Tissue microvascular dynamics as a functional imaging biomarker of the FLASH effect in radiotherapy

Ultra-high dose rate radiotherapy (UHDR) has shown promise in reducing normal tissue damage (the so-called FLASH effect) compared to conventional dose-rate irradiation (CDR), while maintaining tumor control. However, the spatiotemporal dynamics underlying this effect remain incompletely characterized in vivo, particularly in studies relying on endpoint or sparsely sampled measurements. In this study, we used optical coherence tomography (OCT) to perform dense longitudinal imaging of epidermal and microvascular responses following single-fraction irradiation of mouse thighs with CDR and UHDR. OCT enabled simultaneous, non-invasive quantification of epidermal thickness and vascular volume density (VVD), with histological correlation. At 20 Gy, CDR induced a transient increase in VVD, followed by a reduction and incomplete recovery, whereas UHDR produced a more controlled response, with stabilization near baseline by week 25. At 30 Gy, CDR resulted in a marked reduction in angiographic signal, with partial recovery, whereas UHDR showed a more gradual decline followed by a restoration toward baseline. At both doses, UHDR vascular density remained significantly higher than CDR at late time points. Epidermal responses paralleled vascular dynamics: UHDR-treated skin demonstrated thicker and more organized epidermis at day 10 and greater long-term preservation. Histology confirmed improved structural integrity under UHDR compared to CDR. These results demonstrate that OCT enables longitudinal, in vivo characterization of coupled epithelial and microvascular responses to radiation and reveal distinct modality-dependent response trajectories. By capturing both structural and functional dynamics in the same tissue over time, this approach provides insight not accessible through endpoint analyses. The findings support a role for vascular dynamics in normal tissue sparing associated with UHDR irradiation and highlight OCT as a translational tool for monitoring radiation-induced tissue response.