S1996
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Conclusion: This study demonstrated that increased mean blood dose to the lung, heart, aorta and PA were significantly associated with reduced overall survival in patients with LA-NSCLC. Introducing dose constraints for aorta and PA was feasible without compromising target coverage or clinical standards. These findings suggest that targeted sparing of blood bearing orangs may help improve patient outcomes. References: [1] Beekman C, et al. A stochastic model of blood flow to calculate blood dose during radiotherapy. Phys Med Biol. 2023;68(22):10.1088/1361-6560/ad02d6.[2] Shin J, Xing S, McCullum L, et al. HEDOS-a computational tool to assess radiation dose to circulating blood cells during external beam radiotherapy based on whole- body blood flow simulations. Phys Med Biol. 2021;66(16):10.1088/1361-6560/ac16ea.[3] Zarepisheh M, Hong L, Zhou Y, et al. Automated intensity modulated treatment planning: The expedited constrained hierarchical optimization (ECHO) system. Med Phys. 2019;46(7):2944-2954. doi:10.1002/mp.13572 Keywords: Immune-sparing, dose to blood Digital Poster 4388 Beyond the Gradient Index: Independent Validation of a Volume-Adaptive V50% Model for Quality Assurance in LINAC-based SRS Tuba Tekin 1,2 , Lara Aylin Caglayan 1 , Stephan Garbe 1 , Fabian Kugel 1 , Jasmin Holz 1 , Thomas Müdder 1 , Gustavo Sarria 1 , Davide Scafa 1 , Christina Leitzen 1 , Julian Layer 1,3 , Cas Dejonckheere 1 , Shari Wiegreffe 1 , Eleni Gkika 1 , Youness Nour 1 1 Department of Radiation Oncology, University Hospital, Bonn, Germany. 2 University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Oldenburg, Germany. 3 Institute of Experimental Oncology, University Hospital, Bonn, Germany
fraction intracranial SRS plans (February 2024 – May 2025) was retrospectively analyzed. Target volumes ranged from 0.19 to 4.58 cm ³ . All plans were created on a TrueBeam STx LINAC (HD120-MLC, 6 MV FFF) using Eclipse™ (v18.6.06) with Acuros XB (v15.6.06) following a standardized institutional protocol. The previously published power-law model was applied a priori without modification:Optimal limit: V50% = 4.10 ⋅ TV0.833Clinically acceptable limit: V50% = 5.05 ⋅ TV0.803Each plan was categorized as optimal, acceptable, or suboptimal according to these thresholds. Statistical analysis evaluated agreement between predicted and measured V50% values. Suboptimal plans were reviewed qualitatively to identify whether deviations were due to anatomical constraints or technical optimization potential. Results: The median target volume was 0.60 cm ³ (IQR 0.40– 1.29 cm ³ ). The model correctly classified 135 of 157 plans (86%) as optimal (18.5%) or clinically acceptable (67.5%). Twenty-two plans (14%) exceeded the acceptable limit. A strong correlation between target volume and V50% was confirmed (r = 0.981, p < 0.001), consistent with the original derivation dataset (r = 0.994). Suboptimal plans exceeded the acceptable threshold by a median of 11%. Qualitative review revealed that in several cases, the increased dose spill could be linked to modifiable technical factors rather than unavoidable clinical constraints. Conclusion: This independent temporal validation confirms the volume-adaptive V50% model as a robust and practical QA tool for LINAC-based SRS. By providing objective, patient-specific benchmarks, the model overcomes the inherent limitations of the Gradient Index and allows reliable identification of plans with optimization potential. Its routine implementation can support consistent plan quality across planners and institutions, ultimately enhancing safety and treatment standardization in modern SRS practice. Keywords: SRS, brain metastases, radionecrosis Enhancing Target Coverage and Organ Sparing in Endometrial Cancer VMAT Using Normal Tissue Objective Optimization Mouna Ben Rejeb, Roua Toumi, Raja Oueslati, Ghada Abdessatar, Farah Ben Aissa, Lilia Ghorbal, Lotfi Kochbati Radiation Oncology, Abderrahman Mami Hospital, Ariana, Tunisia Purpose/Objective: The Normal Tissue Objective (NTO) is an optimization tool in the Eclipse treatment planning system (TPS) Digital Poster 4415
Purpose/Objective: In stereotactic radiosurgery (SRS), minimizing
intermediate dose spill (V50%) is essential to reduce the risk of radionecrosis. Conventional gradient-based metrics such as the Gradient Index (GI) are strongly volume-dependent and therefore limited as objective quality assurance (QA) tools. A recently developed, volume-adaptive model based on a power-law function offers patient-specific benchmarks for V50%, defining “optimal” and “clinically acceptable” thresholds. This study aimed to independently validate this model over time in a new cohort of LINAC-based SRS plans. Material/Methods: An independent, consecutive cohort of 157 single-
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