ESTRO 2025 - Abstract Book

S346

Brachytherapy - Physics

ESTRO 2025

3180

Proffered Paper Selenium-75 as a novel high-dose-rate brachytherapy source Oren Tal 1,2 , Jake Reid 1,2 , Jonathan Kalinowski 1,2 , Hamed Bekerat 2 , John Munro III 3 , Andrea Armstrong 4 , Shirin Abbasi Enger 1,2 1 Oncology, McGill university, Montreal, Canada. 2 Lady Davis Institute for Medical Research, Jewish General institute, Montreal, Canada. 3 Technology, Burnley Technology Inc., Seabrook, USA. 4 Nuclear Operations and Facilities, McMaster University, Hamilton, Canada Purpose/Objective: 75 Se (half-life = 119 days, Eγ,avg = 165 keV) offers advantages over 192 Ir (t ₁ / ₂ = 74 days, Eγ,avg = 380 keV) as a brachytherapy source due to its lower gamma energy and longer half-life. Manufacturing challenges hinder its brachytherapy application despite its use in industrial gamma radiography. This study aimed to design, irradiate, and evaluate a low-activity 75 Se source for clinical use. Material/Methods: A 75 Se brachytherapy source was designed and simulated using RapidBrachyMCTPS to evaluate its TG-43 dosimetric parameters and spectral characteristics. The seed featured a vanadium diselenide core (6.7 mm length, 0.55 mm diameter, 5.078 g/cm³) encapsulated in a titanium capsule (9.5 mm length, 0.95 mm outer diameter, 0.55 mm inner diameter, 4.54 g/cm³). A 16.5 mm titanium wire, 0.4 mm in diameter, was attached, with 5 mm included in the simulation. Quality assurance procedures were conducted at the McMaster Nuclear Reactor, including neutron radiography, analysis, and seed activation. Irradiation at a neutron flux of 2.16 × 10¹³ n/cm²s for 24 hours was performed to achieve a target activity of 8.49 mCi . Post-irradiation activity was measured using Capintec CRC-55tR and AtomLab500 detectors, with gamma spectroscopy conducted on an HPGe ORTEC detector. Wipe tests were performed to evaluate potential 75 Se leakage from the titanium capsule. Results: The air-kerma strength per unit activity and dose rate constant for the Se-75 source were 4.756 ± 0.0006 × 10 ⁻ ⁸ U/Bq and 1.119 ± 0.0007 cm ⁻ ² , respectively. Se-75 requires 2.0337 ± 0.00035 times more activity than a 10 Ci Ir-192 source to achieve a similar dose rate at 1 cm in water along the transverse axis. The radial dose function and 2D anisotropy functions are shown in the figure:

Quality assurance assessments revealed minor discrepancies in the source's dimensions and mass, including neutron radiography and neutron activation analysis. Post-irradiation activity was 8.84 ± 0.49 mCi , closely aligning with the expected 8.49 mCi (4.1% difference). Activity measurements for the 75 Se seed over time are shown in the table: