ESTRO 36 Abstract Book

S181 ESTRO 36 _______________________________________________________________________________________________

Material and Methods

deflection of the Bragg peak ranged from 0 cm for 70 MeV to 1 cm for 180 MeV in comparison to no magnetic field. No out-of-plane beam deflection was observed. Exposing the film to 2 Gy at the Bragg peak was estimated to cause a mean dose to the magnets of 20 µGy, which is expected to produce negligible magnetic flux loss. The initial activation was estimated to be below 25 kBq.

We conducted the neutron measurement under the collaboration with National Institute of Standards and Technology (NIST). We employed Bubble detectors (BTI, Canada) to measure the neutron dose and energy spectrum with good spatial resolution. The detectors provide six energy thresholds from 10 keV to 10 MeV allowing to validate dose and the neutron energy spectrum. To simulate neutron scatter, a polyethylene cylindrical phantom was milled and the bubble detectors were placed inside. The phantom was then irradiated with a Californium-252 neutron source to simulate the secondary neutrons. We also simulated the experiment in TOPAS to compute the neutron dose and energy spectrum for comparison (Figure 1). Results

Figure 2 : Simulated dose distribution of a deflected proton beam (180 MeV, 10 7 primary particles) on a film dosimeter. Conclusion A first experimental setup capable of measuring the trajectory of a proton pencil beam slowing down in a tissue-equivalent material within a realistic magnetic field has been designed and built. Monte Carlo simulations of the design show that magnetic field induced lateral beam deflections are measurable at the energies studied and radiation-induced magnet damage is expected to be manageable. These results have been validated by irradiation experiments, as reported in a separate abstract. OC-0344 Experimental validation of TOPAS neutron dose for normal tissue dosimetry in proton therapy patients G. Kuzmin 1 , A. Thompson 2 , M. Mille 1 , C. Lee 1 1 National Cancer Institute, Division of Cancer Epidemiology and Genetics, Rockville, USA 2 National Institute of Standards and Technology, Radiation Physics Division, Gaithersburg, USA Purpose or Objective In the last several years, the popularity and use of proton therapy has been increasing due to its promise of a dosimetric advantage over conventional photon therapy. This is especially of great importance in pediatric patients who have a higher risk of developing late effects. During proton therapy 90% of scatter dose is from neutrons, which can travel out of the treatment field and can be highly biologically effective. In order to conduct epidemiological investigations of the risk of long term adverse health effect in proton therapy patients, it is imperative to accurately assess radiation dose to normal tissue. Tool for Particle Simulation (TOPAS) based on the GEANT4 Simulation Toolkit may be a computational option for normal tissue dosimetry to support large scale epidemiological investigations of proton therapy patients. While previous works have benchmarked TOPAS for proton dosimetry within treatment fields, there is a lack of validation for neutron scatter and energy spectrum. In the current study, we measured the energy spectrum of scattered neutrons using a simple physical phantom coupled with a series of Bubble Detectors irradiated by Californium-252 neutron source.

The measured spectrum was unfolded and shows to be in good agreement with the simulation. On average, the percent difference in the spectrum was less than 31% (Graph 1) and the percent difference of dose was under 23%. The agreement was best at the neutron energies 10 keV – 100 keV (19 %) and worst at 2.5-10 MeV (91 %). Better statistics are needed for the higher energy spectrum region. We plan to conduct the measurement three times to minimize statistical errors and plan to extend the validation to anthropomorphic physical phantoms. Conclusion We validated the dose and energy spectrum of scattered neutrons computed from TOPAS Monte Carlo code by the measurements using Bubble Detector. We plan to utilize TOPAS dose calculation system coupled with patient- specific proton therapy data for normal dose calculations to support epidemiological studies of proton therapy patients. OC-0345 Comparing cranio spinal irradiation planning for photon and proton techniques at 15 European centers E. Seravalli 1 , M. Bosman 2 , G. Smyth 3 , C. Alapetite 4 , M. Christiaens 5 , L. Gandola 6 , B. Hoeben 7 , G. Horan 8 , E. Koutsouveli 9 , M. Kusters 10 , Y. Lassen 11 , S. Losa 4 , H. Magelssen 12 , T. Marchant 13 , H. Mandeville 3 , F. Oldenburger 14 , L. Padovani 15 , C. Paraskevopoulou 16 , B. Rombi 17 , J. Visser 14 , G. Whitfield 13 , M. Schwarz 17 , A. Vestergaard 18 , G.O. Janssens 19 1 UMC Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands 2 University Medical Center Utrecht, Radiotherapy, Utrecht, The Netherlands Proffered Papers: Treatment planning applications