Real-time source tracking for HDR brachytherapy using a fibre-optic dosimeter

Purpose: Real-time tracking of source position during high-dose-rate (HDR) brachytherapy remains limited in routine clinical practice, despite its importance for treatment accuracy and patient safety. This study investigates the feasibility of using a CsI(Tl)-based fibre-optic dosimeter for real-time HDR source tracking using a stem-inclusive calibration framework that avoids suppression of non-scintillation optical signals. Methods: A single fibre-optic detector incorporating a CsI(Tl) scintillator was used with an Ir-192 brachytherapy source. A calibration model accounting for all optical signal contributions was applied to relate measured detector voltage to source geometry. Real-time source tracking was performed by comparing the averaged measured voltages with modelled responses to reconstruct the source position. Tracking performance was evaluated for step sizes of 3 mm and 5 mm, dwell times from 5.0 s to 0.5 s, and source–detector separations up to approximately 50 mm. Results: The detector response showed close agreement with the calibration model, with deviations within approximately 4% of the peak signal across the tracking range. Reconstructed source positions closely followed planned dwell locations for all tested configurations. Tracking errors remained within 1.51 mm for most conditions and increased to 2.6 mm only at the largest source–detector separations. Stable localisation performance was maintained across all dwell times, with no clear degradation as dwell time decreased. Conclusions: Real-time HDR source tracking was achieved using a single CsI(Tl)-based fibre-optic detector and a stem-inclusive calibration approach. Millimetre-scale localisation accuracy was obtained without stem signal suppression, additional detectors, or hardware modification, supporting calibration-driven signal interpretation for HDR source position verification.