A QED framework for radiation therapy describing collective energy transfer from clusters/bunches of charged particles at FLASH ultra-high dose rates

Clinical implementation of FLASH proton therapy demands the development of accurate and traceable dosimeters to ensure patient safety. Detectors, such as plastic scintillation dosimeters (PSDs), with their water equivalence, high temporal resolution and dose rate independence, could become the reference dosimeter for FLASH dosimetry, but their response suffer from ionization quenching around the Bragg peak curve in proton and other heavy ion beams. This work presents experimental data fitted with a first-principles quantum mechanical framework that models the ionization quenching phenomenon at FLASH ultra-high dose rates (UHDR). Our approach demonstrates how spatial and temporal particle clustering in dense ionization tracks further decreases ionization yield by channeling energy into non-radiative pathways, such as vibrational phonon modes, compared to conventional dose rates (CDR). Through experimental measurements at MD Anderson's FLASH-capable proton beamline, using a 5-channel PSD, we characterized the response of four plastic scintillators under both CDR and UHDR irradiation conditions. Multi-scale fitting of our theoretical model to experimental data revealed distinct parameter sets that quantify enhanced non-radiative decay contributions under FLASH conditions, with excellent agreement (R^2 > 0.94) across all fits. We identified a continuous transition between two physical regimes: intra-track and inter-track dominant interaction regimes, as a function of linear energy transfer (LET) responsible for quenching mechanism at CDR and UHDR, respectively. This study provides a quantitative foundation for improved interpretation and calibration of PSD signals in FLASH proton therapy.