Purpose: In electron FLASH-RT, precise delivery of the correct number of pulses is critical to accurate dose administration in preclinical radiobiological studies. This work investigates the use of LINAC monitor ion chambers to most precisely control FLASH pulse delivery.
Methods: A Varian Clinac 21EX was modified and tuned to deliver 16 MeV electrons at FLASH dose rates (>100 Gy/s,>0.55 Gy per pulse). Instead of stopping the beam with a custom circuit asserting an external interlock, we use monitor units (MUs) in the primary monitor ion chamber (MU1) in a range of 1-60, which corresponds to a dose range of 2-110 Gy. A calibrated plastic scintillation detector and EBT-XD Gafchromic films were used for online and passive dosimetry, respectively. The plastic scintillation detector also served as a direct pulse counter.
Results: The circuit board interpreting the monitor ion chamber signal is saturated by the ultra-high instantaneous FLASH dose rate (~10⁶ Gy/s), which significantly impacts the relationship between MUs counted and radiation dose delivery. However, this saturation can be exploited to convert the monitor ion chamber system into a real-time internal pulse counter. The number of pulses delivered exhibits a highly reproducible, perfectly linear relationship with programmed MU1 (3.19 pulses/MU, R²=1). In contrast to our custom external circuit, which delivers with a ±1 pulse uncertainty, no pulse delivery uncertainty was observed when using MU1. Given the stable dose rate of our FLASH system (SD = ±1.1%), the dose delivered is linear with MU1 over a large dose range (2-110 Gy) for both scintillator and film measurements.
Conclusion: This study demonstrates that the primary monitor ion chamber can act as a simple, stable and reliable control for precise FLASH pulse delivery.