Speaker
Description
Ultra-high dose-rate (FLASH) radiotherapy presents significant challenges for real-time dosimetry, particularly due to detector saturation and pulse pile-up at elevated pulse repetition frequencies (PRF). Glass fibre-coupled scintillators are well suited to this environment owing to their prompt radioluminescence response and microsecond-scale timing characteristics, which enable pulse-resolved detection. However, under extreme instantaneous dose rates, excessive light yield can compromise detector linearity.
Measurements were performed using a 6 MeV clinical linear accelerator configured for FLASH delivery, producing 4 μs pulses with instantaneous dose rates up to 500 kGy/s and an average dose rate of approximately 50 Gy/s. Each pulse delivered approximately 2 Gy. Experiments were conducted over a total dose range of 10–50 Gy in 10 Gy increments at PRFs of 150 Hz (6.67 ms interpulse interval) and 300 Hz (3.33 ms interval), within a 10 x 10 cm field at 100 cm SSD.
In the baseline configuration (5 mm length, 1.5 mm outer diameter, no optical gap), peak signals approached ~1500–1600 counts per gate, indicating operation near photomultiplier saturation and observable baseline elevation. Geometrical optimisation was investigated by varying scintillator outer diameter (OD), length (L), and optical coupling gap. Introducing a 0.5 mm gap reduced peak amplitude to ~500 counts (~70% reduction), while a 1.5 mm gap reduced peaks further to ~350–420 counts. Halving the OD to 0.75 mm produced comparable amplitudes (~300–380 counts). Increasing length to 10 mm did not fully compensate for signal loss caused by a 2 mm gap, suggesting optical coupling efficiency dominates over active scintillation volume.
Lower-yield geometries demonstrated reduced baseline accumulation and improved pulse separation at both 150 Hz and 300 Hz. These observations indicate that geometrical tuning can align scintillator output with detector dynamic range, mitigating FLASH-induced non-linearities while preserving the intrinsic timing advantages of glass fibre-based systems.