(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: Recent studies have shown that radiation therapy at ultra-high dose rates (>40 Gy/s, FLASH) may significantly reduce normal tissue toxicity compared to conventional dose-rate delivery. The purpose of this work was to 1) demonstrate the feasibility of proton FLASH beam delivery using a gantry-mounted clinical synchrocyclotron; and 2) to investigate the performance of different types of ionization chambers for FLASH proton dosimetry.
Methods: A Mevion HYPERSCAN clinical synchrocyclotron was modified for FLASH beam delivery. Protons with 230 MeV energy were delivered with pulse widths from 1 µs to 20 µs to provide conventional and ultra-high dose rate beam delivery, respectively. A boron carbide block was introduced in the beam to reduce the proton beam range. The number of protons per pulse at various dose rates were measured by a Faraday cup. The absolute dose rates were measured by three different ionization chambers (PPC05 plane-parallel and CC04 and A16 cylindrical chambers) and compared to dose-rates calculated by TOPAS Monte Carlo simulation from the Faraday cup collected charge. In addition, integral depth dose (IDD) was measured with a plane-parallel Bragg ionization chamber.
Results: The cylindrical ionization chambers studied in this work showed significant ion recombination. The average dose rate measured using the plane-parallel chamber showed 101 Gy/s at the entrance and 216 Gy/s at the Bragg peak. Integral depth dose (IDD) measurement with the ion chamber agreed well with the Monte Carlo simulation.
Conclusions: It is feasible to deliver proton FLASH beams, with 100-200 Gy/s average dose rate, using a gantry-mounted proton therapy synchrocyclotron. Extreme caution must be exercised for FLASH dosimetry.
Funding Support, Disclosures, and Conflict of Interest: Townsend Zwart, Miles Wagner, and Daniel Catanzano are employees of Mevion Medical Systems.
Keywords
Dosimetry, Monte Carlo, Protons
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