Seven-probe scintillator dosimeter for treatment verification in HDR-brachytherapy

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.