Demonstration of quasi-real-time intrafractional motion in esophageal cancer radiotherapy provided by ExacTrac X-ray snap verification

OBJECTIVE: To investigate the following hypotheses: (1) ExacTrac X-ray Snap Verification (ET-SV) is an alternative to CBCT for positioning patients with esophageal carcinoma (EC), (2) ET-SV can detect displacement in EC patients during radiotherapy (RT) and (3) EC patients can be feasibly monitored in quasi-real-time with ET-SV during RT. METHODS: Anthropomorphic phantoms and 13 patients were included in this study. CBCT and ET-SV were both implemented before treatment delivery to detect displacement, and their correction results were compared. For the patient tests, positional correction in 3 translational directions and the yaw direction were applied using the ET-SV correction results. The residual error was detected immediately using ET-SV. Finally, to acquire the intrafractional motion, ET-SV was implemented when the gantry was at 0 degrees, 90 degrees, 180 degrees and 270 degrees, respectively. RESULTS: In phantom tests, the maximum value of the difference in displacement between the CBCT and ET systems was 1.16 mm for translation and 0.31 degrees for yaw. According to Bland-Altman analysis of the patient test results, 5% (5/98), 5% (5/98), 5% (5/98), and 4% (4/98) of points were beyond the upper and lower limits of agreement in the AP, SI, LR and yaw directions, respectively. The mean residual error was -0.482 mm, 1.215 mm, 1.0 mm, -0.487 degrees, 0.105 degrees, and 0.003 degrees in the AP, SI, LR, pitch, roll and yaw directions, respectively. The intrafractional displacement ranged from -0.21 mm to 0 mm for translation and from -0.63 degrees to 0.21 degrees for rotation. The mean total translational error for intrafractional motion increased from 0.47 mm to 1.14 mm during the treatment. CONCLUSIONS: The accuracy of ET-SV for EC RT positional correction is comparable to that of CBCT. Thus, Quasi-real-time intrafractional monitoring can be used to detect EC patient displacement during radiotherapy.