ESTRO 37 Abstract book

S887

ESTRO 37

PT and VMAT plans were created with and without accounting for the constraint to the VB (V 95-97% > D 80% ). The plan robustness was evaluated using multiple dose distributions associated with various error scenarios: set- up errors (with the magnitude of 5 mm in 26 directions per VMAT and PBS-PT plans) and range errors (±3% density scaling, resulting in 52 scenarios per PBS-PT plan). Plans were accepted if the V 95% of the CTV > 98% in the voxel-wise minimum evaluation dose. Mean dose (D mean ) and dose-volume statistics were computed to assess CTV coverage and organs at risk (OAR) and body sparing (p<0.05, Wilcoxon). Results For both techniques, the CTV coverage was fulfilled. Significant better sparing of the OARs and body was achieved with PBS-PT for both WT and NBL patients (Table 1). The maximum reduction of the D mean to the OAR was up to 37% for WT patients and 34% for NBL patients. The volume of the body receiving high dose levels (V >95% ) was reduced up to 3% and 1% while the volume of the body receiving low dose levels (V <2Gy ) was reduced up to 31% and 39%, for WT and NBL patients respectively. Moreover, significant differences were found between plans optimized accounting and not accounting for the VB for each delivery technique (Table 2). A reduction of the D mean to the OAR up to 8% and 5% and to the body up to 3% and 1% was denoted for VMAT and PBS-PT plans, respectively.

Conclusion Imaging data collected during RT demonstrated that with an IMRT MRI workflow the PTV margin can be reduced by 3 mm when compared to the clinical VMAT CBCT . Furthermore, the IMRT MRI workflow reduces the doses to the OAR while maintaining the target coverage when compared to VMAT CBCT treatments. The MRI-guided workflow is showing to be promising for pediatrics from whom there is special concern for potential late effects. EP-1645 Dosimetric comparison of VMAT and pencil beam scanning proton therapy for abdominal pediatric tumors F. Guerreiro 1 , E. Seravalli 1 , G.O. Janssens 1 , J.H. Maduro 2 , C.L. Brouwer 2 , E.W. Korevaar 2 , A.C. Knopf 2 , B.W. Raaymakers 1 1 UMC Utrecht, Department of Radiotherapy and Imaging Division, Utrecht, The Netherlands 2 University of Groningen- UMCG, Department of Radiation Oncology, Groningen, The Netherlands Purpose or Objective Dosimetric comparison of robustly optimized volumetric modulated arc therapy (VMAT) and pencil beam scanning proton therapy (PBS-PT) in patients treated with Wilms’ tumor (WT), undergoing flank irradiation after nephrectomy, and neuroblastoma (NBL). Material and Methods VMAT and PBS-PT dose distributions were calculated for 10 WT patients (average 2, range 1-4 years), with prescription doses (PD) ranging between 10.8-14.4 Gy, and 10 NBL patients (average 4, range 1-7 years), with PD of 21.6 Gy. The treatment plans were robustly optimized on the clinical target volume (CTV). A uniform 5 mm patient set-up uncertainty was used for both delivery techniques. Moreover, for the PBS-PT plans a 3% proton range uncertainty was accounted for. VMAT plans were based on a 6MV full-arc while PBS-PT plans on 2-3 posterior fields to avoid uncertainties due to bowel filling and organ motion. To reduce the risk of asymmetric skeletal growth, a homogeneous dose was aimed for the vertebral bodies (VB) adjacent to the CTV (accepted gradient <5 Gy within one VB). As there is no international consensus in how to treat the VB, both PBS-