Author: Alexander Baikalov, Luke Connell, Nicholas Coupey, Nolan M. Esplen, Rebecca Lim, Chinh Nguyen, Emil Schueler 👨🔬
Affiliation: Northwell, The University of Texas MD Anderson Cancer Center, Department of Radiation Physics, The University of Texas MD Anderson Cancer Center 🌍
Purpose: FLASH radiotherapy delivered using electron linear accelerators (LINAC) requires precise control and minimal variation of dose rate and We developed a robust external beam control system (EBCS) for radiofrequency (RF) power optimization and beam monitoring which addresses this need.
Methods: The EBCS was implemented on a converted Clinac 21EX LINAC to support conventional DPP (C-DPP) and ultra-high DPP (UH-DPP) delivery, utilizing either an internal transmission ion chamber (IC) or beam current transformer (BCT) to monitor the beam. The EBCS used the LINAC gating system to hold the beam while beam output and stability were maximized by optimizing the accelerating RF power efficiency through voltage inputs (VEXT) to the automatic frequency control interface. We investigated EBCS performance by characterizing beam off latency, and beam output stability within and between deliveries, the sensitivity to deviations from optimization solutions, and the BCT linearity through a range of gun currents.
Results: The beam off latency was calculated to be (56.7±4.9) µs and all available repetition rates were successfully implemented. The RF optimization was shown to reduce the DPP variability within the first five pulses from 26.7% to below 0.5% for both C-DPP and UH-DPP modes, established using the IC and BCT, respectively. We observed total output was reduced by up to 20% or 80% when VEXT varied from the optimal solution by more than -15% and +20% respectively. The BCT was additionally shown to be linear with dose (R2>0.9999) for all DPP tested (0.007-16.25 Gy/pulse at 98-cm SSD).
Conclusion: This EBCS, capable of delivering reproducible DPP, was developed and implemented on a converted clinical LINAC. Through real-time read-out of the BCT signal and automatic RF optimization, the uncertainty in dose delivery and in DPP within each delivery, was reduced to <0.5%, offering unprecedented precision and accuracy