Room: ePoster Forums
Purpose: Recent reports have shown that very high dose rate radiation preferentially spares normal tissues. We characterized a 20 MeV non-clinical beam at about 36.5 Gy/sec.
Methods: A non-clinical machine was modified to deliver high dose rate 20 MeV beam. IC profiler was used at 100 cm to re-steer the beam (without the electron cone). For detectors, we chose Exradin model A16 chamber due to higher operating bias; PTW model CC04 for infield monitoring chamber and Gafchromic film and TLD detectors since they are dose rate independent. An Arduino board was used to generate the desired gating signal since monitor chamber is not sufficient in FLASH mode.
Results: The reproducibility of delivered dose varied within session. A warm up of 1000 MU and gating delays from 3-9 seconds improved reproducibility. Delays of 3, 6 and 9 seconds to the start of the gate signal reduced output variation by 4.9%, 1.5% and 2.3% respectively. Hence, we chose to use a 6 second delay. After all these efforts, the standard deviation of delivered dose for a gate of 1 second was about 1% open field and about 2% for a 2x2 cm^2 field at the level of the mirror. Output and dose rate fluctuations were observed. Increasing bias voltage from 300 to 629 on A16 decreased the Pion from 1.58 to 1.17, the Ppol increased from 0.94 to 0.87. We relied on Gafchromic film dosimetry to characterize output and profiles (at the mirror) of the FLASH beam and used A16 chamber as a secondary measurement.
Conclusion: We characterized a very high dose rate field for radiobiology experiments. It is important to develop accelerators capable of delivering consistent doses at varying dose per pulse as well as varying pulses per second for radiobiology experiments at these high dose rates.