Seven-probe fiber detector for time-resolved source tracking in HDR-brachytherapy: Pre-clinical experimental...

In vivo dosimetry (IVD) is increasingly recognized as a critical tool for verifying treatment delivery in HDR-brachytherapy (HDR-BT). Time-resolved techniques, such as source tracking, enable real-time error detection and reduce uncertainties in dose delivery. However, to date, multichannel detection systems have been tested only in limited conditions that do not reflect the full diversity of clinical HDR-BT scenarios.

This study evaluates a compact biocompatible Seven-probe Scintillator Detector (7SD) for monitoring HDR-BT treatment sequences across a range of dwell times and source-probe spacings representative of most HDR-BT techniques. The capability of the 7SD to detect source insertion errors is also assessed.

The 7SD comprises seven detection cells made of ${\mathrm{Gd}}_2{O}_2\text{S:Tb}$ , each measuring 0.28 $\pm$ 0.02 mm in diameter and 0.43 $\pm$ 0.02 mm in length, coupled to the microstructured tips of silica optical fibers (110-micron diameter). The probes, spaced 15 mm apart along the fiber axis, are organized into a bundle with a total diameter of less than 0.45 mm. The 7SD was tested in a $50\times 50\times 30$ ${\mathrm{cm}}^3$ phantom using a MicroSelectron 9.1 Ci Ir-192 HDR afterloader connected to a BT stainless steel interstitial needle. Detection signals were acquired at 0.06-second intervals with an sCMOS camera equipped with a chromatic filter to eliminate spurious Cerenkov signals. The probe was experimentally calibrated in 2D without relying on the AAPM-TG43 formalism. Monitoring of dwell times and positions was performed by combining detection signals from all seven probes. Cytotoxicity tests were performed to confirm the biocompatibility of the 7SD RESULTS: A total of 4040 dwell positions were analyzed, covering source-probe spacings from 10 to 36 mm and a source travel range of 62 mm, with dwell times ranging from 0.1 to 19.5 s. The 7SD successfully identified 99.5% of the dwell positions. In cylindrical coordinates, the measured dwell positions deviated from the planned values by 0.224 $\pm$ 0.155 mm (radial) and 0.077 $\pm$ 0.181 mm (axial, source travel axis). The average deviation from planned dwell times was 0.006 $\pm$ 0.061 s. 99.4% of the dwell positions were measured within the 1 mm reliability threshold. The remaining 0.6% of deviations consistently occurred at the initial dwell position of treatment sequences and appear to stem from a systematic source positioning error by the afterloader. Additionally, the detector accurately identified intentional needle mispositioning scenarios with sub-millimeter accuracy.

A 7SD based on ${\mathrm{Gd}}_2{O}_2\text{S:Tb}$ , coupled with an sCMOS camera, demonstrates its suitability for time-resolved IVD in monitoring HDR-BT treatments. The 7SD substantially extends the monitoring volume along both the source's axial path and the orthogonal radial direction, enabling accurate source tracking across treatment geometries representative of a wide variety of HDR-BT procedures. By detecting subtle afterloader malfunctions, the 7SD offers a precise and valuable means of evaluating the integrity of HDR-BT setups and procedures. Its high-resolution monitoring capabilities, confirmed biocompatibility, scalability, and compatibility with standard BT needles, catheters, and applicators underscore its potential for clinical implementation.