294 - In Vivo Dosimetry for Ultra High Dose Rate Irradiation of Rat Spinal Cord with a Novel Scintillation Detector

Purpose/Objective(s): Despite mounting evidence supporting the efficacy of FLASH radiotherapy (RT), most studies rely on acute toxicity, while data on late-responding tissues remain scarce. The rat spinal cord, a late responding organ with a steep dose response relationship, serves as an ideal model to investigate whether FLASH spares late toxicity compared to conventional dose rate (CONV) RT. Accurate in vivo dosimetry (IVD) capable of assessing the dose and dose rate distribution of ultra-high dose rate (UHDR) irradiation is crucial for interpretating experiment results and improving study reproducibility.

Materials/Methods: The tissue equivalent HYPERSCINT RP-FLASH scintillator, with millimeter spatial resolution and millisecond temporal resolution, was used for IVD. Its dosimetric accuracy for UHDR electron beams was quantified against EBT-XD film measurements. The effects of beam energy, field size, dose per pulse (DPP) and pulse repetition frequency (PRF) on scintillator response were evaluated. Following characterization, the probe was inserted into the C2-T2 region of the rat spinal cord, with movement controlled by a micrometer translation stage of 0.01 mm precision. A portable X-ray imager, an X-ray sensor and a radio-opaque ruler localized the probe within the spinal canal. The micrometer reading was validated against the X-ray imaging for rapid estimation of the probe’s location. The C2-T2 spinal cord was irradiated with 18 MeV UHDR with 2x2 cm² filed. Dose and dose rate distribution were measured and compared to Monte Carlo (MC) calculations, using relative dose difference and gamma index analysis for evaluation.

Results: The scintillator exhibited <3% dose accuracy compared to film measurements up to 40 Gy at 1 kHz sampling frequency under UHDR electron irradiation. Its response was minimally dependent on energy (6 and 18 MeV), field size (2x2 to 25x25 cm²), DPP (1 to 2.3 Gy/pulse), and PRF (30 to 180 Hz). After correcting for magnification caused by X-ray divergence, micrometer readings were consistent with X-ray imaging, enabling rapid probe localization. In the C2-T2 region, the dose rate exceeded 350 Gy/s, and 14 mm of the spinal cord received >90% of the prescribed dose, ensuring uniform UHDR irradiation. The relative dose difference remained within 4% of MC calculations, with all measurements passing the gamma index analysis under 1mm/3% criteria.

Conclusion: The tissue-equivalent scintillator system enables accurate, fast-responding, millisecond-resolved UHDR electron dosimetry, with minimal dependence on dose, energy, field size, DPP, and PRF across commonly used electron energies. This study establishes the first UHDR IVD for spinal cord irradiation, to support study reproducibility in evaluating whether FLASH can mitigate toxicity in late-responding tissues.