ESTRO 2023 - Abstract Book

S1680

Digital Posters

ESTRO 2023

Conclusion To our knowledge this is the first study evaluating the stability of the Vantage system. We have shown that this system has a sub-mm stability both pre- and intra-fraction. The main contributor to shift of the target in relation to the stereotactic frame is due to events before the treatment. Based on this result, it can be argued that pre-treatment CBCT images should be acquired, especially for targets close to sensitive OARs, in order to reduce the effect of pre-fraction skull movements.

PO-1926 Multicriteria dose-guided patient positioning correction using iso-center shifts and couch angles

P. Süss 1 , M. Heidgen 1 , H. Krieg 1 , K. Teichert 1 , Y. Zhang 2 , J. Qin 2 , N.J. Yue 2 , K. Nie 2 , S. Jabbour 2 , M. Deek 2 , S. Kim 2

1 Fraunhofer Institute for Industrial Mathematics, Optimization – Technical Processes, Kaiserslautern, Germany; 2 Rutgers Cancer Institute of New Jersey, Department of Radiation Oncology, New Brunswick, USA Purpose or Objective To study the potential of daily online dose-guided adaptations by varying only the iso-center and couch angles. Materials and Methods We previously introduced a method for dose-guided daily iso-center shifts that resulted in Pareto-optimal shifts when trying to reconciliate the daily dose (as predicted by a daily image prior to treatment) with the planned dose. We showed that it was possible in certain cases to improve on the shifts suggested by image-guidance alone. Furthermore, we demonstrated that trade-offs between minimizing the deviations from planned dose in different planning structures do arise and that they need to be addressed. We have extended this method to include the possibility to change the couch angle by 5 degrees in each direction. We also allow a shift in any of the 3 iso-center coordinates by 3mm in each direction. For a fixed treatment couch angle, the set of plans given by shifting the iso-center position alone determines a Pareto front. By varying the angle of the couch by discrete angle steps (of 1 degree) we obtain twenty possible Pareto fronts from which a user (or automated procedure) would have to choose from. We greatly reduce this number of fronts to any a select few by determining promising correction parameters heuristically. Finally, we demonstrate how to deal with multiple Pareto fronts in a straight-forward decision-making procedure. Results We apply this dose-guided positioning correction procedure to 68 retrospective cases of various disease sites (liver, lung, pelvis). We compare V95% of CTV of plans produced using our correction method with the plans that were created by dosimetrists using CBCT- or MV-based image-guided correction alone. The dose-guided correction was done manually by navigating the calculated Pareto fronts and trying to restore the planned CTV coverage. We found that V95% could be improved by about 3% on average. More interestingly, however, in all cases V95% could be restored to at least 88% of prescription using our dose-guided positioning, whereas it could only be restored to 80% in some cases without our method. Conclusion We are able to quickly determine good patient setup corrections (iso-center and couch angle) to correct for changes in patient anatomy. We provide Pareto navigation (a multicriteria decision-making tool) on multiple Pareto fronts to balance trade-offs when trying to restore original plan goals. Alternatively, this decision-making can be automized. There are clinical cases which would benefit greatly (as measured by V95% of the CTV) using this online dose-guided patient positioning correction method. 1 Hiroshima High-Precision Radiotherapy Cancer Center, Radiation oncology, Hiroshima, Japan; 2 Hiroshima High-Precision Radiotherapy Cancer Center, Radiation Oncology, Hiroshima, Japan Purpose or Objective Published organ motion data have been collected from measurements of a limited number of points within the organ, the centroid, or the edge of the organ. We proposed a novel volumetric method for evaluating deformed organ motion using deformable image registration, is called "vector volume histogram (VVH)". In this study, we investigated the inter-fraction reproducibility of the tumor position in expiratory breath-hold (BH) using VVH method. Materials and Methods Subjects comprising 14 patients with lung cancer who were treated with lung stereotactic body radiotherapy (SBRT) under expiratory BH conditions were monitored by the Abches system. The first computed tomography (CT) scan at the expiratory BH was used for treatment planning. The remaining three CT scans were acquired successively without repositioning to simulate inter-BH expiratory BH reproducibility. Gross tumor volume (GTV) was delineated by a physician. The hybrid deformable registration algorithm of RayStation treatment planning system (TPS) was used to deform from the three expiratory BH-CTs to the treatment planning expiratory BH-CT, and calculated the deformation vector field (DVF). DVFs consist of DVFLR (left-right), DVFAP (anterior-posterior), and DVFCC (cranio-caudal). Vectors on each direction are stored in separate arrays. Exported DVF data was converted as the Digital Imaging and Communications in Medicine-Radiation Therapy (DICOM RT). The modified DICOM-RT file was then imported into the RayStation TPS. The VVH calculation approach is a similar concept to that of the dose volume histogram (DVH). To evaluate the inter-fraction BH reproducibility of the tumor positions, we measured the largest motion extent within the organ of LR, AP and CC dimensions and a 3D vector using the VVH method. PO-1927 Evaluation of inter-fractional lung tumor position reproducibility with vector volume histogram H. Miura 1 , S. Ozawa 2 , M. Nakao 2 , Y. Doi 2 , M. Kenjo 2 , Y. Nagata 2

Results