Impact of proton LET and ultrahigh dose-rate on the structural integrity of DNA in solution

Purpose Ultra-high dose-rate (UHDR) irradiation reduces normal tissue toxicity while maintaining tumor control under isodose conditions with respect to conventional dose-rate (CONV) irradiation (the FLASH effect). But its underlying physicochemical mechanism remains highly debated. This study evaluates whether radical-radical recombination can explain this sparing without invoking bulk oxygen depletion by decoupling the roles of track structure (linear energy transfer, LET) and particle fluence and velocity on free-radical kinetics and DNA damage.Materials and methods Highly purified pUC19 plasmid DNA in aqueous oxygenated buffer was irradiated with 142.4 MeV protons at CONV (∼1.0 Gy/s) or UHDR (>455 Gy/s). Samples were positioned in a water phantom and received doses ranging from 0 to 160 Gy at LETd (dose-averaged LET) values of 1.7, 4.4, and 6.5 keV/µm. Strand breaks and formamidopyrimidine-DNA glycosylase (Fpg) and endonuclease III (Nth) -sensitive base lesions were quantified by gel electrophoresis. At low LET, fluence was estimated from Φ = ρD/LET and mean nearest-track distance from dNN = 1/(2√Φ), using the dose delivered during one OH• lifetime (approximately 0.91 µs).Results At LETd = 1.7 keV/µm, the estimated mean nearest-track distance during one OH• lifetime was ∼275 µm at 1.0 Gy/s and ∼13 µm at 455 Gy/s. Even the UHDR value was ∼100 times larger than the ∼0.1–0.13 µm OH• diffusion distance. Above ∼40 Gy cumulative dose, UHDR produced lower strand-break and enzyme-sensitive lesion values than CONV, and the CONV–UHDR difference decreased as LETd increased.Conclusions Proton dose-rate alone is insufficient to predict the FLASH effect-mediated tissue sparing. Our results demonstrate that sparing is a track-structure-dependent kinetic phenomenon. LET, fluence, spill structure, solution chemistry, and oxygen history must be considered together. The data constrain candidate FLASH mechanisms but do not identify a unique chemical pathway.DEFINITIONS Spur: a localized cluster of closely spaced ionizations and excitations.Branch: a lateral secondary-electron trajectory containing one or more additional spurs.Intra-track recombination: reactions among species generated within the same primary-particle track and its branches.Inter-track recombination: reactions between species generated by different primary-particle tracks. Track coincidence requires two tracks to be created close enough in both space and time for their reactive species to meet.Mean nearest-track distance: for randomly and independently positioned, approximately parallel tracks, the average center-to-center distance from one track to its closest neighboring track.