Quantification of esophageal tumor motion and investigation of different image-guided correction strategies

Abstract Purpose To accurately quantify esophageal tumor position variability and to optimize image-guided correction strategies. Material and Methods Esophageal cancer patients receiving chemoradiotherapy (41.4-50.4 Gy in 23-28 fractions combined with carboplatin/paclitaxel) were included in a prospective cohort study (NCT*blinded*). Gold fiducial markers were inserted into the esophageal tumors during diagnostic endoscopic ultrasound. Four-dimensional (4D) planning computed tomography (CT) and daily 4D-cone beam (CB)CT-scans were acquired. Each CBCT was registered to the planning CT using different regions of interest (bone (3D) and carina, diaphragm, clinical target volume (CTV) and fiducial markers (4D)) for alignment and using the fiducial markers as the true tumor position. Subsequently, a planning target volume (PTV) margin accounting for residual uncertainties including the average respiratory motion was calculated for each of these registrations. Results Fifty-six patients with tumors located in the proximal (n=1), mid (n=7) or distal esophagus (n=25) or at the gastroesophageal junction (n=23) were included. The average peak-to-peak respiratory tumor motion was 0.20, 0.92 and 0.34 cm on the planning CT in left-right(LR), cranial-caudal(CC) and anterior-posterior(AP) direction, respectively. The required PTV margin with average motion amplitude, depending on the correction strategy used for image-guidance, ranged from 0.8-1.0 cm, 1.1-1.6 cm and 0.7-0.9 cm in LR, CC and AP direction, respectively. A registration based on the CTV resulted in the smallest PTV margins (0.8, 1.1 and 0.7 cm in LR, CC and AP direction, respectively). For bone-registration the calculated PTV margins were 1.0, 1.3 and 0.7 cm in LR, CC and AP direction, respectively. The registration based on the diaphragm increased PTV margins. Conclusion Substantial and anisotropic position variability of esophageal tumors was observed during radiotherapy, hence non-uniform margins should be considered. Cranial-caudal PTV margins need to be larger than nowadays commonly used. Target positioning during IGRT could be improved with a CTV registration-based correction strategy.