DNA is the primary molecular target of ionizing radiation, and its damage — especially in the form of single- and double-strand breaks — plays a central role in determining the biological response of cells. Under conventional dose rates, radiation-induced radicals interact with DNA over extended timescales, and oxygen efficiently “fixes” the damage, making it permanent. However, irradiation at ultra-high dose rates (FLASH, ≥40 Gy/s) leads to profoundly different radiochemical conditions. Transient oxygen depletion and rapid radical–radical interactions reduce the fixation of DNA lesions, resulting in the experimentally observed sparing of normal tissues. Despite this phenomenon being demonstrated in multiple biological models, its underlying mechanism remains one of the most intriguing open questions in modern radiobiology.
At the microscopic level, the earliest stages of radiation action are governed by the track structure of incoming particles and the subsequent water radiolysis chemistry. Hydroxyl radicals (•OH) are the main contributors to indirect DNA damage, while the competition between oxygen and scavenger-derived radicals determines whether damage becomes permanent. Under FLASH conditions, the balance of these reactions shifts dramatically. Understanding how this shift translates into observable DNA strand breaks requires combining precise physical modelling with experimental measurements of DNA damage.
The presented project investigates these mechanisms by studying plasmid DNA exposed to FLASH proton and electron beams under controlled oxygenation and scavenger conditions. DNA damage is quantified using gel electrophoresis and atomic force microscopy (AFM), allowing separation of supercoiled, nicked, and linear forms as indicators of single- and double-strand breaks. These experimental results are interpreted using advanced Monte Carlo track-structure simulations performed in the TOPAS-nBio framework. The model incorporates heterogeneous and homogeneous water radiolysis chemistry and has been extended to include reactions of DMSO-derived radicals, enabling detailed analysis of radical competition and dose-rate–dependent effects in cellular environment.
By linking experimental measurements with stochastic simulations at the nanoscale, this work aims to determine whether changes in radiochemical kinetics — particularly oxygen depletion and modified radical competition — can quantitatively explain the radioprotective FLASH effect. A mechanistic understanding of this phenomenon is crucial for designing safe and effective FLASH radiotherapy treatments, which may significantly improve the therapeutic window by reducing normal tissue toxicity without compromising tumor control.
The project is carried out in collaboration with IFJ PAN, Jagiellonian University, the Silesian University of Technology, and UCSF.
FUNDING: Narodowe Centrum Nauki (https://www.ncn.gov.pl/ ), Poland (2024/54/E/ST4/00457)