Evaluation of respiratory-induced target motion for esophageal tumors at the gastroesophageal junction

Purpose: To quantify the internal motion margin requirements for radiotherapy of tumors near the gastroesophageal junction (GEJ). Methods and materials: Four-dimensional computed tomography (4DCT) scans were obtained for 25 patients with primary tumors located near the GEJ. The gross tumor volume (GTV) was manually contoured on the exhale-phase image from the 4DCT image set. A deformable image registration method was used to automatically propagate the contours to other phases of the 4DCT images. To quantify target motion, we measured the displacement of the GTV centroid and the variations in the target boundary and volume. Internal margins were calculated in the lateral (RL), anterior-posterior (AP), and superior-inferior (SI) directions. Results: The mean ± standard deviation peak-to-peak GTV centroid motion was 0.39 ± 0.27 cm (range, 0.04-1.09 cm) in the RL, 0.38 ± 0.23 cm (range, 0.10-0.94 cm) in the AP, and 0.87 ± 0.47 cm (range, 0.43-2.63 cm) in the SI directions, respectively. On average, the internal target volume was 72% (range, 9-172%) larger than the GTV defined on a single-phase CT image. Variations in tumor boundaries due to tissue motion and deformation suggested asymmetric margins: 1.0 cm left [toward the stomach], 0.8 cm right, 1.1 cm anterior, 0.6 cm posterior, 1.0 cm superior (toward the distal esophagus), and 1.6 cm inferior (toward the stomach). Conclusion: Because tumors near the GEJ are subject to a marked but asymmetric amount of respiratory-induced intrafractional tumor motion, the use of asymmetric internal margins may be beneficial.