Author: Kristina Bjegovic, Luke Connell, Emil Schueler, Leshan Sun, Lucy Whitmore, Liangzhong Xiang 👨🔬
Affiliation: University of California, Irvine, MD Anderson, Department of Radiation Physics, The University of Texas MD Anderson Cancer Center 🌍
Purpose: The use of FLASH-RT in the clinic would greatly reduce off-target radiation toxicity in normal tissues. However, due to the hypo-fractionated delivery of prescribed doses at FLASH dose rates, real-time dose mapping is necessary to ensure proper tumor coverage. Radiation-induced acoustic dosimetry (RiAD) is a novel real-time, 3D, in situ method for monitoring volumetric dose deposition deep in the body, which maintains dose rate independence even in the FLASH regime and allows for monitoring of the spatial conformity of the delivered dose in each hypo-fractionated pulse.
Methods: A modified FLASH-capable electron beam line (Mevion, >1Gy/pulse) was collimated into 6 distinct shapes. The conformed FLASH beams were monitored first in water and then in 3 mice with a custom 256-channel transducer array. Data was collected with a 256-parallel channel DAQ (Photosound) and all signals were processed in MATLAB. Signals in water were collected in two orientations, allowing for gamma index analysis comparisons to film and Percent-Depth Dose curve generation. For in vivo experiments, mice were held in a 3D printed holder for proper alignment to the transducers, allowing for reconstructed signal overlay onto prior-CT scans.
Results: Reconstructions for individual pulses demonstrated high pass rates (>90%) for 5%/5mm gamma analysis for small collimators (e.g. 1cm diameter circle) and accurate PDD generation in water. Single pulse 3D reconstructions of electron beams were also achieved in vivo, allowing for real-time monitoring of beam conformity in target tissue for the first time.
Conclusion: With the RiAD system, single-pulse irradiation of varying collimation was monitored volumetrically in vivo for a FLASH dose-rate electron beam for the first time. The high-fidelity reconstructions were overlaid on CT, achieving accurate dose mapping and precise localization with respect to clinically relevant topography, giving clinicians access to the precise, qualitative, real-time information needed to implement FLASH-RT in the clinic.