ESTRO 2025 - Abstract Book

S2594

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

2081

Poster Discussion Micro silica beads for verification of profiles and outputs factors of a Leksell Gamma Knife Chris J Stepanek 1 , Jack D Aylward 1,2 , Ronald Hartley-Davies 1 , Lucy Winch 1 1 Medical Physics and Bioengineering, University Hospitals Bristol & Weston NHS Foundation Trust, Bristol, United Kingdom. 2 Medical Physics, School of Applied Sciences, University of the West of England, Bristol, United Kingdom Purpose/Objective: Stereotactic radiosurgery (SRS) using a Gamma Knife (GK) uses finely collimated radiation fields, known as shots, to deliver tightly conforming dose distributions to small benign and malignant targets, with rapid dose fall-off at target boundaries. Dosimetric characterisation of these shots, requires detectors with high spatial resolution and minimal density perturbations. Physicists often rely on radiochromic film for GK dosimetry [1-5]. However, results are markedly affected by quality of the scanner, procedures for handling and processing, inhomogeneities between batches, and uncertainties can be >5% [5]. Herein, we report the use of Microsilica Beads (MSBs) to measure shot profiles and detector output ratios (DORs) to verify the Relative Output Factors (ROFs) from the GK planning system. Material/Methods: MSBs were threaded together to form 1D arrays consisting of 22 beads (1.1mm length x 1.6mm width). Measurements were made inside a modified solid water head phantom on a Leksell Gamma Knife Esprit. Onboard cone beam imaging was used to ensure accurate positioning a central bead at the radiation focus. Exposed MSBs were analysed according to [6]. Measured DORs and profiles were compared to ROFs calculated by Leksell Gamma Planning system, and measured by a MicroLion Type 310180 chamber, a PTW microDiamond Type 60019 and EBT3 radiochromic film. Full-width at half-maximum (FWHM) was calculated from measured profiles and cross-compared.