ESTRO 2020 Abstract Book

S817 ESTRO 2020

collisions, incorrect isocenter shifts and beam mix-up due to predicted absolute table coordinates, which are recorded to the R+V system with the corresponding beams. PO-1513 Overdose areas in whole brain irradiation with simultaneous integrated boost and hippocampal sparing A. Pierelli 1 , M.G. Giri 1 , P.M. Polloniato 1 , N.L.V. Cernusco 2 , R. Micera 2 , A. Muraglia 2 , R. Mazzarotto 2 , C. Cavedon 1 1 Azienda Ospedaliera Universitaria Integrata Verona, Medical Physics, Verona, Italy ; 2 Azienda Ospedaliera Universitaria Integrata Verona, Radiotherapy, Verona, Italy Purpose or Objective To evaluate and compare overdose areas in whole brain irradiation with Hippocampal Sparing (HS) and Simultaneous Integrated Boost (SIB) on multiple metastases (HS-WBRT-SIB) for three intensity modulated techniques: non-coplanar IMRT, non-coplanar VMAT and Tomotherapy. Whole brain volume without metastasis and without the Hippocampal Avoidance Zone (HAZ) was defined (WB*). Differences in terms of dose homogeneity, usually neglected, and in terms of global absolute amount of overdoses in WB* volume were analyzed. Material and Methods The inner clinical protocol used for HS-WBRT-SIB treatment provides a prescription of 30 Gy and 40 Gy in 10 fractions to the whole brain and metastases, respectively, and a simultaneous hippocampus sparing with “near maximum dose” D2% ≤ 16Gy. Ten patients, all with brain metastases, were selected for this study and planned after a review of all the anatomical structures mainly defined using CT and MRI images. The dosimetric data collected from each plan and each modulation technique were first used to determine the WB* coverage, the dose homogeneity index (HI) and the near maximum dose as described by ICRU 83, together with the absolute volume irradiated at high dose levels; the dose to the organs at risk was also considered. To evaluate the significance of the differences between the three modulation techniques, a statistical analysis was performed using the Freidman test for paired samples. Results As regards the organs at risk, the difference for HAZ D2% is statistically significant between Tomotherapy and IMRT and between VMAT and IMRT but not between Tomotherapy and VMAT. The dose reduction is also statistically significant for mean eyes dose and maximum lens dose between Tomotherapy and the other two techniques. WB* is well covered in each technique: we found D95% (mean value ± STD): (28.9±0.4)Gy, (28.9±0.5)Gy, (29.1±0.3)Gy respectively for IMRT, VMAT and Tomotherapy. About the WB* D2%, the results obtained were respectively: 114%, 110% and 107% of WB* dose prescription. Values of HI were 0.18±0.02, 0.13±0.03 and 0.09±0.03, respectively; the absolute volumes (in cc) exceeding 107% of WB* dose prescription were 125±38, 59±27 and 31±15, respectively. The overall treated metastases volume was 14±10 cc.

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