ESTRO 2021 Abstract Book

S422

ESTRO 2021

Conclusion The use of MIPs facilitates more rapid assessment of COVID-19 related lung density changes than CBCT review resulting in less impact on clinical resource. This will provide minimal burden for screening of infections in radiotherapy departments. OC-0535 Comparison of a surface-based (SGRT) vs. a laser-based setup in breast cancer radiation therapy. M. Machiels 1,2 , F. Horácio 1 , N. De Dooij 1 , W. Vingerhoed 1 , D. Nevens 1,2 , C. Billiet 1,2 , P. Meijnders 1,2 , P. Poortmans 1,2 , D. Verellen 1,2 1 Iridium Netwerk, Radiation Oncology, Antwerp, Belgium; 2 University of Antwerp, Faculty of Medicine and Health Sciences, Antwerp, Belgium Purpose or Objective Surface-Guided Radiation Therapy (SGRT) is a rapidly growing technique which uses stereovision technology to track patients’ surface in 3D, for both setup- and motion-management during radiation therapy (RT). In breast cancer RT, the target volume is located directly under the skin, making it in potential a superior technique for patient positioning able to decrease interfractional setup variation as compared to a conventional laser-based setup. In the present study, we aimed to determine the accuracy of a SGRT system for positioning of breast A cohort of consecutive breast cancer patients receiving either tangential or locoregional RT from December 2020 until January 2021 not requiring deep-inspiration breath-hold techniques were included in this study. All twelve patients received skin marking enabling a laser-based setup based on isocenter coordinates during the simulation process. The optical surface scanning system was used for daily setup using a one camera system (Figure 1). Afterwards, daily verification imaging (i.e., kV-kV imaging or cone beam CT; n = 80) and a rigid registration to the reference CT using Varian Offline Review software (Palo Alto, CA) was performed to determine the patient shifts necessary after a surface-based setup. Radiopaque markers were placed on the patients’ skin markers before daily verification imaging. Retrospectively, a conventional laser-based setup was performed on the same acquired verification imaging (n = 80) using the available radiopaque markers to quantify the patient shifts necessary after a laser-based setup. We then quantified per setup-method the systematic error (SE) and the standard deviation (SD) of the random error (sdRE) in the left–right (LR), cranio- caudal (CC), and anterior–posterior (AP) direction. Further, we estimated the SD of SEs (Σ), and the root mean square of sdRE (σ). Comparison between errors was done using a paired Wilcoxon signed-rank test. cancer patients in RT. Materials and Methods

Results The distributions of the absolute mean SE after a laser-based setup and a surface-based sestup are given in Figure 2. Overall , largest Σ was seen in CC direction (7 mm vs 2.5 mm, respectively). The Σ (σ) in the LR/CC/AP direction was 4.6(3.0)/6.2(4.8)/7.0(5.3) mm using a laser-based setup compared to 1.8(2.4)/1.6(2.2)/2.5(2.5) mm when using a surface-based setup. A surface-based setup decreased the