Comprehensive Characterization of Beam Current Transformers for the Performance Evaluation of Flash Linacs Producing Ultra-High Dose Rates for Flash Radiotherapy
Abstract
Purpose
The beam output of linacs producing ultra-high dose rates for FLASH radiotherapy (FLASH linacs) can exhibit significant and sometimes unpredictable variations in response to beam parameter adjustments, differing even among identical models. This study aims to characterize the beam output stability and pulse-structure dependence within and between three IntraOp Mobetron electron FLASH linacs using internal Alternating Current Transformers (ACCTs) for direct, non-invasive beam current monitoring.
Methods
Three IntraOp Mobetrons capable of producing 6 and 9 MeV UHDR (> 800 Gy/s) with dual-ACCTs integrated into the head of the linac were included in this study. For both energies, the absorbed dose at depth of maximum dose (dmax) in a solid water phantom was evaluated against the integral of the beam current measured by the ACCTs using Gafchromic EBT-3 films in triplicates. The output variability with respect to Pulse Width (PW, 0.5 – 4 μs), Pulse Repetition Frequency (PRF, 30 – 120 Hz), and Number of Pulse (2 – 20 pulses) was characterized.
Results
Dose linearity with respect to the ACCT integral charge was confirmed (R2 > 0.999) for all three units. PW dependence varied significantly among the units with one unit showing a non-linear increase in output with increasing PW while another exhibited a linear decrease in output, resulting in an approximately 30% loss of output. Furthermore, output decreased by ~5% as the pulse number increased from 2 to 20, suggesting beam loading effects. Output variation remained within 3% when PRF was adjusted across all units.
Conclusion
ACCTs provide a robust method for the individual characterization of FLASH linacs, revealing unit-specific beam behaviors. The variability in PW dependence across identical models highlights the necessity of commissioning specific beam current monitoring systems for FLASH-enabled units.