Purpose: A fast-imaging technique was developed for the first in vivo Cherenkov emission imaging from an ultra-high dose rate (UHDR) electron beam source (360 Hz repetition rate) at single pulse and 1 millimeter resolution for real time monitoring of delivery.
Methods: A CMOS camera, gated to the UHDR LINAC, imaged the Cherenkov emission profiles pulse by pulse during irradiation of murine limbs and intestinal regions and a tissue equivalent phantom. The intensifier’s effect on image quality was investigated considering signal to noise and spatial resolution. Camera response and Cherenkov emission profiles from individual pulses were quantified spatially and temporally and Cherenkov intensity was compared to a dose rate independent EDGE diode detector to confirm beam output.
Results: An intensifier improved the emission profile’s signal to noise ratio from 15 to 280, with reduced spatial resolution (2.8 to 1.0 line pairs/mm). The profile extended beyond the treatment field edge due to the lateral scattering of the electrons and optical photons in tissue. The CMOS camera with an intensifier detected changes of ~3mm in Cherenkov emission profiles due to expiration and inspiration during part of the murine respiratory cycle sampled. The camera resolved the LINAC’s variability in output agreeing with the diode to within 4%.
Conclusion: This fast-imaging technique can be utilized for in vivo intrafraction monitoring of FLASH patient treatments at single pulse resolution. It can display delivery differences during respiration, and variability in the delivered treatment’s surface profile, which may be perturbed from the intended UHDR treatment especially with pencil beam scanning systems. The technique may leverage the Cherenkov emission surface profile as a quality control and patient monitoring tool to document treatment under FLASH conditions while considering beam parameters such as beam’s per pulse output or spatial profile consistency during delivery.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by Norris Cotton Cancer Center seed funding (core grant P30 CA023108 and shared irradiation service), Thayer School of Engineering seed funding, NIH grants (R01EB023909, R01EB024498, R42CA224646-02), and Department of Medicine (SEAM) Awards Program from the Dartmouth Hitchcock Medical Center and Geisel School of Medicine.