ESTRO 2020 Abstract Book

S880 ESTRO 2020

Physics and Dosimetry, Warrenville, USA ; 4 Northern Illinois University, Department of Physics, DeKalb, USA ; 5 Santa Cruz Institute for Particle Physics- University of California, Physics Department, Santa Cruz, USA ; 6 University of California, Radiation Oncology, San Francisco, USA Purpose or Objective There is great interest in using ultra-high dose rate (FLASH) proton therapy for lung tumors. However, verification and dosimetry are challenging. The purpose of this work was to test a new idea of verifying FLASH proton therapy by analyzing scattered protons. Material and Methods FLASH proton therapy (PT) delivers a high dose of protons in a fraction of a second. Normal tissue toxicity is much less of a concern for FLASH PT. Thus, one may use the plateau beam of the proton beam placing the Bragg peak outside the patient (shoot-through beam). For lung tumors, one could use available proton beam energies to this end. Protons will be scattered in various directions from the tumor due to large-angle scattering. The surrounding low-density lung tissue will lead to less scattering. We placed a 4-cm long, 1.9-cm diameter cylindrical tissue-equivalent insert (1.07 g/cc) into a Styrofoam holder (Fig. 1) and irradiated it with a stationary proton pencil beam of 140 MeV. The two-plane tracking detector of a preclinical proton CT scanner detected scattered protons at a rate of about 1 million particles per second. The energy detector of the scanner provided the trigger signal for data acquisition. We back- projected the registered particle tracks onto the plane containing the beam axis. The profile of the signal was then analyzed and compared to the output from a TOPAS simulation.

Results Fig. 2 shows the back-projected profile of the protons scattered from a tissue-equivalent insert (1,07 cc/g) along the beam axis. The protons had a nominal energy of 140 MeV in this case. The profile shows the position and longitudinal size of the insert with submillimeter accuracy. Further analysis will include irradiation of other tissue- equivalent materials at many beam energies and

Conclusion From this first analysis, the use of NHFT in combination with surface scanning monitoring allows safely treating breast and lung cancer patients with prolonged voluntary and stable BH over multiple fractions of treatment. NHFT is therefore a feasible approach for supporting prolonged patient breath holds. PO-1615 Detection and analysis of scattered protons for verification of FLASH lung tumor proton therapy M. Garbacz 1 , R. Schulte 2 , V. Bashkirov 2 , M. Gao 3 , M. Pankuch 3 , C. Sarosiek 4 , R.P. Johnson 5 , J. Ramos Mendez 6 , A. Rucinski 1 , P. Olko 1 1 Institute of Nuclear Physics PAN, Proton Radiotherapy Group, Krakow, Poland ; 2 Loma Linda University, Department of Basic Sciences, Loma Linda, USA ; 3 Northwestern Medicine Chicago Proton Center, Medical