S1658
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
exhibited enhanced sensitivity in the optically OSL registration mode. Signal degradation after 48 hours of immersion was below 5% for all samples, indicating high environmental stability. These results suggest that the optimal material choice depends on specific application requirements rather than a single performance metric. Conclusion: TL/OSL detectors based on Ce³ ⁺ -doped garnet crystals represent a promising platform for ionizing radiation detection in liquid radiopharmaceutical environments. Their high radiation resistance, chemical stability in aqueous solutions, and reliable TL/OSL response indicate strong potential for use in radiopharmacy monitoring and dosimetry applications in nuclear medicine. Future work should focus on integrating these materials into compact detection systems capable of simultaneous evaluation of different radiation types under clinical and regulatory conditions. References: S. Witkiewicz-Łukaszek, V. Gorbenko, A. Mrozik, P. Bilski, e. a. Rad. Measur.128 (2019) 106J. Winiecki, S. Witkiewicz-Łukaszek, P. Michalska, S. Jakubowski, e.a. Materials, 15 (2022) 7861. Keywords: scintillator, detector, dosimetry Digital Poster Highlight 2906 Development of a Water-Filled Phantom for End-to- End Dosimetry Audits on MRI-Linacs Kate L Francis 1 , Andrew Alves 1 , Rachel Williams 2 , Silvio Malfitana 3 , Ariadne Brodmann 4 , Glen Newman 4 , Rhonda L Brown 1 1 Australian Clinical Dosimetry Service, Australian Radiation Protection and Nuclear Safety Agency, Yallambie, Australia. 2 Assessment & Emergency Preparedness, Australian Radiation Protection and Nuclear Safety Agency, Yallambie, Australia. 3 Property and Facilities, Australian Radiation Protection and Nuclear Safety Agency, Yallambie, Australia. 4 Townsville Cancer Centre, Queensland Health, Townsville, Australia Purpose/Objective: End-to-end dosimetry audits are essential for quality assurance in radiotherapy. MRI-Linacs introduce unique challenges, including the need for MR-visible phantoms and the impact of air gaps on dose measurements in magnetic fields. This work presents the development of a novel, MR-visible, water-filled phantom designed to overcome these limitations and enable robust evaluation of adaptive treatment workflows. Material/Methods: Custom water-filled Perspex inserts were designed to
fit within the existing CIRS thorax phantom (CIRS Inc. Norfolk, USA), filled with demineralised water to ensure MR visibility and water-equivalent conditions. The design accommodates Farmer chambers (PTW 30013), microDiamond detectors (PTW 60019), and EBT4 Gafchromic film (Ashland ISP Inc.,Wayne,NJ,USA), replicating current audit capabilities. Small water-filled cylinders facilitate IGRT and phantom alignment, a relocatable farmer chamber insert enables point dose measurements at varying depths and a central insert is used to measure soft tissue-like SABR treatment. Adaptive positional shifts are assessed by moving the phantom on the treatment couch manually in the sup- inf and left-right directions. An elliptical virtual target volume is provided for planning, with Radiation Therapists instructed to treat to a visible cylindrical target, supporting evaluation of adapt-to-shape workflows. Results: Phantom and detector placement were verified by comparing planning CT and treatment MR images, confirming MR visibility and repeatable positioning. Participating centres reported perceived improved image matching between CT and MR scans compared to previous phantom versions. Online adaptive plans met required constraints (Prescribed dose = 45 Gy/3# to PTV, D95 > 45 Gy, D2 > 49.5 Gy, spinal cord maximum < 10 Gy and CI100 < 1.2). The CT-to-MR transformation matrix was established using IGRT, with film alignment accuracy < 0.3 mm. In conventional linac audits, the mean dose offset was < 2% and mean position offset < 1 mm; all soft tissue cases met these constraints as well as a 5%/2mm gamma criteria with > 95% pass rate. Process refinements, including minimizing air bubbles, were implemented based on trial feedback.
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