Abstract Book

S271

ESTRO 37

OC-0516 Doses to brain structures associated with cognition in photon vs proton therapy of craniopharyngioma L. Toussaint 1 , D.J. Indelicato 2 , Y. Lassen-Ramshad 3 , C.H. Stokkevåg 4 , C. Pedro 5 , R. Mikkelsen 6 , M. Di Pinto 2 , Z. Li 2 , S. Flampouri 2 , A. Vestergaard 7 , J.B.B. Petersen 1 , H. Schrøder 8 , M. Høyer 7 , L.P. Muren 1 1 Aarhus University Hospital, Department of Medical Physics, Aarhus C, Denmark 2 University of Florida, Department of Radiation Oncology, Jacksonville, USA 3 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark 4 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway 5 Instituto Português de Oncologia de Lisboa Francisco Gentil EPE, Department of Radiotherapy, Lisbon, Portugal 6 Aarhus University Hospital, Department of Neuroradiology, Aarhus, Denmark 7 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark 8 Aarhus University Hospital, Department of Pediatrics, Aarhus, Denmark Purpose or Objective Pediatric craniopharyngioma patients are at great risk of developing neurocognitive late effects after radiotherapy (RT). Craniopharyngiomas are typically located in the suprasellar brain, in close proximity to several substructures related to cognition (hereafter referred to as cognitive brain substructures, CBSs). Although proton therapy is often used to treat pediatric craniopharyngioma patients, there is little research evaluating which specific CBSs are spared from unnecessary radiation using this approach. The aim of this study was therefore to explore the pattern of doses delivered to CBSs for contemporary photon versus proton therapy techniques. Material and Methods Ten pediatric craniopharyngioma patients (median planning target volume (PTV) of 31 cm 3 [range: 18-63 cm 3 ]) previously treated with double scattered proton therapy were included in this study. Conventional critical structures (brainstem, optic nerves, chiasm) as well as CBSs (structures of the limbic system and Papez circuit, frontal and temporal lobes, anterior and posterior cerebellum, subventricular zones, corpus callosum and left frontal white matter) were delineated using CT and T1 MRI scans. Volumetric modulated arc therapy (VMAT) and intensity modulated proton therapy (IMPT) plans were generated for each patient using the Eclipse treatment planning system (prescribed dose of 54Gy/CGE, 30 fractions). For the VMAT plans, three non- coplanar arcs were used, while for the IMPT plans the beam orientations were identical to those in the delivered double scattered plans. During plan optimization, only the critical structures typically constrained in clinical routine were considered. For each CBS, we subsequently compared the fraction of volume receiving low (V10Gy), intermediate (V40Gy) and high (V50Gy) doses, based on published literature, as well as the mean doses (Dmean). A Wilcoxon signed-rank test was performed to compare the dose/volume parameters for VMAT vs. IMPT plans. Results All plans fulfilled institutional constraints in terms of target coverage and organ at risk sparing. In general, the volumes of CBSs exposed to low doses (10Gy) were reduced with IMPT compared to VMAT; e.g. from 59% to 35% for the right cingulum. Intermediate and higher dose levels (V40Gy, V50Gy) were comparable (Fig.1). IMPT reduced the mean doses to twelve CBSs, e.g from 16Gy to 2.2Gy for the left hippocampus tail (Fig.2); while the choice of a vertex field resulted in slightly higher mean

doses compared to VMAT plans for some CBSs, including the corpus callosum and subventricular zones.

Conclusion The mean doses and volumes of CBSs exposed to low doses were reduced by the use of IMPT compared to VMAT, while the intermediate and high levels were comparable. The ability of IMPT to spare some of the CBSs was influenced by the choice of beam orientations. OC-0517 Robust optimization is not sufficient to account for anatomical changes in head and neck IMPT M. Cubillos Mesías 1 , E.G.C. Troost 1,2,3,4,5 , F. Lohaus 1,2,3 , L. Agolli 2 , M. Rehm 2 , C. Richter 1,2,3,4 , K. Stützer 1,4 1 OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden- Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany 2 Faculty of Medicine and University Hospital Carl Gustav Carus, Department of Radiotherapy and Radiation Oncology, Dresden, Germany 3 German Cancer Consortium DKTK, Partner site Dresden, Dresden, Germany 4 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology - OncoRay, Dresden, Germany 5 National Center for Tumor Diseases NCT, Partner site Dresden, Dresden, Germany Purpose or Objective Robust optimization in proton therapy considers uncertainties in patient setup and particle range during the plan optimization. In general, however, anatomical changes occurring during the treatment course, potentially causing a degradation of the plan quality, are neglected. The aim of this study was to quantify the influence of these changes on the dose distribution for patients with bilateral head and neck cancer (HNC).