Assessment of Linac photon dose delivery on Cherenkov production

Cherenkov imaging has emerged as a promising real-time optical dosimetry modality for treatment visualization and delivery verification. However, the dependence of Cherenkov emission on photon field size and beam-quality variations remains insufficiently characterized, particularly for highly modulated small-field delivery techniques. This work investigated whether field-size-dependent changes in dose delivery affected the production of Cherenkov photons.

Cherenkov emission from a tissue-equivalent optical phantom was imaged using an intensified complementary metal-oxide-semiconductor (CMOS) camera for rectangular photon fields between 0.5 × 5.0 cm² and 15.0 × 5.0 cm². Monte Carlo simulations using the Tool For Particle Simulation (TOPAS) framework quantified field-size-dependent changes in incident beam energy, contaminant particle spectra, secondary particle generation, and angular transport to Cherenkov production. Contributions from photons, electrons, positrons, and contaminant particles were evaluated alongside the angular distributions of Cherenkov-producing charged particles.

Measured Cherenkov emission per unit dose decreased nonlinearly with decreasing field size, consistent with disruption of optical equilibrium driven by tissue scattering and absorption effects. Simulations demonstrated reduced field sizes increased mean incident photon and contaminant electron energies but produced only a modest change in the energy of Cherenkov-generating particles, confirming that Cherenkov yield remains strongly dose-correlated. Secondary electrons generated within tissue dominated Cherenkov production across all field sizes, while secondary electrons originating in the accelerator head contributed measurably only in superficial regions of large fields.

Field size dependent Cherenkov response reflects changes in particle and optical transport, highlighting considerations for Cherenkov imaging of highly modulated treatments.