Electron Ultra-High Dose-Rate (FLASH) beam monitoring and control through beam current transformers
PD-0905
Abstract
Electron Ultra-High Dose-Rate (FLASH) beam monitoring and control through beam current transformers
Authors: Emil Schueler1, Kevin Liu2, Allison Palmiero1, Nitish Chopra1, Ziyi Li3, Sam Beddar1
1MD Anderson Cancer Center, Radiation Physics, Houston, TX, USA; 2MD Anderson Cancer Center, Radiation Physics, Houston, tX, USA; 3MD Anderson Cancer Center, Biostatistics, Houston, TX, USA
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Purpose or Objective
There is now overwhelming pre-clinical evidence that using ultra-high dose rates (UHDR, >40 Gy/s) compared to conventional dose rates (~0.1 Gy/s) results in significantly lower levels of toxicity post treatment while tumor control remains isoeffective. While we have the capability of delivering these dose rates with current technology, we do not have a way to control our treatment delivery beyond counting pulses, which will hinder the safe clinical translation of FLASH radiation therapy (RT). Here we present our work on the use of beam current transformers (BCTs) that could replace the standard transmission ion chambers in UHDR beams for beam monitoring and control.
Material and Methods
The Mobetron (IntraOp Medical) was updated with two internal BCTs at different positions within the head of the linac. The BCTs were evaluated against known dose-rate independent dosimeters (Gafchromic EBT3 film, TLDs, and OSLDs) and calibrated to a reference point. The possible dependencies against dose, mean dose rate, instantaneous dose rate, pulse width, and dose per pulse was investigated. Furthermore, the reproducibility, long term stability, and response time of the entire system was evaluated to determine the use of BCTs for active beam control of eFLASH deliveries.
Results
The BCTs were found to be independent of mean- and instantaneous dose rate and showed a linear response to dose and dose per pulse up to the highest values investigated (8 Gy/pulse, >1000 Gy/s). The BCTs showed excellent reproducibility and long-term stability. The overall response time of the entire system (beam-off latency) was on the order of 1-2 ms but was limited by the read-out equipment. Higher temporal resolution equipment is currently being implemented which will likely bring these values down significantly.
Conclusion
The BCTs were found to be capable of accurately capture all the essential beam parameters for each individual pulse delivered. The two BCTs were calibrated individually to allow redundancy in beam monitoring and control. The system was found to be fast enough for beam control of pulse deliveries up to 500 Hz. Future work is focused on integrating the signal processing into the control system for full beam control.