C Duzenli*, C Mendez, M Petric, J Sweeney, D Ta, T Karan, BCCA Vancouver Centre, Vancouver, BCCA

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  C Duzenli

PO-GePV-T-42 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: To build on previous experiments and improve reproducibility of electron FLASH delivery on a conventional linear accelerator, a pulse-gating circuit was constructed and tested with several dosimeters.

Methods: FLASH mode was achieved by operating a Varian iX linear accelerator with 15MV photon operating parameters, with the target and flattening filter retracted and a 12MeV electron foil in the beam’s path. Pulse gating was achieved by an Arduino-based circuit interfaced with the linac gating module and a scintillator-photodiode sensor that was in the beam’s path to count pulses. Dosimeters were placed at a source-to-surface distance of 47-47.5cm, with 0-5mm of build up and 2-3cm backscatter. Measurements were performed in open fields and on cut-outs ranging from 1x2cm2 to 3x3cm2. Doses were measured using a 0.01cc volume ion chamber, optically stimulated luminescence dosimeters (OSLDs), Gafchromic MD film and a novel plastic scintillation detector and Hyperscint spectral analysis. Film profiles and PDD were used to confirm dosimetry measurements were performed in a region receiving >90% of maximum dose.

Results: A dose rate of 225 ± 11Gy/s was achieved in FLASH mode. The pulse-gating method delivered the specified pulse count in 89% of irradiations and was within 2 extra pulses for 11% of the beams, as confirmed by scintillator. Corrected ion chamber, OSLD and scintillator doses showed excellent agreement for a range of 0-60Gy, while film showed saturation effects past 30Gy. The dose per pulse varied from 1.1Gy/pulse (7 pulses delivered) to 1.3Gy/pulse (40 pulses delivered). Oscilloscope and scintillator readings indicated consistently lower amplitude of the first pulse compared with subsequent pulses.

Conclusion: A prototype pulse-gating method for eFLASH deliveries on a conventional linear accelerator was implemented. Further efforts will aim to reduce circuitry delays resulting in extra pulse delivery and compensation for pulse height for more consistent dose delivery.

Keywords

Dosimetry

Taxonomy

TH- External Beam- Electrons: Development (new technology and techniques)

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