# Unraveling the Redox Mechanisms Underlying FLASH Radiotherapy: Critical Dose Thresholds and NRF2-Driven Tissue Sparing

## TL;DR

This study uncovers how FLASH radiotherapy triggers tissue sparing by activating NRF2 pathways and reducing oxidative stress, with a focus on dose thresholds and redox mechanisms to optimize therapeutic outcomes.

## Contribution

It reveals that a lower FLASH dose can still achieve tissue sparing if a threshold is met and elucidates the redox mechanisms involving NRF2 activation and ROS reduction.

## Key findings

- FLASH-RT enhances peroxyl radical recombination reducing ROS.
- Activation of NRF2 pathway promotes tissue protection.
- A reduced dose threshold is critical for sparing effect.

## Abstract

FLASH radiotherapy (FLASH-RT) achieves tumor control comparable to conventional dose-rate irradiation (CONV-RT) while significantly reducing radiation damage to normal tissues. However, the physical conditions triggering the FLASH sparing effect remain unclear, and mechanisms related to oxidative stress and redox regulation are poorly understood. This study utilizes a murine acute intestinal toxicity model to investigate how beam parameters influence the FLASH sparing effect and tumor control using innovative FLASH-RT and CONV-RT combined irradiation. Results demonstrate for the first time that a substantially reduced FLASH dose can still elicit sparing effect, provided a total dose threshold is met. Kinetic simulation and experimental validation demonstrate that FLASH-RT enhances peroxyl radical recombination, reducing reactive oxygen species (ROS) and malondialdehyde levels. Antioxidant interventions further confirm the essential role of free radicals. RNA sequencing and molecular analyses reveal that FLASH-RT activates the nuclear factor E2-related factor 2 (NRF2) antioxidant pathway while suppressing extracellular signal-regulated kinases (ERK) signaling, thereby enhancing cellular redox defenses, reducing apoptosis, and mitigating ROS-mediated tissue injury. These findings highlight the feasibility of optimizing the FLASH-RT therapeutic window through redox modulation and provide a foundation for developing free radical-targeted strategies to improve its therapeutic efficacy.

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Source: https://tomesphere.com/paper/2508.20510