2081 Micro silica beads for verification of profiles and outputs factors of a Leksell Gamma Knife

Stepanek, Chris J.; Aylward, Jack D.; Hartley-Davies, Ronald; Winch, Lucy

Abstract

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.
Results: Measured profiles are shown in Fig. 1, DORs, ROFs and profile FWHM are shown in Table 1. The 8mm MSB DORs
were within 0.5% of the ROFs calculated using the microdiamond detector, film and planning system. The 4mm MSB
DORs were within 3% of the ROFs calculated using planning system and film, and microdiamond. Uncertainties on
these measurements were greatest for radiochromic film (5.2% for 4mm) smallest for the micro diamond (0.6% for
4mm) with the MSBs in-between (3.6% for 4mm). Film ROFs with significant variation were seen throughout the
literature [1,3,5]. FWHM calculated using MSB-measured profiles were within 0.5mm of those measured by film,
planning system and others [1,2]. The use of onboard imaging to position the beads was essential for DOR
measurement accuracy and reproducibility. The use of MSBs was limited to z-axis measurements due to the design
of the solid water phantom.
Conclusion:
The MSBs provided satisfactory measurements of DORs and shot profiles and can serve as a lower cost and more
efficient method of dosimetry.

• Presentation Conference Type: Conference Abstract
• Conference Name: European Society of Radiation Oncology Congress
• Start Date: May 2, 2025
• Acceptance Date: May 21, 2025
• Online Publication Date: May 21, 2025
• Publication Date: May 31, 2025
• Deposit Date: Jun 18, 2025
• Journal: Radiotherapy and Oncology
• Print ISSN: 0167-8140
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 206
• Issue: S1
• Pages: S2594-S2597
• DOI: https://doi.org/10.1016/s0167-8140%2825%2900832-1
• Public URL: https://uwe-repository.worktribe.com/output/14578683
• Additional Information: This article is maintained by: Elsevier; Article Title: 2081 Micro silica beads for verification of profiles and outputs factors of a Leksell Gamma Knife; Journal Title: Radiotherapy and Oncology; CrossRef DOI link to publisher maintained version: https://doi.org/10.1016/S0167-8140(25)00832-1; Content Type: simple-article; Copyright: Copyright © 2025 Elsevier Ireland Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.