Purpose: Dosimetric control of electron FLASH radiotherapy systems demand pulse level control, as each pulse delivers a non-trivial fraction of the overall dose. In our delivery system, an initial beam current ramp results in a variable dose per pulse, limiting the accuracy of dose determination through digital pulse counting. Here we describe our initial attempts at describing this effect through analog dose integration using an external diode.
Methods: We created a 10MeV FLASH beam that delivers roughly 200Gy/s at the mylar window of the machine. We placed an external diode into the beam and connected a custom low-noise amplifier circuit which delivers a time-dependent analog signal to an oscilloscope and a digital pulse counting signal to an Arduino Uno. We used the Arduino to interrupt the pulse forming network of the linac after a preset number of pulses and attempted to relate pulse height as measured on the oscilloscope to delivered dose. We used radiochromic film to measure absolute dose.
Results: Our diode circuit saturated at the 6th pulse in all trials. We, therefore, delivered pulse trains of 2, 3, 4, and 5 pulses and plotted integrated pulse height (V) vs dose (Gy). The goodness of fit of the linear curve was R2=0.9965. Removal of the 5 pulse data point improved the fit to R2=1. This is likely due to the 5th pulse nearing diode saturation.
Conclusion: Preliminary tests indicate a simple proportional relationship between pulse height when not near diode saturation. Tuning the diode amplifier circuit and performing analog integrating of pulse heights should provide better dosimetric beam control than our current digital pulse counting system.