S2777
RTT - RTT contouring, target definition, and treatment planning
ESTRO 2026
alone (p=0.043), representing a reduction of 3.73 Gy. The mean lung dose was 15.39±6.58 Gy with PET-CT vs 17.38±5.89 Gy with CT alone (p=0.024). The maximum spinal cord dose was significantly reduced: 28.79±9.12 Gy with PET-CT vs 41.63±6.82 Gy with CT alone (p=0.008), representing a gain of 12.84 Gy. Conclusion: Our study demonstrates that the integration of 18F- FDG PET-CT significantly improves dosimetric planning in esophageal cancer. PET-CT enables a significant reduction in GTV (-18.9%) and PTV (-8.3%) while improving PTV coverage. Most importantly, it allows significant dose reductions to critical OARs, particularly to the spinal cord (-12.84 Gy), heart (-3.73 Gy), and lungs (-1.99 Gy), thus optimizing the safety profile of treatment without compromising tumor efficacy. Keywords: 18F-FDG PET-CT,Esophageal Cancer, Target Volume Digital Poster 1933 Isotoxic Dose Escalation of Large and/or Multifocal HCC Thijanushan Thavarajah 1 , Richard Hugtenburg 2,3 , Adam Selby 4 , Rhiannon Evans 5 , Owen Nicholas 4,6 1 Department of Medical Physics, Swansea University, Swansea, United Kingdom. 2 Swansea University Medical School, Swansea University, Swansea, United Kingdom. 3 Department of Medical Physics and Clinical Engineering, Swansea Bay University Health Board, Swansea, United Kingdom. 4 South West Wales Cancer Centre, Swansea Bay University Health Board, Swansea, United Kingdom. 5 Velindre Cancer Centre, Velindre University NHS Trust, Cardiff, United Kingdom. 6 Faculty of Medicine, Health and Life Science, Swansea University, Swansea, United Kingdom Purpose/Objective: Hepatocellular carcinoma (HCC) is the most common primary liver malignancy. While stereotactic ablative radiotherapy (SABR) is an established treatment, UK guidelines restrict its use to small-volume disease ( ≤ 6 cm) (Radiologists, 2024), limiting curative options for patients with larger or multifocal tumours. These cases are often excluded due to higher risk to adjacent organs of interest and poorer local control. Several studies show a dose–response relationship for SBRT in HCC, with improved outcomes at a biologically effective dose (BED ₁₀ ) ≥ 75 Gy(Robbins et al., 2019; Su et al., 2016). An international consensus further defines ablative external beam radiotherapy as BED ₁₀ ≥ 80 Gy (Yanagihara et al., 2024)Isotoxic dose escalation enables adaptive planning that raises tumour dose while maintaining safe limits for the
C., Huger S., Bruand, M. et al. Artificial intelligence contouring in radiotherapy for organs-at-risk and lymph node areas. Radiat Oncol. 2024 Nov 21;19:168. doi: 10.1186/s13014-024-02554-y Keywords: APRT, AI, APRT-AI contouring
Digital Poster Highlight 1708
Impact of 18F-FDG PET-CT on Target Volume Delineation and Organ-at-Risk Sparing in Esophageal Cancer. Hicham Gazanayi, Hoda Ibroun, Oumayma Mezouari, Khadija Saadi, Hanae Dlaimi, Wafae Merssetti, Soukaina Morchid, Nabila Sellal Radiotherapy, University hospital center Mohammed VI, Tangier, Morocco Purpose/Objective: In precision radiotherapy for esophageal cancer, CT alone may include peri-tumoral inflammation, risking organ overdosage. 18F-FDG PET-CT identifies metabolically active tumor tissue, potentially optimizing delineation. Study objective: evaluate PET- CT's impact on target volumes defnition and OARs doses versus CT alone. Material/Methods: We included 20 patients treated for esophageal cancer at the Radiotherapy Department of Mohammed VI University Hospital in Tangier between January 2023 and January 2025. All patients received concurrent chemoradiotherapy. Target volumes (GTV, CTV, PTV) were delineated twice according to two modalities: CT alone and CT + 18F-FDG PET-CT fusion. Delineation followed a strict protocol: CTV T = GTV T + 5 cm longitudinally + 2 cm radially; PTV = CTV + 8 mm (upper/middle third tumors) or 1 cm (lower third tumors); CTV N = GTV N + 5 mm and PTV N = CTV N + 1 cm. Two dosimetric plans were created for all patients using VMAT technique, 6 MV, for a total dose of 50.4 Gy in 28 fractions of 1.8 Gy. Volumetric and dosimetric data were reported and compared. Statistical analysis was performed using SPSS v25. Results: The mean GTV volume with CT alone was significantly larger than with PET-CT: 84.1±49.5 vs 68.2±56.9 cm ³ (p=0.043), representing an 18.9% reduction. The PTV showed a significant reduction (777.0±285.8 cm ³ vs 712.4±242.1 cm ³ ; p=0.031), representing a mean decrease of 64.6 cm ³ (8.3%). Regarding target volume coverage, the mean D95% was 48.35 Gy (96.0%) with CT alone compared to 48.67 Gy (96.6%) with PET-CT (p<0.05). The conformity index remained comparable: 0.96±0.02 with CT alone vs 0.99±0.01 with PET- CT.Regarding organs at risk, the mean heart dose was 17.93±9.12 Gy with PET-CT vs 21.66±9.45 Gy with CT
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