Author: Luke Connell, Stephen F. Kry, Malcolm R. McEwen, Bryan R. Muir, James Renaud, Emil Schueler, Hayden Scott, Paige A. Taylor 👨🔬
Affiliation: National Research Council, The University of Texas MD Anderson Cancer Center, UT MD Anderson Cancer Center, Department of Radiation Physics, The University of Texas MD Anderson Cancer Center 🌍
Purpose: To demonstrate the capability of a robust calorimeter to accurately determine the absorbed dose delivered by an electron FLASH beamline.
Methods: Electron FLASH beams of 6 MeV and 9 MeV were produced using an IntraOp Mobetron unit - 4us pulse width, 90 Hz pulse repetition frequency, ~ 2 Gy per pulse. A calorimeter based on Bourgouin et al (2020) was fabricated from aluminum and the core temperature was determined using two thermistors connected to a high-accuracy 8.5 digit DMM. The thermistor resistance was converted to temperature through a calibration step and knowledge of the aluminum specific heat capacity yielded absorbed dose.
Two investigations were carried out: in the first, dose linearity was assessed by delivering between 1 and 8 pulses using the 9 MeV beam. In the second, the aluminum material upstream of the core was varied to obtain the dose as a variation of measurement depth for both 6 MeV and 9 MeV beams. A consistent measurement protocol was applied to control cumulative thermal effects and standard analysis methods were used to determine the absorbed dose.
Results: It was found that temperature-time traces were likely impacted by RF generation of the accelerator prior to beam-on but the effect appeared to be consistent. The typical standard uncertainty in a set of six runs was 0.5 %, providing accurate results in limited time. The performance was independent of day, energy or measurement depth. The calorimeter demonstrated excellent pulse linearity, yielding a dose per pulse at a water equivalent depth of 0.7 g cm-2 of 1.9 Gy.
Conclusion: Operation of a portable calorimeter system for the accurate (sub-1 %) dosimetry of FLASH beams from a Mobetron unit has been demonstrated, allowing for real-time dose determination and the calibration/investigation of other dosimeters.