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.

Clinical use of iterative 4D-cone beam computed tomography reconstructions to investigate respiratory tumor motion in lung cancer patients.

BACKGROUND: Cone beam computed tomography (CBCT) provides means for respiratory resolved volumetric imaging of the thorax. However, merely sorting the acquired projections into respiratory phases and performing a series of conventional three-dimensional (3D) reconstructions lead to clinically prohibitive reconstruction artifacts. This problem can be mitigated by iterative 4D reconstruction. We present a clinical evaluation of two iterative 4D-CBCT reconstruction algorithms during stereotactic body radiation therapy. MATERIAL AND METHODS: Two types of iterative 4D-CBCT reconstructions were performed utilizing: 1) total variation (TV) minimization; and 2) optical flow (OF) based deformable registration between phases. The reconstructions were initially evaluated on a lung phantom with a moveable target insert. Subsequently, 4D-CBCT reconstructions were performed for 19 patients on 2-3 CBCT projection datasets previously acquired for conventional 3D-CBCT reconstruction (∼650 half-fan projections per scan in a full one-minute gantry rotation). The 4D reconstructions were imported into a treatment planning system, where the gross tumor volume (GTV) was delineated and used to extract the tumor motion amplitude. RESULTS: For both phantom and patient scans, the iterative 4D-CBCT reconstructions had sufficient quality for GTV delineation when the breathing period was faster than 3.5 seconds (15 of 19 patients), but not for slower breathing periods (4 patients). The 3D tumor motion amplitude for the patients was significantly lower (p = 10(-6), Wilcoxon signed rank test) in the OF reconstructions (mean 4.0 mm) than in the TV reconstructions (mean 5.3 mm). CONCLUSION: TV and OF iterative 4D-CBCT reconstruction of the thorax in a lung phantom and for 19 patients was demonstrated from standard CBCT scans and used to estimate the daily lung tumor motion.