calypso-based reconstruction (PICCS: 4.7±3.2mm, FDK: 4.8±3.3mm). The difference between all the methods was not significant for every performed t-test for the SI, LR and AP directions.

Conclusion: All 4DCBCT reconstruction methods failed to represent the full tumour motion range, but performed similarly. For a safe treatment in spite of motion exceeding the motion range from the images, adequate ITV-to-PTV margins or a real-time treatment adaptation directly tackling motion peaks and unpredictable motion need to be chosen.

Funding Support, Disclosures, and Conflict of Interest: The LIGHT-SABR trial is partially funded by Varian Medical Systems.

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