S1630
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
References: [1] Maeyama et al. “A diffusion-free and linear-energy- transfer-independent canocomposite Fricke gel dosimeter”. Radiat. Phys. Chem. 96 (2014), 92–6. DOI: 10.1016/j.radphyschem.2013.09.004.[2] Uecker et al. “Real-time MRI at a resolution of 20 ms”. NMR in Biomed. 23 (2010), 986–94. DOI: 10.1002/nbm.1585.[3] Bayer et al. “Carbon ion mono-energetic and spread- out Bragg peak measurements using nanocomposite Fricke gel dosimeters with LET-independent response”. Radiat. Meas. 176 (2024), 107175. DOI: 10.1016/j.radmeas.2024. 107175. Keywords: Absolute dosimetry, Fricke gel, carbon ion beam Proffered Paper 786 Investigating Superoxide Radical Yield Under Ultra- High Dose Rate Irradiation: Effects of Beam Parameters on Water Radiolysis Aashini Rajpal 1,2 , Alexandra Schmidt 3 , Charlotte Robert 4,5 , Eric Deutsch 4,5 , Gérard Baldacchino 3 1 Inserm U1030, Molecular Radiotherapy and Therapeutic Innovation, Gustave Roussy, Villejuif, France. 2 Innovation and technology, THERYQ, Paris, France. 3 LIDYL, CEA Saclay, University Paris Saclay, Gif- sur-Yvette, France. 4 Inserm U1030, Molecular Radiotherapy and Therapeutic Innovation, Université Paris-Saclay, Gustave Roussy, Villejuif, France. 5 Radiotherapy department, Gustave Roussy, Villejuif, France Purpose/Objective: The distinct response of healthy and cancerous tissues to ultra-high dose rate (UHDR) radiation has sparked interest in its underlying mechanisms. One hypothesis involves the dynamics of the concentration of Reactive Oxygen Species (ROS). Our work focuses on the superoxide radical (O2• ⁻ ), which contributes to oxidative stress and initiates chain reactions. Its experimental detection is limited due to its short lifetime, pH dependence, low concentration, and low absorbance coefficient. However, some studies have used Monte-Carlo simulations to investigate its production kinetics.1,2 This work aims to experimentally measure superoxide yield as a function of pulse repetition frequency (Freq) and pulse width (PW), to elucidate the effect of beam temporal structure on the FLASH effect. Material/Methods: The experiments were performed with 10 MeV electrons (FLASHKNiFE - THERYQ, France). For the real- time detection (setup @ Figure1), Cytochrome C (CytC) molecule was used as specific probe of O2• ⁻ . Upon its reduction with O2• ⁻ , it reveals a characteristic absorption signal @ 550 nm (Figure 2a). To eliminate
the possibility of reduction due to other species, irradiation was performed with and without Superoxide Dismutase (SOD). The O2• ⁻ yield was estimated by subtracting reduced CytC yields, with and without SOD. Yields were systematically studied as a function of dose per pulse (DPP, varied via PW), and mean dose rate (MDR, varied via Freq), to assess dependence of yield on beam temporal structure. The dosimetry was performed using the flash diamond (fD) and Fricke solution.
Results: The superoxide yield from water radiolysis was experimentally measured after irradiation with 10 MeV FLASH electron beam, under O2 saturation. A difference of 0.42±0.03 molecules/100eV (Figure 2b) was observed in the O2• ⁻ yield between FLASH and CONV dose rate. However, a negligible variation (0.22±0.12 molecules/100eV) was seen when changing the DPP, at constant MDR (not shown here).
Conclusion: This work experimentally quantified superoxide yield upon water radiolysis using a highly sensitive optical setup dedicated for real-time radical detection. Under oxygen-saturated conditions, O2• ⁻ production was lower at UHDR than that observed at conventional dose rates. Across the tested DPP range at a fixed MDR of 213 Gy/s, O2• ⁻ yield remained constant, however, a small reduction was observed when increasing MDR at fixed DPP of 2.13 Gy. Further investigations on biologically relevant media with varying O2 concentration are expected in the future as well as in in-vitro models to determine the extracellular O2• ⁻ and linking biological effects to the underlying radical production mechanisms.
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