Intrafraction esophageal motion in patients with clinical T1N0 esophageal cancer

Aim To investigate the intrafraction movement of the esophagus using fiducial markers. Background Studies on intrafraction esophageal motion using the fiducial markers are scarce. Materials and methods We retrospectively analyzed patients with clinical T1N0 esophageal cancer who had received fiducial markers at our hospital between July 2007 and December 2013. Real-Time Position Management System to track the patient's respiration was used, and each patient underwent three-dimensional computed tomography of the resting expiratory and inspiratory level. We used the center of the marker to calculate the distance between the expiratory and inspiratory breath-holds, which were measured with the radiotherapy treatment planning system in three directions: left–right (LR), superior–inferior (SI), and anterior–posterior (AP). The movements at each site were compared with the Kruskal–Wallis analysis and Wilcoxon rank sum test with a Bonferroni correction. Results A total of 101 patients with 201 fiducial markers were included. The upper, middle and lower thoracic positions had 40, 77, and 84 markers, respectively. The mean absolute magnitudes of the shifts (standard deviation) were 0.18 (0.19) cm, 0.68 (0.46) cm, and 0.24 (0.24) cm in the LR, SI, and AP directions, respectively. From the cumulative frequency distribution, we assumed that 0.35 cm LR, 0.8 cm SI, and 0.3 cm AP in the upper; 0.5 cm LR, 1.55 cm SI, and 0.55 cm AP in the middle; and 0.75 cm LR, 1.9 cm SI, and 0.95 cm AP in the lower thoracic esophagus covered 95% of the cases. Conclusions The internal margin based on the site of esophagus was estimated.