S1674
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
gate start, stop, and duration, as well as beam-on time outside the intended window and the overall gating efficiency, were determined.
Oncology, Calvary Mater Newcastle Hospital, Newcastle, Australia
Purpose/Objective: Whilst initially designed for imaging, the primary role of flat-panel megavoltage imaging in radiotherapy is now for dosimetry and quality assurance purposes. They are poorly suited for dosimetry applications due to their energy-dependent scintillator response. In this work a novel design is investigated to produce multiplexed contrast imaging and water-equivalent dosimetry in a single irradiation. Material/Methods: An optical blocking layer (grid or chequerboard pattern) is placed between the scintillator and photodiode array. The dosimetry signal (direct signal) is formed in the blocked regions through direct energy deposition in the photodiode array while the imaging signal (indirect signal) is formed from scintillation photons in the unblocked regions. A Monte-Carlo model was developed in the TOPAS system for a Varian TrueBeam linac with an aS1200 imager to investigate direct and indirect energy deposition with blocking configurations. Subsequently, IMRT field direct images measured with a complete blocking layer [1] were used to model sampled direct images with gamma comparison to the full resolution images. Phantom and anthropomorphic indirect images were similarly investigated. Results: The design was found to be well suited for multiplexed imaging and dosimetry. TOPAS model results for optical edge and grid blocking patterns aligned with the pixel matrix yielded only ~0.1% of total indirect optical signal in the first pixel under the blocking. TOPAS results for direct signal reproduced fully blocked direct signals (Figure 1 left panels). Results for indirect signal showed penumbral falloff of the indirect signal over 4–5 pixels (~1.3–1.7 mm) (Figure 1 right panels) which is correctable with image uniformity correction.Gamma pass-rates for 25 IMRT fields with varying grid widths from 3 to 10 pixels varied from 99.9% to 96.2% at 3%/1mm criteria with superior results for chequerboard blocking patterns. Simulations of indirect image quality with the PIPSpro QC3V phantom showed maximum contrast reduction of 15% in high spatial resolution. Reconstructed phantom images are indiscernable from original images (Figure 2).
Results: For the exhale gating strategy, mean [max] errors for gate start, stop, and duration were 50 ms [183 ms], 5 ms [141 ms], and −34 ms [236 ms], respectively (fig.2). Although 70% of gates showed some beam-on outside the intended window, this accounted for only 2.2% of total irradiation time. The gating efficiency was 96.1%.For the mid-position strategy, the respective mean [max] errors were 43 ms [182 ms], −103 ms [235 ms], and −145 ms [256 ms] (fig.2). Beam-on outside the window occurred in 32% of gates, representing 6.9% of the total treatment time. The gating efficiency was 58.2%.
Conclusion: By combining dose-based position encoding with the fast temporal response of a plastic scintillation detector, we established an independent, measurement-driven method for MR-linac gating QA. The gating system of the 1.5T MR-Linac demonstrated high temporal accuracy with only minimal beam-on time outside the intended gating window and confirming compliance with system specifications. Keywords: Gating, dosimetry
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A novel megavoltage detector design to produce water-equivalent beam dosimetry and high quality anatomical contrast Louie Murcia 1 , John Holdsworth 1 , Karen Livesey 1 , Samuel Blake 2 , Jericho O'Connell 3 , Peter Greer 1,4 1 Physics, University of Newcastle, Newcastle, Australia. 2 Radiation Oncology, Verspeeten Family Cancer Centre, London, Canada. 3 Radiation Oncology, University of Washington, Seattle, USA. 4 Radiation
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