Abstract Book

S317

ESTRO 37

Results SPArc significantly improved the target coverage under interplay effect and reduced the dose to lung and other critical structures. More specifically, all patients planned via SPArc maintained at least 95% of ITV covered by 95% of prescription dose with hotspots less than 115%. Only half of the patients in the IMPT could achieve the similar coverage. In terms of normal tissue sparing, SPArc reduced the average integral dose by 17.0% (p = 0.01) and V5, V10, V20, V30, and mean lung dose by an average of 3.9% (p < 0.01), 4.0% (p < 0.01), 3.5% (p < 0.01), 3.0% (p < 0.01), and 1.9 Gy (RBE) (p < 0.01) respectively compared to RO-IMPT. In regards to other critical structures, SPArc could reduce the max dose to the cord by 8.7 Gy (RBE, p = 0.02), mean dose to esophagus by 1.7 Gy (RBE, p = 0.04), and mean dose to the heart by 0.7 Gy (RBE, p = 0.23) in comparison to RO-IMPT.

Heidelberg, Germany 6 Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden, Department of Radiation Oncology, Dresden, Germany Purpose or Objective Proton therapy (PT) is expected to benefit greatly from integration with magnetic resonance (MR) imaging due to its sensitivity to anatomical variations. Consequently, the concept of MR-guided PT (MRPT) receives increased interest. Previous studies on MR-guided photon therapy (MRXT) have reported local dose enhancement of up to 40% at tissue-air interfaces caused by the electron return effect (ERE) in transverse magnetic fields. For MRPT, however, no consensus on the magnitude and hence the clinical effect of the ERE can be found in the scarce literature. The objectives of this study were 1) to experimentally confirm the ERE for PT by measurements and 2) to determine its magnitude for clinically relevant proton energies and MR field strengths by simulation. Material and Methods Measurements were performed with a collimated 200 MeV proton beam traversing a PMMA phantom made of one or two 10 mm vertical slabs. Dose was measured with GafChromic EBT3 films (PMMA equivalent thickness 0.312 mm) using two experimental setups: (A) one film sandwiched between two slabs – for normalization – and (B) two films attached to the distal end of one slab, i.e., at effective distances of 0.467 and 0.156 mm from the air interface. Film irradiation was performed under the same conditions without and within a transverse magnetic field (0.92 ± 0.02 T). All measurements were repeated 4 to 8 times and the entire experiment was performed twice. In the Monte Carlo simulations (Geant4, V10.3), the proton beam shaping devices, magnetic field and PMMA slabs were modeled in detail. The EBT3 films were simulated as PMMA slabs and dose was scored in 50 µm bins in PMMA from 0 to 1000 µm distance from the air interface. In additional simulations, the field strength was varied between 0.35 and 1.5 T for a 210 MeV beam as well as beam energy between 90 and 210 MeV at 1 T. The dose enhancement ratio (DER) was defined as dose with divided by dose without magnetic field: D B /D. Results Significant dose enhancement was measured with magnetic field compared to no field (p<0.01) close to the PMMA-air interface and confirmed by repeated experiments. The dose enhancement decreased with increasing distance from the interface (Fig. 1). Good agreement was achieved between measured and simulated dose both with and without magnetic field. The DER was largest in simulations with strong magnetic fields increasing from about 2.0% in the presence of a 0.35 T field up to 7.4% for a 1.5 T field near the interface (Fig. 2a). A decrease of the proton energy resulted in a decreasing DER (Fig. 2b).

Conclusion SPArc maintained the target coverage and improved the normal tissue sparing for lung cancer patients with moderate target motion. SPArc has the potential to be safely and effectively implemented into clinical practice to treat mobile tumors in lung patients. OC-0604 Simulation and experimental verification of magnetic field induced proton dose enhancement effects A. Lühr 1,2,3 , L.N. Burigo 4,5 , S. Gantz 1,3 , S. Schellhammer 1,3 , A.L. Hoffmann 1,3,6 1 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology - OncoRay, Dresden, Germany 2 German Cancer Consortium DKTK, Partner Site Dresden, Dresden, Germany 3 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 4 German Cancer Research Center DKFZ, Division of Medical Physics in Radiation Oncology, Heidelberg, Germany 5 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO,