Hybrid Monte Carlo model for efficient tissue Cherenkov emission estimation to assess changes from beam size, energy and incidence

Purpose: Cherenkov imaging in radiation therapy provides key knowledge of the delivery of treatment plans, but light-tissue interactions alter the emitted spectral signal and cause the modeling of emission relative to dose in highly modulated treatment plans to be complex. Methods: A 2-stage Monte Carlo approach to modeling Cherenkov emission was developed that leverages a traditional treatment planning system with an optical Monte Carlo simulation to provide a widely useable and efficient tool for modeling every beam control point for delivery interpretation of highly-modulated treatment plans. The emitted optical spectra were estimated for 6, 10, 15MV photon beams, 6 MeV electron beams, beam incidence in tissue, and square field sizes from 1 cm to 20 cm. The model was validated through comparison of measured Cherenkov emission from a blood and intralipid optical phantom. Results: The resulting hybrid model provides an efficient method of estimating Cherenkov emission for linac beams, showing a clear trend of decreasing emission intensity with increasing beam energy and strong emission intensity variation with beam type. The largest change in observed intensity was from altering field size, with a 76 % intensity decrease when going from 20 cm down to 1 cm square. The model showed agreement with experimental detected Cherenkov with an average percent difference of 6.2 % with the largest difference at the very smallest beam sizes. Conclusion: The model potentially allows for modeling entire modulated treatment plans with high computational efficiency and is a key step to translate delivered dose and observed Cherenkov in highly modulated situations.