ESTRO 38 Abstract book

S1059 ESTRO 38

beam time: 100% (no beam interruptions), 90% and 50% ( Fig. 1 ). Results Post-treatment analysis of recorded motion curves was done assuming upper and lower thresholds of 3 mm above normal inhale and below normal exhale, respectively. Normal inhale and exhale was defined as the average maximum and minimum block positions, respectively, based on the first four respiratory cycles following setup. Results are summarized in Fig. 2 , illustrated by a recorded curve. Thresholds are shown as red dotted lines. 19% and 23% of treatment sessions had gradual intra-fractional drift or sudden motion, respectively, which would lead to beam interruption given the above mentioned thresholds. Gamma passing rates for plans delivered to the ArcCheck phantom, using different DC, are shown in Table 1 . A 3%/3 mm criterion was used. Gamma passing rates for plan delivery using different DC differed less than 0.5%, even with only a 50% DC – the latter implying an unlikely high rate of beam interruptions.

It seems feasible to use the TB RGS as a system detecting patient motion during delivery of FB lung SBRT – with the additional benefit of pausing the beam when motion exceeds pre-defined thresholds. Repeated beam interruptions during delivery appear to have little dosimetric impact. EP-1944 Automated respiratory cycle binning for liver 4D-MR imaging B. L'Homel 1 , L. Parent 1 , O. Bieri 2 , Z. Celicanin 3 , P. Cattin 4 , S. Ken 1 1 Institut Universitaire du Cancer de Toulouse, Department of Engineering and Medical Physics, Toulouse, France ; 2 University Hospital Basel, Department of Radiology, Basel, Switzerland ; 3 Intuitive Therapeutics SA, Radiotherapy and Radiosurgery Planning Treatments, Saint-Sulpice, Switzerland ; 4 University of Basel, Department of Biomedical Engineering, Basel, Switzerland Purpose or Objective Stereotactic body radiation therapy (SBRT) has proven its benefit for local control of liver lesions 1 . Because high dose per fraction is delivered on small volume, very accurate lesion segmentation is necessary and respiratory gating enables safe margin reduction on the planning target volume (PTV). MRI is the preferred imaging modality for detection and characterization of hepatic lesions, but it is prone to motion artifacts if breathing is not managed. As external device for breathing movement detection may be inaccurate, the tracking of the sub- diaphragmatic organs movements with very fast 2D sagittal images appeared to be an interesting retrospective strategy using a previously validated 4D-MRI sequence for the liver 2 . In this study, an automated method was developed to sort images according to the respiratory phases with the sub-diaphragmatic organ movements. Material and Methods The 4D-MRI acquisition is performed with an experimental sequence (bSSFP TrueFISP 3 ) on a 1.5T system (Magnetom Aera, Siemens). This sequence allows very fast interleaved axial and sagittal 2D acquisition (0.44 sec/slice) during free breathing. In order to correctly sample the entire breathing magnitude, 20 axial acquisitions are collected every 2.5 mm in the cranio-caudal direction. A sagittal slice (the navigator) is always acquired at the same position in the imaging volume and at 0.44 sec from the associated axial slice. According to the sub-diaphragmatic organs position on navigators, axial slices are automatically sorted in 6 phases (0% = inspiration, 16%, 33%, 50% = expiration, 66%, 83%). Automatic binning is performed by comparing the relative translation (white arrow on Figure 1) of each navigator (green slice, Figure 1 ) regarding a reference navigator (purple slice).

ɣ passing rate (%): 100% DC

90% DC 50% DC

6 FFF VMAT 99.5 10 FFF static 97.5

99.5 97.4

99.0

97.4 Table 1: Gamma passing rates for plans delivered to the ArcCheck phantom using different DC.

The sequence was evaluated on 5 volunteers with an audio coaching (5 and 6 sec period) and 2 patients without audio coaching. Results Retrospective axial slices binning according to their positions in the breathing cycle was achieved by automated motion tracking of sub-diaphragmatic organs on navigators. All axial slices of the 6 respiratory phases were imported into the treatment planning system (TPS) Eclipse 13.7 (Varian) allowing the reconstruction of the sagittal and coronal images. The 50% phase appears as the

Conclusion