Room: Track 2
Purpose: To show for the first time video-rate images of visible light emission excited by scanned proton pencil beams in biological tissue ex vivo. Spatial and spectral properties of the visible emission was investigated, and results compared against Monte Carlo simulations to examine the light emission mechanisms.
Methods: 240 MeV proton pencil beams were delivered to porcine tissue (layer composition: 0.5 mm skin, 7 mm fat, 20 mm muscle) by Varian ProBeam system. Visible emission from tissue was collected by 50 mm f/0.95 lens and detected by intensified CMOS camera. Emission spectra were recovered using a set of optical bandpass filters. Bi-alkali and GaAs intensifiers were used to maximize detection efficiency in blue and red spectral range, respectively. Camera collected the scanned beam at 100 frames per second, visualizing each individual spot. Theoretical proton dose profiles were acquired from RayStation treatment planning system; proton-excited Cherenkov emission was simulated in GAMOS environment.
Results: Upon proton irradiation we observed a broadband light emission covering a range of 400-800 nm, peaked at 630 nm. Comparing the spatial cross-beam intensity profiles against Raystation TPS dose profile, the light emission matched TPS at the level of >90% accuracy of full width at half maxima, but with elevated halo intensity. Similarity of cross-beam intensity profiles for all wavelengths suggest superficially-weighted origin of light emission. Based on our findings we hypothesize that we observed Cherenkov radiation excited by high-energy secondary electrons, which is further supported by our Monte-Carlo simulations.
Conclusion: For the first time we observed visible light emission from tissue irradiated by scanned proton pencil beams. In analogy to Cherenkov in vivo imaging during conventional radiotherapy, this work could enable a path towards real time imaging and direct localization of beam entrance in relation to patient’s anatomy during proton therapy.
Funding Support, Disclosures, and Conflict of Interest: The authors acknowledge the Irradiation Shared Resource at the Norris Cotton Cancer Center at Dartmouth with NCI Cancer Center Support Grant P30 CA023108 and the NIH Grant R01 EB023909. Brian Pogue reports financial interest in DoseOptics LLC, a company developing cameras and software for the use of Cherenkov imaging.