  Purpose: To present the performance of a multi-point plastic scintillation
detector (mPSD) as a tool in vivo dosimetry in brachytherapy. Methods: A
previously optimized three-point sensor system was used for in vivo HDR
brachytherapy measurements (using the scintillators BCF-60, BCF-12, and
BCF-10). The light detection system of the mPSD consisted of compactly
assembled photomultiplier tubes (PMTs) and dichroic mirrors and filters to
achieve a highly sensitive scintillation light collection. The PMT signals were
recorded using a NI-DAQ board at a rate of 100 kHz. Dose measurements covering
a range of 0.5 to 10 cm from the 192Ir source were carried out according to
TG-43 U1 recommendations in order to: (1) characterize the system's response in
terms of angular dependence; (2) obtain the relative contribution of
positioning and measurement uncertainties to the total system uncertainty; (3)
assess the system's temporal resolution; and (4) track the source position in
real time. Results: The positioning uncertainty dominated close to the source,
whereas the measurement uncertainty dominated at larger distances. A maximum
measurement uncertainty of 17 % was observed for the BCF-60 scintillator at 10
cm from the source. The average best compromise between positioning and
measurement uncertainties were reached at 17.4 mm. The detector further
exhibited no angular dependence. The system provided an average location with a
standard deviation under 1.7 mm. The maximum observed differences between
measured and expected source location was 1.82 mm. Dose deviations remained
below 5% in all the explored measurement conditions. With regard to dwell time
measurement accuracy, the maximum deviation observed at all distances was 0.56
s. Conclusions: The performance of the system demonstrated that it could be
used for real-time dose, position and dwell time measurements during HDR
brachytherapy.
