ESTRO 2021 Abstract Book

S1457

ESTRO 2021

Now, setup errors have been grouped together into a single one, simplifying the current treatment delivery protocol. Moreover, we have been able to establish a more realistic tolerance (2 mm).

Conclusion We have developed an in-house software that enables us to treat SBRT patients under a unique tolerance instead of the former treatment delivery with 6-DoF correction. Furthermore, with the arisen IGRT protocol we reduce the number of required CBCT registrations in comparison to the independent shift analysis, providing a greater consistency throughout the target localization process. PO-1734 Accuracy evaluation of ExacTrac X-ray Snap Verification for the oesophageal cancer radiotherapy J. Duan 1 1 Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Department of Radiation Oncology, Jinan, China Purpose or Objective In this prospective study, we investigated the following hypotheses: (1) ExacTrac X-ray Snap Verification (ET- SV) is an alternative to CBCT for positioning patients with oesophageal carcinoma (EC); (2) ET-SV can detect displacement in EC patients during radiotherapy (RT); and (3) it is feasible to carry out quasi-real-time monitoring for EC patients during RT. Materials and Methods Anthropomorphic phantoms and 13 patients with EC who underwent RT were enrolled in this study. CBCT and ET-SV were both implemented before treatment delivery to detect displacement. The ET-SV correction results were compared with the CBCT correction results. For the patient tests, positional correction in 3 translational directions and the yaw direction were applied using the ET-SV correction results. Immediately, the residual error was detected using ET-SV. Finally, to acquire the intrafractional motion, ET-SV was implemented when the gantry was at 0°, 90°, 180° and 270°. Results In the phantom test results, the maximum value of the difference in displacement between the CBCT and ET systems was 1.16 mm for translation and 0.31° for yaw. In the 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, and LR directions and yaw, respectively. The mean residual error was -0.482 mm, 1.215 mm, 1 mm, -0.487°, 0.105°, and 0.003° 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° to 0.21° for rotation. The mean total translational error for intrafractional motion increased from 0.47 mm to 1.14 mm during the treatment. Conclusion The accuracy of ET-SV for EC RT positional correction is comparable to that of CBCT. Quasi-real-time intrafractional monitoring can detect EC patient displacement during radiotherapy. PO-1735 A systematic review on motion, margins and delineation variability in thoraco-abdominal pediatric RT K. Meijer 1 , I. van Dijk 1 , S. Huijskens 1 , J. Daams 2 , C. Windmeijer 1 , B. Balgobind 1 , A. Bel 1 1 Amsterdam UMC, University of Amsterdam, Radiation Oncology, Amsterdam, The Netherlands; 2 Amsterdam UMC, University of Amsterdam, Medical Library, Amsterdam, The Netherlands Purpose or Objective For radiotherapy of thoracic and abdominal tumors, safety margins are applied to address geometrical uncertainties caused by e.g. set-up errors, inter- and intrafractional organ motion, and delineation variability.