S2008
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
clinical practice, when compared with VMAT plans, RC3D delivered between 5% to 20% higher for the GTV D98% value.For GTVs up to 35 cm ³ , GTV D98% value ranged between 1.07 and 1.3 of prescribed dose; for larger volumes, the range was from prescrbied dose up to 1.1 of prescribed dose.The proximity of ribs, vertebrae, or others organs of interest was found to induce variations in GTV D98%, with a median of 1.08 of the prescribed dose compared with 1.12 of prescribed dose in other cases. Comparable trends were observed for GTV D50%. Conclusion: This study demonstrates that for comparative assesments of lung tumors treated with SBRT, systematic reporting of GTV D98% and D50% values is crucial for dosimetric evaluation.The combination of GTV D98% and D50% appears to be an appropriate surrogate for evaluating plan quality and dose conformity, potentially outperforming standard PTV- based prescriptions. Keywords: SBRT, GTV density, Quality metric Radiation Necrosis Risk Prior to SRS: A Nomogram for Tumor Boards and Patient Consultation Based on Lesion Size, Number, and Systemic Therapies. Youness Nour 1 , Tuba Tekin 1,2 , Lara Caglayan 1 , Christina Leitzen 1 , Davide Scafa 1 , Julian Layer 1,3 , Cas Stefaan Dejonckheere 1 , Shari Wiegreffe 1 , Andrea Glasmacher 1 , Laura Kersting 1 , Stephan Garbe 1 , Fabian Kugel 1 , Jasmin Holz 1 , Thomas Müdder 1 , Gustavo Sarria 1 , Eleni Gkika 1 1 Department of Radiation Oncology, University Hospital, Bonn, Germany. 2 Medical Radiation Oncology, University Clinic, Oldenburg, Germany. 3 Institut of experimental Oncology, University Clinic, Bonn, Germany Purpose/Objective: Stereotactic radiosurgery (SRS) is an established standard for treating brainmetastases. A significant late toxicity is radiation necrosis (RN), the risk of which is markedlyincreased by modern systemic therapies, especially immune checkpoint inhibitors (ICIs).Patients desire quantified risk data, yet often receive only vague estimates. The aim of thisproject was to develop a visual, pre-planning communication tool (nomogram) to estimateindividual RN risk, support shared decision-making, and provide a standardized basis forcounselling and multidisciplinary tumor board decisions regarding potential systemic therapypauses. Material/Methods: The tool uses two primary inputs: MRI-based lesion characteristics(Target Volume, TV) and the type of systemic therapy.1. Dosimetry Prediction: A validated power-law model (defined as the clinical upperlimit: V Digital Poster 4655
trial. BMC Cancer 20, 380 (2020). https://doi.org/10.1186/s12885-020-06876-4[2] Reinhardt, Philipp et al. “Does omitting the clinical target volume in stereotactic radiotherapy for NSCLC affect oncologic outcomes? A retrospective analysis.” Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology vol. 211 (2025): 111092. doi:10.1016/j.radonc.2025.111092 Keywords: Target undercoverage, SBRT/SABR Lung Stereotactic Body Radiotherapy: Factors Inducing variability in the delivered D98% and D50% Dose to the GTV — Focus on Small Volumes Mohamed Maouche, Brian Baron, Marie Blanchet, Cyrielle Mella, Maxime Loo, Anne-Helene Josse, Maelle Seychelles, Hamza Ait Itto, Raissa Yebga Hot, Clement Chevillard, Mostafa Ghafooryan Sangchooli, Wissal Jbara Bouzgarrou, Roxane Lahady, Arnaud Veret, Rezart Belshi Radiotherapy, Institut Curie, Saint-Cloud, France Digital Poster 4637 Purpose/Objective: The aim of this study is to quantitatively assess the coverage outcome of the iGTV/GTV as a function of different anatomical and technical parameters. Patients were treated either under free-breathing conditions (4D-CT) or a deep inspiration breath-hold (DIBH) technique. We analyze the influence of 4 parameters: the mean mass density (MMD) of the tumor and lung parenchyma, the GTV volume, the treatment technique (3D-CRT / VMAT) and the GTV location within the lung. Material/Methods: The treatment planning system used was Pinnacle v16.2 (Collapsed Convolution Cone algorithm). CT acquisitions were performed with a slice thickness of 1-2mm using a BigBore CT scanner (Philips). The planning target volume (PTV) was defined as GTV plus a 0.5 cm margin. A calculation grid size of 0.1cm was applied for iGTV below 2.7 cm ³ , and 0.2 cm for larger
volumes.To assess the impact of MMD, a representative clinical case was analyzed,
characterized by a GTV of 0.37 cm ³ , a tumor MMD of 0.46 g/cm ³ , and a homolateral lung MMD of 0.109 g/cm ³ . The following parameters were varied:Lung MMD: from 0.109 to 0.4 g/cm ³ Tumor MMD: from 0.46 to 1.2 g/cm ³ GTV volume: from 0.014 to 2.085 cm ³ with identical MMDIn addition, 94 clinical treatment plans were reviewed to evaluate the effect of tumor volume on GTV D98% and D50%. Results: For small GTVs, an increase in pulmonary or tumor MMD led to respective variations of -7% and +8% in GTV D98%, and -1% and +17% in GTV D50%.In our
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