ESTRO 2020 Abstract Book

S127 ESTRO 2020

particle beam line, the current treatment planning strategies are severely challenged. Particles are deflected in magnetic fields, modifying beam paths towards and within the patient. Compensation methods and new dose calculation engines are therefore required. This work aims to develop a research treatment planning system for proton beams in transverse magnetic fields. Material and Methods An in-house developed pencil beam (PB) algorithm for MR guided proton therapy (MRgPT) was implemented in the open source matRad research treatment planning system (TPS). The TPS was then utilized to generate treatment plans (TP) in homogeneous phantoms and three patient cases (prostate, liver and brain). The PB model was based on GATE/Geant4 Monte Carlo (MC) simulations in the clinical proton energy range (62.4 – 215.7 MeV) for a dipole magnet and field strengths between 0 and 1 T. The new dose calculation engine in matRad was benchmarked against MC simulations and dose measurements for both single beams and spread-out Bragg peaks (SOBP). Absolute dose was measured with a Roos chamber (PTW, Freiburg, Germany) in a water phantom for magnetic field strengths 0 and 1T. Finally, TPs generated for magnetic field strengths of 0 and 1T were compared. Dose volume histograms (DVH), plan quality indicators (D 2% , D 50% ,D 90% ,V 95% and V 105% ) and the difference between dose distributions using a global gamma index criteria of 2%/2mm were selected as assessment parameters. Results Dose calculations in water of the TPS agreed well with MC simulations, showing gamma index pass rates higher than 99% and 96% for single beams and SOBP, respectively. Deviations between measured, simulated and calculated depth dose distributions were smaller than 2% in the Bragg peak regions for both magnetic field strengths. From a clinical point of view, comparable TPs were obtained for B=0 and B=1T in phantom and patient cases, (see Fig.1-2). Plan quality indicators in the target agreed within 1.5% for all the analysed cases, except for the liver case where deviations up to 2.5% were observed.

Fig. 1 Dose distributions for a prostate patient case for B = 0T (left), B = 1T (middle) and their difference (right). Results are normalised to a prescribed dose to the PTV of 68 Gy.

Conclusion Carbon ions and photons have distinct physical and biological advantages. Joint optimization of both modalities yields the optimal combination of carbon ions and photons that may best exploit each modality's advantage and improve on a naive combination of the two modalities. PH-0244 Treatment planning for MRI guided proton therapy: dose calculations in transverse magnetic fields. F. Padilla Cabal 1 , A.F. Resch 1 , D. Georg 1 , H. Fuchs 1 1 Medical University of Vienna- Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Department of Radiation Oncology, Vienna, Austria Purpose or Objective Magnetic resonance (MR) image guidance is expected to enhance the clinical potentials of particle therapy. Among the several hurdles of integrating an MR in a clinical