ESTRO 36 Abstract Book

S148 ESTRO 36 _______________________________________________________________________________________________

the transponder motion and treatment delivery were used to calculate the motion-induced geometrical errors during beam-on in the actual gated treatments and in simulated non-gated standard treatments with CBCT-guided setup to the mean transponder centroid position before each fraction. The observed motion was used to reconstruct the actually delivered CTV dose distribution with gating and the would-be dose distribution without gating. Results Fig. 1 shows the internal tumor motion during a single fraction. Due to drift and respiratory motion the mean (+/- SD) geometric error during non-gated treatment at this fraction (Fig1A) would have been 1.3mm (1.7) LR, 5.0mm (7.7) CC, and -2.0mm (1.8) AP. The gated treatment, including 5 couch shifts to counteract drift (Fig1B), reduced the errors to 0.7mm (0.7) LR, 0.4mm (1.9) CC, and -0.1mm (0.9) AP. Fig. 1C shows the CC geometrical errors for all patients. The mean (range) number of couch corrections for drifts during each gated fraction was 2.8 (0-7). The mean duty cycle during gated treatment was 60.8% (31.7-72.7%). As shown in Fig 2A, gating markedly reduced the population based PTV margin needed for intrafraction motion. Motion-including dose- reconstruction provided the CTV-DVHs of all fractions of planned, actual gated delivered, and simulated non-gated delivered doses. Mean CTV-DVHs are shown in Fig 2B. Note the large DVH variation for non-gated treatments. The mean (range) reduction in CTV D 95 relative to the planned dose was 0.9 percent points (0.1-2.3) with gating and 6.8 percent points (0.9-29.6) without gating. Conclusion Gating based on internal motion monitoring markedly reduced geometric and dosimetric errors in liver SBRT compared to non-gated standard treatment. Results of the full trial (15 patients) are expected for presentation at ESTRO.

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