S132
Brachytherapy - Physics
ESTRO 2026
brachytherapy, 10(6), 551–558.[3] Rivard, M.J., Coursey, B.M., et al. (2004), Update of AAPM Task Group No. 43 Report: A revised AAPM protocol for brachytherapy dose calculations. Med. Phys., 31: 633-674.
Digital Poster 2923
Commissioning and End-to-End Validation of Micro- Silica-Bead TLDs for Internal In-Vivo Dosimetry of Brachytherapy William Hamblyn 1 , Amani Chowdhury 1 , Shakardokht M Jafari 2 , Fatemeh Nazari 2 , Rachel Wills 1 , Gerry Lowe 1 , Thomas Hague 1 , Peter Hoskin 1,3 1 Cancer Centre, Mount Vernon Hospital, Northwood, United Kingdom. 2 Trueinvivo, Trueinvivo, Portsmouth, United Kingdom. 3 Division of Cancer Sciences, University of Manchester, Manchester, United Kingdom Purpose/Objective: Routine in-vivo dosimetry (IVD) in brachytherapy represents a key aim toward improving patient safety and quality assurance. Micro-silica bead thermoluminescent dosimeters (TLDs) offer a promising solution [1,2]. This work reports the commissioning of such beads for use in HDR brachytherapy and subsequent validation through end-to-end testing and gamma analysis, with the aim of establishing a practical framework for clinical implementation. Material/Methods: Micro-silica TLD beads (1.6 mm diameter) were commissioned for internal in-vivo dosimetry of brachytherapy through a three-stage calibration: (1) reference irradiation with 6 MV photons, (2)192Ir calibration at 2 cm, and (3) determination of energy correction factors (ECFs) using spiral templates varying source-detector distances. Commissioning evaluated energy dependence, CT dose contribution, and uncertainty. All bead configurations were manufactured by TRUEInvivo® (Portsmouth, UK), who also provided the readout service.
Following commissioning end-to-end tests were performed in representative clinical geometries. Beads were positioned on a rectal probe suspended in water, while a continuous string of beads were loaded into 2- and 3-way urinary catheters in an anthropomorphic phantom. CT imaging, treatment planning, and irradiation followed clinical workflow using BrachyVision and Bravos (Varian Medical Systems Inc., Palo Alto, USA). An in-house script automated the averaged dose-weighted ECFs for clinical plans. Measured doses were then compared with TPS calculations, and urinary data were evaluated using gamma analysis with varying criteria. Results: The TLDs demonstrated a reproducible calibration factor, with a 1.5% difference between two calibrations at 2 cm from the source. The CT scan dose contribution was approximately 0.03Gy per pelvic scan, negligible in high-dose regions but relevant for low-dose areas. For the end-to-end tests, the urinary bead arrays were divided into ‘deep’ and ‘shallow’ halves as it is suspected the reduced scatter near the phantom surface led to systematic differences between TPS and TLD dose measurements. The mean gamma pass rate (3%/3 mm, 10% threshold) was 90.4% for deep beads and 79.3% for shallow beads. For deep beads, the mean absolute TPS-TLD differences were 0.2Gy for the 2-way catheter and the 3-way catheters, with maximum deviations of -0.6Gy and +1.0Gy, respectively. For the rectal end-to-end tests, the first setup showed a mean absolute difference of 0.5Gy and the repeat 0.4Gy, with maximum discrepancies of 1.7Gy and 1.5Gy.
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