ESTRO 2025 - Abstract Book

S347

Brachytherapy - Physics

ESTRO 2025

The stable activity validated the neutron flux and irradiation protocol and highlighted titanium's suitability as an encapsulation material. Short-lived contaminants like 24 Na (3.6 μCi) , 48 Sc (9.12 nCi) , and 187 W (0.814 μCi) had no dosimetric impact, decaying rapidly within acceptable limits. Leakage tests confirmed no 75 Se leakage. Conclusion: A low-activity 75 Se brachytherapy source was successfully developed and irradiated. Dosimetry calculations using TG-43 formalism demonstrated its potential as an alternative to 192 Ir . Post-irradiation tests confirmed the source's integrity and viability.

Keywords: Brachytherapy, vanadium diselenide, gamma

3215

Digital Poster Physical Dose and BED Correction to GammaTile® Brachytherapy with External Radiation Planning Brian S Jang, Kathryn E Dusenbery, Margaret A Reynolds, Lindsey Sloan, David Sterling, Clara Ferreira Department of Radiation Oncology, University of Minnesota, Minneapolis, USA Purpose/Objective: GammaTile® is an FDA-cleared brachytherapy platform of collagen tiles embedded with radioactive Cs-131, surgically placed after maximal safe resection for brain tumors. In treating glioblastoma (GBM), GammaTile® brachytherapy delivers 60 Gy to tissue within 5 mm of the tiles, decreasing exponentially beyond this margin, providing insufficient dose in tissue at risk of residual disease. Postoperative external beam IMRT can be added across this brachytherapy gradient using inverse planning tools. Planning across the brachytherapy-EBRT junction region is challenging as simple addition calculates only physical dose (PD). A biologically effective dose (BED) calculation potentially corrects PD in the junction region. This report compares EBRT planning using PD versus BED. Material/Methods: Ten patients with newly diagnosed IDH wild type GBM underwent maximal safe tumor resection and GammaTile® implantation, followed by 20 fractions of IMRT EBRT after 4 weeks. Brachytherapy PD of 60 Gy was delivered to 5 mm expansion from the resection cavity into brain parenchyma. From post-op CT and MRI scans, HR-PTV (resection cavity and residual T1 enhancing area) and LR-PTV (residual T2 FLAIR areas) were outlined. The planning goal was for HR-PTV to receive 55.6 Gy and LR-PTV to receive 44.9 Gy cumulatively from brachytherapy and IMRT. A proprietary algorithm converted brachytherapy PD to BED on a voxel-by-voxel basis using dose rate, time, and alpha/beta ratios. Dose summations of brachytherapy and EBRT doses were generated for HR-PTV and LR-PTV using both PD and BED. Differences in 95% of HR-PTV receiving at least 55.6 Gy (D95) and LR-PTV receiving D95 of 44.9 Gy were compared.