ESTRO 2020 Abstract book

S932 ESTRO 2020

comparison

with

TOPAS

simulations.

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.

Conclusion The profile of scattered protons back-projected onto the isocenter plane parallel to the particle tracker of a pCT system provides high-resolution tumor position and beam fluence monitoring during high-dose-rate delivery of protons. This technique appears suitable for intra- treatment monitoring of FLASH therapy with shoot-through beams. PO-1616 Quantitative evaluation of prostate SABR verification workflow using triggered kV-imaging and CBCT G. Antal 1 , Á. Gulybán 2 , K. Kisiván 1 , A. Farkas 1 , F. Lakosi 1 1 Somogy County Kaposi Mor Hospital, Dr. József Baka Diagnostic- Oncoradiology and Research Center, Kaposvár, Hungary ; 2 Europa Hospitals, Department of Radiation Oncology, Brussels, Belgium Purpose or Objective During stereotactic ablative radiotherapy (SABR) rigorous verification strategy is required to ensure safe treatment delivery. We aimed to evaluate our clinical workflow for prostate SABR using triggered kV imaging (TkVI) in combination with repetitive CBCTs by focusing on intra- and inter-fractional changes on target coverage. Material and Methods Ten patients were treated with VMAT based SABR after gold marker implantation for a total dose of 36.25 Gy in 5 fractions. Following verification workflow was applied: 1) pre-radiotherapy CBCT (pre-CBCT) with gold marker-based correction; 2) treatment delivery with TkVI (≥3mm threshold for interruption/correction) 3) post-CBCT. Prostate, rectum, bladder were delineated on each CBCT. Difference in target coverage (D98) compared to planning CT were analyzed in three different clinical scenarios: 1) pre-CBCT 2) post-CBCT volumes with applied correction and 3) post-CBCT without TkVI. Intrafractional OAR dose differences using D10cc were also calculated. Results On average two beam interruptions (range: 0-10) were required during the 16±12 min treatment sessions. The average [min,max] D98 deviation from planned dose were modest in scenarios 1-2 (-0.14% [-0.51,0.32] and -0.32% [- 0.97,-0.1]) compared to -1.89% [-11.0,-0.1] without TkVI. In two cases the absence of TkVI would have led to a total of -11% and -5.6% target coverage loss caused by a single large and two intermediate shifts. The mean intrafractional changes of bladder D10cc were small, however variation were reduced by using TkVI (0.01±0.1Gy vs. -0.01±0.03Gy). For rectum D10cc no relevant difference were observed (-0.11±0.23Gy vs. - 0.14±0.33Gy). Conclusion Gold marker-based prostate SABR with triggered imaging and pre/post-treatment CBCT was successfully implemented. Dosimetric evaluation showed maintained target coverage through the whole clinical workflow underlying the importance of intrafractional imaging.

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