  Purpose: We introduce a novel concept of a compact multiprobe scintillator
detector and demonstrate its applicability in HDR-brachytherapy. Our fabricated
seven-probe system is sufficiently narrow to be inserted in a brachytherapy
needle or in a catheter.
  Methods: Our multiprobe detection system results from the parallel
implementation of a miniaturized scintillator detector at the end of a bundle
of seven fibers. The resulting system, which is narrower than 320 microns, is
tested with a MicroSelectron 9.1 Ci Ir-192 HDR afterloader, in a water phantom.
The detection signals from all seven probes are simultaneously read with an
sCMOS camera (at a rate of 0.06 s). The camera is coupled to a chromatic filter
to cancel Cerenkov signal induced within the fibers upon exposure. By
implementing an aperiodic array of six scintillating cells along the bundle
axis (one probe is kept bare to assess the stem effect), we first determine the
range of inter-probe spacings leading to optimal source tracking accuracy.
Then, three different source tracking algorithms involving sequentially or
simultaneously all the scintillating probes are tested and compared. In each
case, dwell positions are assessed from dose measurements and compared to the
treatment plan. Dwell time is also determined and compared to the treatment
plan.
  Results: The optimum inter-probe spacing for an accurate source tracking
ranges from 15 mm to 35 mm. The optimum detection algorithm consists of adding
the readout signals from all detector probes. In that case, the error to the
planned dwell positions is of 0.01+/-0.14 mm and 0.02+/-0.29 mm at spacings
between the source and detector axes of 5.5 and 40 mm, respectively. Using this
approach, the average deviations to the expected dwell time are of
-0.006+/-0.009 s and -0.008+/-0.058 s, at spacings between source and probe
axes of 5.5 mm and 20 mm, respectively.
