Emission-associated energy deposition and Cherenkov-dose proportionality in megavoltage photon beams

Abstract

Background: Quantitative Cherenkov-to-dose conversion depends on beam spectrum, field size, depth, and material properties. Purpose: To distinguish dose contributions with and without accompanying Cherenkov production and assess their implications for calibration. Methods: Geant4 simulations paired dose and local Cherenkov distributions in water for flattened 6, 10, and 15 MV and flattening-filter-free (FFF) 6 and 10 MV beams with $3 \times 3$ and $10 \times 10$ cm$^2$ fields. Energy deposition was tagged by whether its transport step produced at least one Cherenkov photon. Component fractions were calculated over the phantom. Distributions were independently normalized within a shared reference region and compared using voxelwise ratios, lateral profiles, and spatial partitioning. Results: Emission-tagged fractions were 37%-61%, increasing with beam energy and lower for FFF beams. Fractions differed by less than 1.3 percentage points between field sizes. Emission-tagged ratios clustered near unity, with 98.4%-99.9% between 0.95 and 1.05 across the evaluated regions, whereas total and non-emission-tagged ratios varied with depth and lateral position. Emission-tagged profiles generally showed smaller residual differences. Conclusions: Emission-tagged deposition closely tracked local Cherenkov production. Departures of the total Cherenkov-to-dose ratio followed spatial variations in the emission-tagged dose fraction, indicating where calibration needs correction. The classification reflects actual photon production and depends on transport settings; dedicated electron-energy scoring is needed to isolate sub-threshold contributions.