Purpose: To explain why the Cherenkov intensity emitted from MV x-ray delivery in tissue does not appear to change with beam energy in radiotherapy. The study developed a quantitative theoretical understanding of the effects of beam and tissue properties on the reemitted light.
Methods: The reemitted Cherenkov signal was theoretically derived with a diffusion theory solution and experimentally measured as a function of the dose build-up slope and tissue optical properties over a range of clinical x-ray energies. Diffuse liquid tissue phantoms of various scattering and absorption properties were irradiated with 6, 10, and 18 MV x-rays. Each phantom was imaged with an intensified camera, and irradiated top-down for entrance surface imaging, as well as from underneath for exit surface imaging. Phantoms were built with the thickness of the depth of d(max) per energy. The reemitted Cherenkov intensity was calculated as the mean pixel intensity of a ROI at each phantom’s surface.
Results: At low energy, a higher surface dose and steeper build-up slope result in a greater reemitted Cherenkov signal per unit dose, a ~46% increase at 6 versus 18 MV. However, at high energy, more Cherenkov photons are generated per unit dose, resulting in a ~44% increase in the reemitted signal at 18 versus 6 MV. These competing effects yield an approximately constant reemitted Cherenkov signal from the surface of the phantoms at different energies. With increasing scattering and absorption properties of the phantoms, the reemitted Cherenkov signal increased ~10% and decreased ~20%, respectively.
Conclusion: The results of this study explain why the reemitted Cherenkov signal during radiotherapy is not strongly dependent on beam energy, despite greater Cherenkov photon generation at higher energies. Quantifying the effects of beam and tissue properties is a progressive step towards verifying surface dose delivery with Cherenkov imaging during radiotherapy treatments in real-time.
Funding Support, Disclosures, and Conflict of Interest: B. Pogue and P. Bruza are partially employed by DoseOptics LLC, manufacturing the C-Dose cameras provided for this research. D. Alexander is a paid consultant for DoseOptics LLC outside of this work.