ESTRO 2020 Abstract book

S194 ESTRO 2020

improve reproducibility, we need to compensate for the gradual lung deflation that causes motion during PBHs.

OC-0340 Intrafraction motion during SBRT in deep inspiration breath-hold for liver metastases L.B. Stick 1 , I.R. Vogelius 1 , S.N. Risum 1 , M. Josipovic 1 1 Rigshospitalet, Department of Oncology, Copenhagen, Denmark Purpose or Objective To assess intrafraction motion in patients with liver metastases treated with stereotactic radiotherapy (SBRT) in deep inspiration breath-hold (DIBH). Material and Methods Ten patients treated with liver SBRT in DIBH in a pilot study between July 2018 and August 2019 were analyzed. Prior to imaging for treatment planning, three gold markers were percutaneously implanted in the tumour vicinity using ultrasound guidance. Patients were trained in voluntary DIBH with visual guidance (RPM system). Gating window of 2.5-3.0 mm was used for all DIBH imaging and treatment delivery. Three DIBH CTs were acquired during imaging for radiotherapy and the DIBH CT with median position of the fiducial markers was used for treatment planning. DIBH CBCTs were performed before (pre- treatment CBCT) and after (post-treatment CBCT) each of the three treatment fractions. Daily patient position correction was performed online using rigid registration (x, y, z and yaw) on fiducials. During the volumetric modulated arc treatment, planar 2D kV images were acquired for every 10° of the gantry rotation for 5 patients. It was only possible to evaluate the CC position on these images due to 2D geometry. Results All patients had one liver metastasis. In one patient, two fiducials were placed close together and appeared as one on CT and CBCT. In another patient, one marker was split in two parts during implementation and appeared as two markers on CT and CBCT. Median 3D distance from marker to center of GTV on the planning CT was 3.1 cm (range 0.4 to 5.0 cm). The median 3D difference in marker position between the three DIBH CTs was 0.3 cm (range 0.0 to 0.9 cm) for all patients. The median 3D difference in marker position between the pre-treatment CBCT and the post- treatment CBCT was 0.3 cm (range 0.3 to 1.4 cm). Stability examination on pre-treatment repeat DIBH CTs was not sufficient to guarantee per-treatment stability cf. patient 3 and 4 in Figure 1. The mean time between the pre-treatment CBCT and post-treatment CBCT was 14 min (range 8 to 20 min). For planar kV images it was not possible to evaluate marker position in one patient due to lack of image contrast. The maximum difference in marker position (CC direction) on planar kV images during one fraction was between 0.8 to 1.3 cm for the four patients and maximum difference was between 0.3 to 1.0 cm within one DIBH (see Figure 2). The markers moved cranially within each DIBH in three out of four patients.

Conclusion Difference in marker position between pre-treatment CBCT and post-treatment CBCT varied substantially between patients. Marker motion of up to 1.0 cm in CC direction was seen during a single DIBH despite use of narrow external gating window and visual feedback. OC-0341 Seminal vesicle motion tracking in 3D cine-MR during MR-Linac prostate treatments D. De Muinck Keizer 1 , T. Willigenburg 1 , M.D. Den Hartogh 1 , J.R.N. Van der Voort van Zyp 1 , B.W. Raaymakers 1 , J.J.W. Lagendijk 1 , H.C.J. De Boer 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands