ESTRO 2021 Abstract Book

S495

ESTRO 2021

plans on all delineated respiratory phases. Robustness towards centre specific setup and range errors was evaluated on the mv scan. The plan was recalculated on a delineated surveillance 4DCT (sCT) acquired at fraction 10 using rigid registration on the vertebral column to evaluate the combined effect of anatomical changes and respiration. Results In all plans, V95%iCTV > 99% for the nominal plan and V95%iCTV > 97% for all respiratory and robustness scenarios for both PT and XT were in agreement with predefined constraints. For the OARs, all plans met maximum constraints on spinal cord and body dose in the nominal plan (D0.05cc<45Gy and D0.05cc<110%) and all scenarios (D0.05cc<50Gy and D5cc<107%). For the nominal plans, the dose to lungs and heart varied considerably between centres (Fig1). The difference in MLD and mean heart dose between each pair of PT and XT plans was in median [range] 4.4 Gy [1.2;8.1] and 8.2 Gy [2.0;20.2], respectively. Recalculation on the sCT let to a large decrease in V95%iCTV for patient 3 (Fig2). For each patient, the mean value of the median V95%iCTV of all scenarios for the six centres at sCT for PT and XT was 96.3% and 98.6% (patient1), 98.6% and 92.2% (patient2), 89.7% and 79.5% (patient3), and 99.4% and 99.9% (patient4). Patient 2 and 3 had large anatomical changes leading to larger underdosage for XT than for PT. One PT plan showed overdosage of the spinal cord (D1cc>45Gy) at sCT and two XT plans showed overdosage of the body (D5cc>107%) at sCT.

Conclusion Treatment planning and robustness evaluation for the European randomized PROTECT phase III trial were evaluated in six centres. Most centres met the constraints for the nominal treatment plans, even though large variability in some OARs was found. XT plans were less robust towards anatomical changes than PT plans. OC-0632 Radiation dose variations in treatment plans for pediatric ependymoma L. Toussaint 1 , P. Brandal 2 , A. Embring 3 , J. Engellau 4 , M. Egeberg Evensen 2 , R. Griskeviskius 5 , J. Hansen 6 , H. Hietala 7 , M. Jørgensen 8 , P. Kramer 9 , I. Kristensen 10 , K. Lehtio 7 , H. Magelssen 2 , M. Vestmø Maraldo 11 , K. Marienhagen 12 , U. Martinsson 13 , S. Peters 9 , S. Plaude 9 , D. Sendiuliene 14 , B. Smulders 11 , J.M. Søbstad 12 , L. Vaalavirta 15 , A. Vestergaard 1 , B. Timmermann 9 , Y. Lassen-Ramshad 1 1 Danish Centre for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark; 2 Oslo University Hospital, Department of Oncology, Oslo, Norway; 3 Karolinska University Hospital, Department of Oncology, Stockholm, Sweden; 4 Skane University Hospital, Department of Oncology, Lund, Sweden; 5 National Cancer Institute, Department of Medical Physics, Vilnius, Lithuania; 6 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark; 7 Oulu University Hospital, Department of Oncology and Radiotherapy, Oulu, Finland; 8 Rigshospital, Copenhagen University Hospital, Department of Oncology, Copenhagen, Denmark; 9 West German Proton Therapy Centre Essen, Essen University Hospital, Essen, Germany; 10 Skane University Hospital, Department of Oncology, Lund, Sweden; 11 Rigshospital, Copenhagen University Hospital, Department of Oncology, Copenhagen, Denmark; 12 University Hospital of North Norway, Department of Oncology, Tromsø, Norway; 13 Uppsala University Hospital, Department of Oncology, Uppsala , Sweden; 14 National Cancer Institute, Department of External Beam Radiotherapy, Vilnius, Lithuania; 15 Helsinki University Hospital, Department of Oncology, Helsinki, Finland

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