S130
Brachytherapy - Physics
ESTRO 2026
Napolitano, Stephanie Parker, Deborah Schofield, Koren Smith, Ellen Yorke, Michelle Wells. Strategies for effective physics plan and chart review in radiation therapy: Report of AAPM Task Group 275, Med. Phys. 47 (6), June 2020; 0094-2405/2020/47(6)/e236/37.
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Development of a routine single-point dose measurement for built COMS ophthalmic applicators Evan Keane, Christy Alekchander, Anne Downes Physics, St. Luke's Radiation Oncology Network, Dublin, Ireland Purpose/Objective: The aim was to develop and validate a routine single- point dosimetry check for constructed COMS ophthalmic plaques loaded with I-125 seeds. The objective was to detect construction deviations, particularly silastic insert thickness variations, which have been previously reported [1], to provide measurements traceable to treatment planning system calculations and to define an actionable acceptance tolerance for routine clinical quality assurance Material/Methods: A 20 mm COMS plaque was loaded with IsoSeed I25.S16 (Eckert & Ziegler BEBIG, Germany) sources according to the Eye Physics Plaque Simulator treatment plan. Dosimetric measurements used GafChromic EBT4 film with a prescription dose of 250 cGy, placed at a prescription depth of 7.11 mm, consisting of an in-house developed 6.97 mm thick wax phantom and a 0.14 mm active film layer depth. Two measurement geometries were implemented: (a) a spherical cap wax phantom designed to replicate the eye curvature and accommodate the largest plaque size for rapid single-point dose checks, and (b) a reference water tank geometry to provide dose measurements inclusive of full backscatter. Profiles were extracted from the film, and the dose at the prescription depth was determined for both geometries.Independent Monte Carlo simulations were performed in Topas to model both measurement configurations ((b) and (d) in Figure 1) and calculate dose distributions for a single I-125 seed. The dose ratio between the wax and water geometries was computed to derive a simulated wax-to-water correction factor for validation against experimental measurements.
Results: Central-axis film readings in the wax phantom were consistently higher than those measured in water at the 7.11 mm prescription depth. The experimentally determined factor was 0.92. The Monte Carlo simulations (Figure 2) predicted a correction factor of 0.90 ± 0.02 (k=1), consistent with the measured value within combined uncertainties. Expanded uncertainties were determined as follows: depth-dose 8.5% (k=2), point-dose measurement 4.7% (k=2), correction factor 8.6% (k=2), resulting in an overall point-dose-to-water expanded uncertainty of approximately 9.6% (k=2). End-to-end verification on a representative plaque and independent TPS comparison demonstrated agreement within these bounds.
Conclusion: A practical film-based single-point dosimetry check for constructed COMS plaques was developed, validated with Monte Carlo and independent TPS comparisons, and implemented with an automated analysis workflow. The simulated and measured correction factors agree within uncertainty. Considering the measured uncertainties and published sensitivity of apex dose to silastic thickness [1], a ±10% acceptance tolerance is recommended for this method of routine point dose measurement of constructed plaques prior to clinical use. Keywords: Dosimetry, Uveal Melanoma, COMS plaque References: [1] Oare, Courtney C., et al. "On the importance of quality assurance (QA) for COMS eye plaque Silastic inserts: A guide to measurement methods, typical
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