  Purpose: This study aims to optimize and characterize the response of a mPSD
for in vivo dosimetry in HDR brachytherapy. Methods: An exhaustive analysis was
carried out in order to obtain an optimized mPSD design that maximize the
scintillation light collection produced by the interaction of ionizing photons.
Several mPSD prototypes were built and tested in order to determine the
appropriate order of scintillators relative to the photodetector, as well as
their length as a function of the scintillation light emitted. Scintillators
BCF-60, BCF-12 and BCF-10 constituted the mPSD sensitive volume.Each
scintillator contribution to the total spectrum was determined by irradiations
in the low energy range.For the best mPSD design, a numerical optimization was
done in order to select the optical components that better match the light
emission profile. The optimized dosimetric system was used for HDR
brachytherapy dose determination. The system performance was quantified in term
of signal to noise ratio and signal to background ratio. Results: It was
determined that BCF-60 should be placed at the distal position, BCF-12 in the
center and BCF-10 at proximal position with respect to the photodetector.This
configuration allowed for optimized light transmission through the collecting
fiber, avoiding inter-scintillator excitation and self-absorption effects.The
optimized luminescence system allowed for signal deconvolution using a
multispectral approach, extracting the dose to each element while taking into
account Cerenkov stem effect.Differences between the mPSD measurements and
TG-43 remain below 5%. In all measurement conditions, the system was able to
properly differentiate the produced scintillation signal from the background
one. Conclusions: A mPSD was constructed and optimized for HDR brachytherapy
dosimetry, enabling real time dose determination, up to 6.5cm from the 192Ir
source.
