ESTRO 36 Abstract Book

S146 ESTRO 36 2017 _______________________________________________________________________________________________

algorithms. This work investigates the feasibility of a- priori estimation and correction of OOPM. Material and Methods Data from a thoraco-abdominal numeric MRI phantom developed in-house were used 2 . A 10-phases 4DMRI, simulating the planning dataset, was registered to the exhale volume using 3D optical flow 3 , thus measuring in- plane motion (IPM 3D P ) and OOPM P along the three orthogonal slices intersecting in the GTV. In addition, IPM 2D P was obtained with 2D slice-to-slice optical flow 3 registration and the difference C = IPM 3D P − IPM 2D P represented the phase-specific a-priori correction. A 36-frames volume sequence (duration 5.4s) represented treatment data: sagittal/coronal/axial slices simulated cine-MRI sequences, whereas 3D volumes served as ground-truth. The diaphragm position measured on each sagittal slice was used to identify the corresponding breathing phase within the 4DMRI. Each axial and coronal slice of the sequence was registered to the corresponding exhale slices of the 4DMRI (IPM 2D T ) and the phase-specific correction was applied (IPM COR T = IPM 2D T + C). The average end-point distances (EPD) against ground-truth IPM (obtained through 3D registration) were measured with and without correction. OOPM was estimated for each frame as OOPM P measured in the corresponding 4DMRI phase. Finally, the planning GTV was propagated from the 4DMRI exhale phase to each treatment frame using: (1) IPM 2D T with OOPM = 0 and (2) IPM COR T combined with OOPM P . Dice indexes against ground-truth GTVs were calculated for both scenarios. The sagittal slice, showing OOPM < 1 mm, was excluded from the analysis. Results GTV motion amplitude was (4.0, 1.7, 0.2) mm (SI, AP, LR) in the 4DMRI and (5.1, 1.2, 0.6) mm in treatment data. Fig.1 reports EPDs and Dice indexes as a function of the ground-truth OOPM. On average, the a-priori correction/estimation approach resulted in EPD reduction and in Dice index increase with respect to the scenario without IPM correction and OOPM estimation (Tab.1).

Conclusion A-priori information from 4DMRI provides a breathing phase-specific approximation of OOPM and can be used to correct OOPM in slice-to-slice registrations. Such procedure significantly improved GTV position estimation when relevant OOPM is observed, i.e. on the axial slice. The corrected IPM represents a more accurate approximation of the motion field that would be measured if full 3D volumes were acquired and registered in real- time to the planning data. Future work should focus on robustness to inter-fraction variations in patients’ data. [1]Mutic et al 2014 Semin Radiat Oncol [2]Paganelli et al 2015 MICCAI [3]Zachiu et al 2015 PMB PV-0283 Gated liver SBRT based on internal electromagnetic motion monitoring E. Worm 1 , M. Høyer 2,3 , R. Hansen 1 , L.P. Larsen 4 , B. Weber 1 , C. Grau 1,3 , P. Poulsen 1,3 1 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark 2 Aarhus University Hospital, The Danish Centre for Particle Therapy, Aarhus, Denmark 3 Aarhus University, Institute of Clinical Medicine, Aarhus, Denmark 4 Aarhus University Hospital, Department of Radiology, Aarhus, Denmark Purpose or Objective To present our results with the new technique of respiratory gated liver SBRT based on internal electromagnetic motion monitoring. The study presents the geometric and dosimetric improvements in treatment accuracy of the gating compared to standard CBCT-guided Thirteen patients with primary liver cancer or metastases had three electromagnetic transponders (Calypso) implanted near the target and received three-fraction gated liver SBRT at a TrueBeam Linac. The PTV was created by a 5mm axial and 7mm (n=10) or 10mm (n=3) cranio-caudal (CC) expansion of the CTV as defined on an exhale breath-hold CT. A mean homogenous dose between 45 and 61.8Gy was prescribed to the CTV using 7-field IMRT or 3D conformal planning. The PTV was covered with 67% of the prescribed dose. Treatment was delivered in free-breathing but gated to the exhale breathing phase according to the continuously monitored (25Hz) transponder centroid position. Gate ON windows were set to +/- 3mm LR/AP and +/-4 mm CC around the exhale position of the transponders. The couch was adjusted remotely if baseline drifts above ~1mm of the exhale transponder position occurred. Post-treatment, log files of non-gated treatment. Material and Methods