High-Flux Entangled Photon Generation via Clinical Megavoltage Radiotherapy Beams for Quantum Imaging and Theranostics

Traditional sources of quantum-entangled photons, such as spontaneous parametric down-conversion, suffer from low flux, limiting clinical applications. We investigated whether clinical megavoltage (MV) radiotherapy beams can serve as dual-purpose sources, delivering therapeutic dose while simultaneously generating high-flux entangled photon pairs for quantum ghost imaging, quantum theranostics (QTX), and related applications. Using GEANT4 Monte Carlo simulations, we modeled water-equivalent phantoms containing 2 cm spherical tumors loaded with gold nanoparticles (AuNPs, 10 mg/mL). Clinical spectra at 6, 10, and 15 MV were simulated. Key metrics included 511 keV photon-pair yield, positronium lifetimes (time-resolved), Doppler and energy broadening (energy-resolved), signal-to-noise ratios (SNR), and entanglement retention at depths from 5 to 15 cm. Pair yields scaled with beam energy and AuNP concentration, reaching about 10^8 pairs per Gy per cm^3 in AuNP-loaded tumors. Positronium lifetime shifts (about 100 ps) reflected tumor oxygenation and reactive oxygen species, offering potential functional biomarkers. Doppler (1-3 keV) and AuNP-induced line broadening (0.5-1 keV) produced distinct spectroscopic signatures of electron density, tissue composition, and nanoparticle uptake. Time-resolved QTX revealed oxygenation and ROS levels, energy-resolved QTX probed tissue density and atomic composition, and combined modes enabled comprehensive theranostic imaging. Depth-dependent simulations achieved voxel SNRs exceeding 100. These findings demonstrate that clinical MV beams can serve as practical, high-flux entangled photon sources. Integrated time- and energy-resolved QTX supports functional and spectroscopic tumor imaging, enables nanoparticle optimization, and expands quantum imaging platforms from optical (eV) to therapeutic (hundreds of keV) regimes.