Diffenderfer, E. S. | Search bioRxiv

bioRxiv2026

Decreased Damage for proton FLASH vs Conventional Dose Rates in Mouse Jejunum Shown by Quantitative Assessment of γ-H2AX

Purpose: FLASH radiation with ultra-high dose rate delivery is less damaging to normal tissue than conventional radiation ( <1 Gy/s). Since radiation depletes oxygen (ROD), this damage reduction might occur via the oxygen effect. ROD experiments have shown an oxygen-independent reduction in dose effectiveness at FLASH dose rates. However, prior in vivo ROD measurements relied on extracellular oxygen probes that could not penetrate cell membranes, leaving intracellular effects unresolved. To investigate the ROD hypothesis more directly, we developed a novel three-component immunohistochemical assay with algorithmic image processing to quantitatively compare DNA damage following FLASH and conventional irradiation in mouse jejunum. Methods: Mice received intravenous EF5 2 hours before proton irradiation at FLASH (103.63 +/- 17.2 Gy/s) or conventional (0.73 +/- 0.1 Gy/s) dose rates of 2.5 Gy or 5 Gy, with unirradiated controls. Mice were euthanized 30 minutes post-irradiation, and 10 cm of jejunum was frozen as a 'Swiss Roll', sectioned, stained, and imaged. Tissue sections were stained for {gamma}-H2AX, DRAQ5, and EF5 to assess DNA double-strand breaks, total DNA content, and hypoxia, respectively. An in-house algorithm identified individual cell nuclei and registered each nucleus with its corresponding {gamma}-H2AX and EF5 signals, enabling quantitative measurement of DNA damage as a function of local tissue hypoxia. Results: Hypoxia was greatest in the villi and, to a lesser extent, the outer jejunal musculature, with substantial inter-animal variation. DNA damage decreased in hypoxic regions. FLASH enhanced the hypoxia-associated reduction in DNA damage compared with conventional dose rate and, separately, revealed an oxygen-independent reduction in DNA damage, suggesting an additional FLASH sparing mechanism. Conclusion: Current results suggest that FLASH compared to conventional dose rate radiation caused less DNA damage with increasing effect at low oxygen levels, a result consistent with ROD as a mechanism. Pronounced tissue heterogeneity in murine jejunum requires further studies to segment the effect for each tissue type.

biophysics↗

bioRxiv2024

FLASH proton reirradiation, with or without hypofractionation, mitigates chronic toxicity in the normal murine intestine, skin, and bone.

Background and purposeThe normal tissue sparing afforded by FLASH radiotherapy (RT) is being intensely investigated for potential clinical translation. Here, we studied the effects of FLASH proton RT (F-PRT) in the reirradiation setting, with or without hypofractionation. Chronic toxicities in three murine models of normal tissue toxicity including the intestine, skin, and bone were investigated. Materials and methodsIn studies of the intestine, single-dose irradiation was performed with 12 Gy of Standard proton RT (S-PRT), followed by a second dose of 12 Gy of F-PRT or S-PRT. Additionally, a hypofractionation scheme was applied in the reirradiation setting (3 x 6.4 Gy of F-PRT or S-PRT, given every 48 hrs). In studies of skin/bone of the murine leg, 15 Gy of S-PRT was followed by hypofractionated reirradiation with F-PRT or S-PRT (3 x 11 Gy). ResultsCompared to reirradiation with S-PRT, F-PRT reduced intestinal fibrosis and collagen deposition in the reirradiation setting and significantly increased survival rate, demonstrating its protective effects on intestinal tissues. In previously irradiated leg tissues, reirradiation with hypofractionated F-PRT created transient dermatitis that fully resolved in contrast to reirradiation with hypofractionated S-PRT. Lymphedema was also alleviated after a second course of radiation with F-PRT, along with significant reductions in the accumulation of fibrous connective tissue in the skin compared to mice reirradiated with S-PRT. The delivery of a second course of fractionated S-PRT induced tibial fractures in 83.3% of the mice, whereas only 20% of mice reirradiated with F-PRT presented with fractures. ConclusionThese studies provide the first evidence of the sparing effects of F-PRT, in the setting of hypofractionated reirradiation. The results support FLASH as highly relevant to the reirradiation regimen where it exhibits significant potential to minimize chronic complications for patients undergoing RT.

cell biology↗