Room: AAPM ePoster Library
Purpose: To characterize dose-rate, gun-ramp, and monitor chamber performance during FLASH electron radiotherapy using a clinical linear accelerator.
Methods: An Elekta Synergy linear accelerator with high dose-rate electron (HDRE) mode was modified to deliver an electron beam at maximum accelerator beam current. Additional signal attenuation was added between the monitor-chamber and the dose-cards to bypass dose-rate interlocks. An amplifier circuit was created to facilitate the analog readout of fluctuations in voltage generated in an external diode detector during beam-on. Gun-set and gun-standby parameters were altered to achieve shortest gun-ramp as determined by the diode readings. Monitor chamber calibration values and external attenuation were varied to test the feasibility of using the standard dosimetry systems during FLASH delivery.
Results: As expected, increasing gun-set values increased instantaneous dose-rates. Keeping the gun-standby value at a standard offset provided a full gun-ramp time of roughly 1 s. The shortest gun-ramp time (~<300ms) occurred when gun-standby was set to the same value as the gun-set value, markedly increasing average dose-rate. No beam leakage was seen during the initial tuning period of the magnetron. Additional signal attenuation of 1M? on all 8 monitor chamber channels was sufficient to bring signals from the chamber to within the acceptable range of the control system. With the altered gun parameters, dose-rates of >100 Gy/s were seen at the mylar window.
Conclusion: A rapid beam ramp time is possible if gun-standby values match gun-target values. The monitor chambers are capable of stopping the beam with fractional MU precision at FLASH dose-rates. The variation in monitor chamber collection efficiencies during the gun-ramp, and thus the variation in delivered dose, remains to be investigated.