Room: Track 3
Purpose: A feasibility study of modifying a standard clinical linear accelerator to deliver electron FLASH (eFLASH) dose rates (>50 Gy/second) for irradiation of cell suspensions and 3D spheroids as well as in vivo irradiations of tumour-bearing mice.
Methods: To achieve FLASH dose rates, a standard linac was set up to 10 MV photon mode with an electron foil in the beam path instead of the target. An Arduino-controlled circuit was used to control the linac gating interface to produce millisecond beam-on times. The eFLASH dose rate was evaluated at several source-to-surface distances (SSD) with a PTW microDiamond and a 0.01 cc volume Wellhöfer CC01 ion chamber.
Characterization of the eFLASH beam (PDD, profiles and output stability) was done with the CC01 ion chamber and Gafchromic film. Due to fluctuations in dose rate, particularly for very short (~10 ms) beam deliveries, a reference ion chamber was used to determine doses delivered at the central axis for sample irradiations with both FLASH and standard clinical 9 MeV beams and validated with optically-stimulated luminescence dosimeters (OSLD) and Gafchromic film. Doses of 5, 10, 15, 20 and 25 Gy were delivered for all experiments.
Results: The experimental samples were placed at the level of the X jaws (47 cm SSD) resulting in dose rates up to 300 Gy/s at the point of measurement. The eFLASH PDD curve was comparable to a clinical, standard set-up 9 MeV beam. The eFLASH field size was ~20x20 cm² as measured at FWHM on film profiles, with a central ~10x10 cm² area that received =90% of the intended dose. Passive dosimetry showed good agreement with ion chamber doses, except for OSLD under response above 20 Gy.
Conclusion: An eFLASH experiment was successfully performed on a clinical linear accelerator. Sources leading to dose rate instability warrant additional investigation.