Author: Julie Colnot, Eric Deutsch, Christelle Huet, Danahé LeBlanc, Pauline Maury, Charlotte Robert, Imane Saïd Mansour, Cathyanne Schott, Francois Therriault-Proulx 👨🔬
Affiliation: INSERM U1030, Gustave Roussy, University Paris-Saclay, LDRI, IRSN, Medscint, THERYQ / INSERM U1030, Gustave Roussy, University Paris-Saclay, Université Paris-Saclay, Gustave Roussy, Inserm U1030, Radiothérapie Moléculaire et Innovation Thérapeutique 🌍
Purpose: FLASH radiotherapy, using ultra-high dose rates (UHDR, >100 Gy/s), spares healthy tissues while maintaining tumor control compared to conventional radiotherapy (CONV). However, UHDR dosimetry is challenging due to saturation of conventional reference dosimeters. Plastic scintillation detectors (PSD) offer a promising solution, enabling real-time dose measurements with a nanosecond temporal resolution. This study evaluates the HyperScint RP-200 PSD (MedScint) for UHDR electron beam dosimetry by investigating the calibration parameters, which involve not only determining the dose-signal proportionality coefficient but also quantifying and subtracting additional signals, and by comparing its response to established UHDR dosimeters.
Methods: PSD calibration was performed on a SARRP (Xstrahl) and a Clinac 2300 C/D (Varian) under CONV conditions. Calibration conditions (dose, particle type, and time delay between calibration and measurement) were evaluated in CONV and dose measurements were compared to an Advanced Markus ionization chamber (IC,PTW). The PSD, calibrated in CONV, was then evaluated under UHDR using the FLASHKNiFE system (FK, THERYQ) across a range of pulse width (PW, 0.5-4 µs) and repetition frequency (PRF, 10-250 Hz). Doses (0.5–10 Gy) were measured in triplicate and compared to a FlashDiamond (fD, PTW) and EBT3 films (Ashland).
Results: Spectral and Cherenkov calibrations remained robust even after PSD removal from the reader (< 0.1% variation). A sensitivity loss with accumulated dose (~1% every 100 Gy) was observed requiring the dose calibration to be performed close to dose measurement. Cherenkov calibration was unaffected by beam type (9 MeV electrons, 6 MV photons) (discrepancies < 0.1%) and dose calibration by the dose level used (< 0.3%). PSD deviations from fD and films were within 2.55%, with linear response with PW and no PRF dependence.
Conclusion: The PSD provides accurate, reliable dosimetry under UHDR conditions, demonstrating robust performance across varying PW and PRF.