S1675
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Digital Poster 3955 Implementation of TMI/TMLI techniques using an O-Ring treatment system: evaluation of plan robustness and delivery consistency Rixy Johana Plata, Alvaro Alejandro Ruiz, Filippo Marangoni, José Luis Rodríguez, Jhonalbert Aponte, Julia Fontana, Marcelo Ribeiro Radiotherapy, Fundación Arturo López Pérez (FALP), Santiago de Chile, Chile
Purpose/Objective: To evaluate geometric robustness, delivery
consistency, and patient-specific quality assurance (QA) in Total Marrow Irradiation (TMI) and Total Marrow and Lymphoid Irradiation (TMLI) treatments delivered on the Varian Halcyon G system. Material/Methods: A retrospective analysis was carried out on nine patients treated with TMI/TMLI using a multi-isocenter (7–9 isocenters) approach. Patient-specific QA was performed prior to the first fraction using the EPID detector. The institutional action level was set in accordance with the TG218 report for global gamma normalization (2%/2 mm, 10% threshold, 95% tolerance).For the first five cases, absolute dose point measurements were performed for each subplan on the MVP phantom using an A1SL ion chamber to verify dose calculation accuracy.Plan robustness was assessed against ±5 mm cranio-caudal shifts by quantifying the impact on maximum (Dmax) and mean (Dmean) doses to the lungs and kidneys. This analysis was performed to establish a clearer criterion for patient setup accuracy and treatment delivery during clinical implementation.Post-treatment, EPID transit dosimetry was evaluated by comparing transit images from each fraction with those from the first session to assess delivery consistency and detect potential interfraction deviations. Additionally, beam-on times and total door-to-door treatment times were recorded for each patient. Results: All treatment plans met institutional pretreatment QA criteria, achieving gamma passing rates above 97%.Point dose measurements showed differences within ±3% compared to calculated doses.Longitudinal overlaps of up to 5 mm resulted in maximum Dmax increases of approximately 17% and Dmean variations of 3–4% for lungs and kidneys.Transit dosimetry analyzed with 2%/2 mm, 10% threshold achieved gamma passing rates above 90% in 84% of fractions. When analyzed with a 3%/3 mm criterion using the same threshold, results exceeded 90% for 98% of the evaluated fractions, suggesting stable and reproducible delivery. A total of 225 transit datasets were analyzed, including all treatment sessions and multiple isocenters per plan.The average beam-on
Figure 1. TOPAS results for grid-pattern w=7 pixels (2.4 mm) and w=14 pixels (4.7 mm) . Direct signal (left) is scored in the blocked regions, while indirect signal (right) is scored at all regions. Comparisons are to fully blocked (direct) and unblocked (indirect).
Figure 2. (top-left) Simulated 6-pixel width blocking image; (top-right) full indirect image; (bottom-left) image reconstructed from blocking image; (bottom- right) image signal profiles. Conclusion: The design produces water-equivalent dosimetry images for patient treatment verification and linear accelerator quality assurance applications and high quality anatomical imaging. References: [1] Gustafsson H et al. Direct dose to water dosimetry for pretreatment IMRT verification using a modified EPID. Medical Physics, 2011. 38(11): p. 6257-6264 Keywords: design, electronic portal imaging, multiplexed
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