Patient setup error and day-to-day esophageal motion error analyzed by cone-beam computed tomography in radiation therapy

Little has been reported on the errors of setup and daily organ motion that occur during radiation therapy (RT) for esophageal cancer. The purpose of this paper was to determine the margins of esophageal motion during RT. Methods and materials. The shift of the esophagus was analyzed in 20 consecutive patients treated with RT for esophageal cancer from November 2007. CT images for RT planning were used as the primary image series. Computed tomography (CT) images were acquired using an Elekta Synergy System, equipped with a kilovoltage-based cone-beam CT (CBCT) unit. The subsequent CBCT image series used for daily RT setup were compared with the primary image series to analyze esophageal motion. CBCT was performed before treatment sessions a total of 10 times in each patient twice a week. The outer esophageal wall was contoured on the CBCT images of all 200 sets. Results. In the 200 sets of CBCT images, the mean (absolute) ± standard deviation (SD) of setup errors were 2 +/− 2 mm (max, 8 mm) in the lateral direction, 4 +/− 3 mm (max, 11 mm) in the longitudinal direction, and 4 +/− 3 mm (max, 13 mm) in the vertical direction. Additionally, the mean ± SD values of daily esophageal motion comparing the CBCT with RT planning CT were 5 +/− 3 mm (max, 15 mm) in the lateral direction and 5 +/− 3 mm (max, 15 mm) in the vertical direction. Conclusions. Our data support the use of target margins (between the clinical target volume and planning target volume) of 9 mm for day-to-day esophageal motion and 8 mm for patient setup in all directions, respectively.