Localized normal tissue-sparing effects of proton FLASH radiotherapy in a preclinical lung irradiation model

OBJECTIVES: FLASH radiotherapy (FLASH-RT), characterized by ultra-high dose rate irradiation (>40 Gy/s), has demonstrated the potential to spare normal tissues while maintaining tumour control. Most proton and electron FLASH studies have focused on whole-organ irradiation, and the normal tissue-sparing effects of high-dose proton FLASH-RT in localized thoracic settings remain unclear. METHODS: A preclinical mouse model was developed to evaluate localized high-dose (60 Gy) proton FLASH irradiation to the left lung using spot-size transmission at FLASH (500 Gy/s) or conventional (2 Gy/s) dose rates. Lung and skin responses were assessed by histology, flow cytometry, and enzyme-linked immunosorbent assays. RESULTS: FLASH-irradiated lungs exhibited decreased pneumonitis and fibrosis compared to conventional irradiation, with faster resolution of tissue damage. Skin toxicity, including epidermal thickening and dermal fibrosis, was significantly reduced after FLASH-RT. At the molecular level, FLASH-RT reduced oxidative stress and inflammatory injury, demonstrated by lower Nrf2 activation, reduced 8-OHdG levels, and decreased MPO expression. Systemically, FLASH-RT led to lower neutrophil-to-lymphocyte ratios and decreased serum IL-6, TNF-α, and IFN-γ, indicating reduced inflammation. CONCLUSIONS: Our findings provide the first evidence that proton FLASH-RT at ablative dose levels (>60 Gy) confers localized protection against radiation-induced lung and skin injury in a preclinical setting. These results support the potential of high-dose proton FLASH-RT for thoracic application, though further studies are needed to establish dose-response relationships and optimize clinical beam configurations. ADVANCES IN KNOWLEDGE: High-dose proton FLASH-RT preserves lung and skin, and mitigates oxidative and inflammatory responses, offering insights into mechanisms underlying the FLASH effect.