Abstract Book

ESTRO 37

S506

to estimate the margin accounting for inter-fractional motion as 2.5Σ+0.7σ. Results Two markers were lost before treatment start. The remaining 59 markers were visible during the full treatment course. Fig 1 shows the intrafractional motion at one fraction for a patient with three markers implanted over a 10cm length of the oesophagus. The cranio-caudal (CC) motion was dominated by respiration and nearly identical for the three markers, while the left- right (LR) and anterio-posterior (AP) respiratory motion and the cardiac motion in all directions varied more among the markers (Fig 1). The 2 nd -98 th percentile motion range had a mean [range] over all markers of 2.7mm [0.4-8.7mm] (LR), 8.8mm [1.0-21.6mm] (CC) and 4.0mm [0.5-19.0mm] (AP) for the full motion during CBCT acquisition with mean ranges of 2.0mm (LR), 8.0mm (CC) and 3.2mm (AP) for breathing motion and 0.9mm (LR), 1.6mm (CC) and 1.4mm (AP) for cardiac motion. Fig 2 shows the interfractional motion for a patient with four markers implanted. A 9mm lateral shift occurred between F3 and F5 for the most cranial marker (M1) due to an abrupt lateral shift of the oesophagus, whereas only minor shifts occurred for the more caudal markers. Oesophageal lateral shifts of varying size were observed throughout the treatment course for this patient, while the AP and CC positions were more stable. Daily marker shifts relative to bones were in general quite large for the patient group with Σ of 2.9mm (LR), 3.0mm (CC), and 2.4mm (AP), and σ of 2.4mm (LR), 2.4mm (CC), and 1.6mm (AP). It resulted in interfraction motion margins of 8.9mm(LR), 9.3mm(CC), and 7.1mm(AP). Conclusion Detailed 3D intra- and interfractional motion of fiducial markers in oesophageal cancer patients were obtained from daily CBCT projection images. Large interfractional shifts correlated to positional changes of the oesophagus were observed in some patients

Conclusion The WED, CAR and MCN models mostly coincided spatially in biological dose to the brainstem and substructures. The LWD and RBE 1.1 reported overall lower doses, however, with similarly shaped isodose curves. The largest differences between the isodoses occurred at 50 Gy(RBE) and above.

Poster: Physics track: Intra-fraction motion management

PO-0934 Detailed mapping of time-resolved 3D intra- and inter-fractional oesophageal tumour motion L. Hoffmann 1 , J. Bertholet 2 , M. Nordsmark 2 , I. Kruhlikava 3 , B.L. Helbo 4 , M.L.S. Schmidt 1 , P.R. Poulsen 2 1 Aarhus University Hospital, Department of medical physics, Aarhus, Denmark 2 Aarhus University Hospital, Department of oncology, Aarhus, Denmark 3 Aarhus University Hospital, Department of surgery, Aarhus, Denmark 4 Aarhus University, Department of physics and astronomy, Aarhus, Denmark Purpose or Objective Oesophageal cancer radiotherapy struggles with low survival and high toxicity. A better understanding of target motion and deformation during treatment is needed in order to increase treatment accuracy. The purpose of this study was to investigate the time-resolved 3D internal motion of oesophageal tumours throughout the radiotherapy treatment course by analysing the 3D trajectories of implanted markers during daily setup CBCT scans. Material and Methods Twenty-one oesophageal cancer patients had 2-7 fiducial markers implanted in the oesophagus in or near the tumour before 23-27 fractions (F) radiotherapy with daily set-up CBCT using bone match. In total 61 Visicoil or Gold Anchor markers were implanted. The 3D marker trajectories during each CBCT scan were estimated from the segmented 2D marker positions in the CBCT projections by a probability-based method resulting in 1- minute trajectories at 11Hz. The motion was separated into a breathing and a cardiac component by frequency analysis and motion ranges were extracted. The daily mean setup error relative to the bony anatomy was used