ESTRO 36 Abstract Book

S467 ESTRO 36 _______________________________________________________________________________________________

Image analysis: The two dynamic MR series were analyzed separately to quantify typical one minute tumor displacements along two orthogonal directions; superior (S), inferior (I), anterior (A) and posterior (P). All time- points affected by non-respiratory associated tongue motion or deglutition were manually discarded from the analysis. One time-point was selected as the reference and all other points were non-rigidly registered to the reference using a validated optical flow algorithm [1]. Motion fields were computed for all the pixels inside the tumor and combined into a single distance metric by assessing the maximum contour coordinates (FIG1-A+B). Typical 10-minutes displacements were investigated by computing the difference Results Mean maximum 1-minute tumor displacements amounted to 2.08 (SD 2.34) mm in (S), 2.26 (SD 1.48) mm in (I) and 1.66 (SD 0.93) mm in (A); 1.65 (SD 1.23) mm in (P) (FIG1- D). However, there was strong inter-subject variability within the laryngeal and oropharyngeal subgroups, with laryngeal tumors exhibiting periodic displacements up to 14 mm in (S) (FIG1-C). The typical 10 minute shifts were smaller than 2 mm for all patients (not shown in figure), with means values of 0.62 (SD 0.44) mm in (S) ; 0.64 (SD 0.58) mm in (I); 0.49 (SD 0.51) mm in (A); 0.41 (SD 0.36) Although tumor displacements were small, there were three subjects that exhibited resting-state displacements larger than 5 mm. This suggests individualized ITVs for the laryngeal tumors and oropharyngeal tumors, instead of applying 5 mm margins in both I and S directions for laryngeal tumors. The 10-minutes intrafraction shift was smaller than 2 mm across all the patients and directions and did not show any outliers. mm in (P). Conclusion

the 4DCBCTs using implanted Calypso beacons in the lung as ground truth. Material and Methods 4DCBCTs were reconstructed from projections for treatment setup CBCT for 1-2 fractions of 6 patients using the prior image constrained compressed sensing (PICCS) method and the FDK (Feldkamp-Davis-Kress) method. For both methods reconstructions were performed based on the internal Calypso motion trajectories (three beacons per patient) or an external respiratory signal (Philips Bellows belt). The Calypso beacons were segmented for all 10 bins of the 4DCBCTs and the beacon centroid motion compared to the motion range from the images. Paired t- tests were performed on the mean size of excess beacon motion and the proportion of scanning time with motion larger than represented in 4DCBCT in order to identify a superior reconstruction method. Results All methods for 4DCBCT reconstruction failed to capture sudden motion peaks during scanning and underestimated the actual beacon centroid motion (see Fig. 1), which is a result of phase-based binning and averaging the images in a bin. For the SI direction in general, reconstructions using the belt signal, led to a representation of a larger motion range (PICCS: 4.88±3.30mm, FDK: 4.81±3.35mm) than the Calypso-based reconstruction (PICCS: 4.71±3.22mm, FDK: 4.76±3.29mm). However, the difference was not significant, as for none of the other directions. For comparison also the Calypso motion during the treatment exceeding the 4DCBCT motion range is shown in Figure 1.

Figure 1. Proportion of intra-CBCT (solid lines) and intra- treatment (dashed lines) motion in SI direction larger than the motion represented in the reconstructed 4DCBCT for reconstruction with a) PICCS Belt, b) PICCS Calypso, c) FDK Belt and d) FDK Calypso. Conclusion All 4DCBCT reconstruction methods failed to rep resent the full tumour motion range , but performed similar. Thus, the belt as an external surrogate is sufficient for 4DCBCT reconstruction. 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. While the 4DCBCT is not able to capture and predict the whole motion range of a treatment fraction, it serves as a valuable tool for accurate patient setup. PO-0860 Characterization of a novel liquid fiducial marker for organ motion monitoring in prostate SBRT R. De Roover 1 , W. Crijns 2 , K. Poels 2 , R. Peeters 3 , K. Haustermans 1,2 , T. Depuydt 1,2 1 KU Leuven - University of Leuven, Department of Oncology, Leuven, Belgium 2 University Hospitals Leuven, Department of Radiation Oncology, Leuven, Belgium 3 University Hospitals Leuven, Department of Radiology, Leuven, Belgium

PO-0859 Impact of 4DCBCT reconstruction algorithm and surrogate on motion representation E. Steiner 1 , C.C. Shieh 1 , V. Caillet 2 , N. Hardcastle 2 , C. Haddad 2 , T. Eade 2 , J. Booth 2 , P. Keall 1 1 University of Sydney, Radiation Physics Laboratory- Sydney Medical School, Camperdown, Australia 2 Northern Sydney Cancer Centre- Royal North Shore Hospital, Radiotherapy Department, St Leonards, Australia Purpose or Objective Lung tumour motion exceeding the observed motion from planning 4D computed tomography (4DCT) is of concern in stereotactic ablative body radiation therapy (SABR). 4D cone-beam CT (4DCBCT) facilitates verification of tumour trajectories before each treatment fraction and an accurate patient setup. This work aims to assess the impact of the selection of the reconstruction algorithm and surrogate for binning on the motion representation in