ESTRO 2025 - Abstract Book

S366

Brachytherapy - Physics

ESTRO 2025

the response of a commercial single-point plastic scintillation detector (PSD) for in-vivo dosimetry in HDR brachytherapy.

Material/Methods: The PRB-0057 plastic scintillation detector (PSD) consisted of a 1-mm diameter, 1-mm long scintillating fiber coupled to an 8-m long clear optical fiber of the same diameter connected to the Hyperscint-RP200 (Medscint, Québec, Canada) spectrometer for signal acquisition (Jean, Therriault-Proulx and Beaulieu, 2021). An Iridium-192 source ( S k =20988U ) was delivered using a Flexitron HDR afterloader (Elekta, Stockholm, Sweden). Measurements were performed in a 48x48x48 cm 3 IBA Blue-Phantom 2 water tank. Source and detector were surrounded by at least 20 cm of water for consistency with the TG-43U1 formalism (Rivard et al. , 2004). Source-to-detector displacement along the x/y-axes (within the source plane, =90˚) was controlled using the Common-Control-Unit (CCU). Source-to-detector distance stability due to the CCU/afterloader reproducibility were explored. First, the uncertainty in source positioning for multiple source-transits to the planned dwell-position at 1.5 cm from the PSD was extracted. The uncertainty in the detector position was assessed from 1.5 to 5.5 cm from the source. Measurements were performed in three series, each consisting of five light yields at each position. All uncertainties were quantified in terms of standard deviation. TG43U1 was used to convert yield to positional differences. Second, acquisition frequencies were set to 1 and 10 Hz for performance evaluation. Overall system performance was quantified in terms of Signal-to-Noise Ratio (SNR) and Signal-to-Background Ratio (SBR) as functions of the dose rate. Results: At 1.5 cm, the afterloader source delivery uncertainty was 0.19%, corresponding to a geometric source positional uncertainty Δr of 14.25 μm. The CCU positional accuracy, as calculated from light output using TG43U1, ranged from 12.8 μm at 2.5 cm to 1.16 mm at 5.5 cm. The SNR remained above the Rose criteria (>5) for dose rates below 1.05 mGy/s at both 1 and 10 Hz. A minimum SBR, around 85, was observed at 7.5 cm from the source ( Fig.1 ).