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margins and its dosimetric impact when using MRI-guided intensity modulated radiation therapy (IMRT MRI ) instead of the clinical VMAT CBCT workflow. Material and Methods Imaging data, including 4D-CT, CBCT and MRI-scans acquired during planning and treatment of 15 patients (average 3, range 1-8 years), were used to estimate the PTV expansion for each workflow. Both intra- and inter- fraction tumor bed motion (using four clips as surrogates) and patient positioning systematic (∑) and random (σ) errors were determined individually for each direction and patient. The average PTV expansion (PTV m ) was calculated using the van Herk recipe (2.5∑+1.7σ). For the dosimetric comparison, RT plans were generated for both delivery techniques using 5 margin scenarios (PTV m ± 0, 1 and 2 mm). All plans, with prescription doses (PD) ranging from 10.8-25.2 Gy, were optimized to achieve the same PTV coverage (D98% > 95%PD). Furthermore, the IMRT MRI plans were optimized with a 1.5T transverse magnetic field turned on to realistically model a MRI-linac treatment. Plan quality was evaluated by assessing mean dose (D mean ) and dose-volume statistics of the PTV and OAR (p<0.05, Wilcoxon). Results The calculated PTV m was 7 mm in cranio-caudal (CC) and 6 mm in anterior-posterior (AP) and left-right (LR) directions for the VMAT CBCT and 4 mm in CC and 3 mm in AP and LR directions for the IMRT MRI (Table 1). For both delivery techniques in all scenarios, PTV coverage was fulfilled. A reduction of the D mean to the OAR up to 18% was achieved with the IMRT MRI when using the estimated PTV m : D mean to the contralateral kidney was reduced from 4.3 ± 2.5 to 3.2 ± 2.2 Gy (p=6e-5), to the liver from 6.0 ± 3.1 to 5.0 ± 2.9 Gy (p=6e-5), to the spleen from 7.1 ± 8.1 to 6.1 ± 7.4 Gy (p=1e-4) and to the pancreas from 12.2 ± 5.6 to 10.9 ± 5.8 Gy (p=1e-4). The maximum reduction of the D mean to the OAR between the largest and smallest margin scenarios was up to 19% for VMAT CBCT (range 0.1- 2.7 Gy) and 21% for IMRT MRI (range 0.2-3.0 Gy).

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).