Identification of Dwell-Times with In-Vivo Scintillation Detectors In Brachytherapy
Abstract
Purpose
Plastic Scintillation Detectors (PSD) have shown great potential for dose measurements, but extraction of dwell-time in particular sub-seconds is plagued with small signal. This study aims to evaluate inorganic scintillator (ISD) performances in terms of Signal-to-Noise Ratios (SNR) and to identify the accuracy of the scintillators in detecting dwell-times.
Methods
The PSD PRB-0080 (1×3 mm3, Medscint, Québec, Canada) and ISD (0.5×0.5 mm3 Gd2O2S:Pr,Ce,F), were coupled to 1 mm×8 m optical fibers to acquire light-yield at 20 Hz with the Hyperscint-RP200 spectrometer from Medscint. For SNR measurements, detectors were positioned at 1.3 and 1.5 cm, respectively from an Iridium-192 Flexisource (Elekta, Netherlands; SK = 29032 and 17639 U). A robotic arm moved the detectors up to 10 cm from the source in respectively 0.3 cm and 0.5 cm steps along the source transverse plane. For dwell-times verification, the ISD was positioned in the longitudinal plane at 1.6 cm from the source (SK = 29003 U) which moved up to 5.6 cm from the detector in 0.5 cm steps. Planned dwell-times were: 0.3 and 1 s. All measurements were replicated three times, made in 48×48×48 cm3 water tank, respecting TG-43 conditions.
Results
The Rose criterion (SNR > 5) is respected up to 8.92×10−3 cGy/s for the PSD and up to an extrapolated dose rate of 8.345×10−4 cGy/s for the ISD. All 9 dwell-times of 0.3 and 1 s were accurately identified with the ISD with a mean and standard deviation of 0.30 ± 0.03 s and 1.0 ± 0.1 s, respectively.
Conclusion
Results show a significant increase in SNR with the ISD, allowing acquisition of light yield up to a factor of 10 lower at 20 Hz. This configuration allows dwell-time extraction down to 0.3 s. A 1×1 mm ISD will allow for measurements of smaller dwell-times, down to 0.1s.