ESTRO 2020 Abstract book

S867 ESTRO 2020

plans, reoptimized clinical plans and seven out of ten original 10-arc plans. Wilcoxon signed-rank test was used to evaluate differences with the clinical plans, a p-value below 0.05 was considered significant. Results Dose statistics for the high-dose and low-dose PTV are shown in Table 1. V 100% and D 98% were significantly different between clinical and trajectory plans.

Conclusion HS-WBRT-SIB treatment with a modulation technique involves overdose areas not negligible within the WB* volume if compared with the overall treated metastases volume; these areas are progressively reduced in the three techniques IMRT, VMAT and Tomotherapy in this order, simultaneously the homogeneity within WB* has improved. PO-1514 Non-coplanar trajectory VMAT for treatment of spinal metastatic tumors J. Van Timmeren 1 , E. Lamaj 1 , S. Ehrbar 1 , M. Guckenberger 1 , S. Tanadini-Lang 1 1 University Hospital Zürich and University of Zürich, Department of Radiation Oncology, Zürich, Switzerland Purpose or Objective The use of non-coplanar arc fields has shown to allow for dose reduction to organs at risk (OARs), without compromising PTV coverage, in brain, liver and lung, showing potential for reducing OAR dose in spinal SBRT. Dynamic couch movements during gantry rotation (referred to as trajectory VMAT) may enable further dose reduction to healthy tissue and potentially reduces treatment times compared to static non-coplanar couch positions. This study aims to compare treatment plan quality of trajectory VMAT plans against delivered clinical plans for spinal metastatic disease. Material and Methods Ten patients with lumbar spine metastases treated with SBRT were included in the study. Depending on epidural involvement, prescription dose was either 40Gy/20Gy in 5 fractions or 48.5Gy/30Gy in 10 fractions. All patients were re-planned with dynamic gantry- and couch rotations, using three full non-coplanar arcs and additionally two full coplanar arcs. Trajectories were defined manually such that during continuous gantry rotation, the couch rotates towards desired couch angles and back to 0° to prevent collisions, while maximizing time at the non-zero angles. Couch angles were selected based on the MU-contribution in a 10-arcs plan with couch kicks at 15° intervals, ranging from 30° to 90° and from 330° to 270°. Plans were optimized using a research version of Eclipse that allows optimization with complex couch trajectories. Normalization was performed based on spinal cord PRV sparing (D 1.0cc of 23.8Gy or 35Gy) and a D 2% of maximal 107%. In case no 80% PTV coverage could be achieved, a D 2% of 109% was allowed. RTOG’s conformity index (CI) and DVH parameters were compared between trajectory VMAT

Dose statistics to OARs are summarized in Figure 1. Overall, dose to the kidneys was not significantly different between the techniques. Increased kidney sparing could only be reproduced for two patients using the trajectory technique.