S2778
RTT - RTT contouring, target definition, and treatment planning
ESTRO 2026
suggesting potential to expand SABR eligibility for patients currently excluded under UK guidance. References: Diez P et al. (2022). UK 2022 consensus on normal tissue dose-volume constraints for oligometastatic, lung and HCC SABR. Clin Oncol, 34(5), 288–300.Royal College of Radiologists (2024). Hepato-pancreato- biliary cancer (4th ed.). RCR.Robbins JR et al. (2019). SBRT for HCC: practice patterns, dose selection and survival factors. Cancer Medicine, 8(3), 928–938.Su T-S et al. (2016). Long-term analysis in combined TAE and SBRT versus SBRT monotherapy for unresectable HCC > 5 cm. BMC cancer, 16(1), 834. Yanagihara TK et al. (2024). Defining Minimum Treatment Parameters of Ablative Radiation Therapy in Patients With HCC: An Expert Consensus. Practical Radiation Oncology, 14(2), 134-145. https://doi.org/10.1016/j.prro.2023.08.016 Keywords: Dose escalation, Treatment planning, Organ sparing Should the Vacuum Immobilization Device Be Included in the Body Contour? Dosimetric Impact on PTV Coverage in Pediatric Craniospinal Irradiation Raquel Rocha 1 , Gabriel Farinha 1 , Daniela Branco 1 , Daniela Saraiva 1,2 , Armanda Monteiro 1 , Lígia Osório 1 1 RTT, ULS São João, Porto, Portugal. 2 Student, University of Vigo, Porto, Portugal Digital Poster 1950 Purpose/Objective: External devices used in radiotherapy, such as the Vacuum Bag (VB), can affect the dose distribution. Although Treatment Planning Systems (TPSs) allow inclusion of these structures within the Body Contour (BC), some TPSs do not automatically model immobilisation devices and manual contouring is time- consuming.This study evaluates the dosimetric impact of the VB in Craniospinal Irradiation (CSI) planning. Given the large Planning Target Volume (PTV) and anatomical region encompassed by the VB, dose distributions were compared with and without the VB in the patient BC. Material/Methods: A total of 21 paediatric patients treated with CSI using VMAT (6 MV, Analytical Anisotropic Algorithm, version 15.6.04, Eclipse TPS; Varian Medical Systems, Palo Alto, CA, USA) were retrospectively selected. Patient positioning was achieved using a thermoplastic mask and VB. Treatment plans were performed with VB included in the BC, setting the HU threshold to − 950 HU, with manual contour adjustments as needed. A second calculation was generated with the BC threshold redefined at − 450 HU, including only the
organs of interest. This strategy may enhance SABR efficacy and broaden eligibility, particularly for large or multifocal disease. Proton beam therapy (PBT), with its characteristic Bragg peak, may further improve the therapeutic ratio compared with photon-based volumetric modulated arc therapy (VMAT).The objective was to evaluate whether escalation using VMAT and PBT, guided by current dose recommendations for the organs of interest (Diez et al., 2022), can safely increase tumour dose and potentially expand SABR eligibility in patients with large or multifocal HCC. Material/Methods: Five patients with large or multifocal HCC were retrospectively replanned. All cases were planned in five fractions. A single planner created both VMAT (Pinnacle, 6 MV FFF photons) and PBT (Eclipse, 226 MeV protons) plans using identical CT datasets, contouring, and motion management to ensure comparability. Dose escalation was performed in 5 Gy increments until either the mean healthy liver constraint ( ≤ 15.2 Gy) was reached or an organ of interest exceeded guidance. PBT plans incorporated robust optimisation (±5 mm setup, ±3.5% range) to account for delivery uncertainties. All plans were evaluated using isodose distributions, DVHs, and BED ₁₀ were calculated. Results:
All plans met mandatory dose guidance. PBT improved sparing of the liver, stomach, and duodenum, allowing higher BED ₁₀ delivery than VMAT. PBT reduced bowel maximum and liver mean doses by 1.5 Gy and 6 Gy, respectively. In several cases, bowel proximity and chest wall limit did restrict further escalation. Conclusion: Both VMAT and PBT achieved safe isotoxic escalation. While PBT demonstrated a more favourable balance between tumour dose and normal tissue sparing, VMAT also enabled meaningful dose escalation,
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