Setup strategies and uncertainties in esophageal radiotherapy based on detailed intra- and interfractional tumor motion mapping

Background and purpose Detailed knowledge of target motion is important for improved accuracy and decreased toxicity of esophageal cancer radiotherapy. This study uses the 3D trajectories of implanted markers during setup CBCT scans to investigate the intra- and interfractional tumor motion in esophageal cancer radiotherapy. Material and methods For 21 esophageal cancer patients with implanted fiducial markers, 60-s 3D marker trajectories were estimated from the 2D marker positions in the projections of daily setup CBCT scans by a probability-based method. The motion was separated into respiratory and cardiac components by frequency analysis and motion magnitude (2nd–98th percentile) was extracted for each marker. The mean motion was calculated over all markers. The daily mean setup interfraction error for bony-anatomy and soft-tissue setup was used to estimate the margin accounting for interfractional motion. Results A total of 1036 marker trajectories were extracted using 427 CBCT scans and 63 markers. The mean motion magnitude over all markers was 2.9 mm (left–right (LR)), 8.8 mm (cranio-caudal (CC)) and 4.1 mm (anterior–posterior (AP)) for the full motion during CBCT acquisition with mean magnitudes of 2.7 mm (LR), 8.4 mm (CC) and 3.5 mm (AP) for respiratory motion and 1.0 mm (LR), 1.5 mm (CC) and 1.4 mm (AP) for cardiac motion. Substantial daily marker shifts relative to bones resulted in margins of 8.9 mm (LR), 9.5 mm (CC), and 7.3 mm (AP). Soft-tissue based setup in and near the CTV combined with rescanning of patients with anatomical changes reduced the margins to 6.9 mm (LR), 6.8 mm (CC), and 5.6 mm (AP). Conclusions Esophageal tumor motion was mapped with unprecedented detail throughout the radiotherapy course. Respiratory motion dominated and was largest in the CC direction. Soft-tissue matching and an adaptive strategy reduced interfractional margins by 2–3 mm compared to bony-anatomy matching.