S617
Clinical – Head & neck
ESTRO 2026
included. Patients underwent planning MRI in RT position acquired on a 1.5T MRI scanner. sCTs were generated from a dedicated T1 VIBE-Dixon sequence using certified software (MRI Planner 2.5.0, Spectronic Medical). Target volumes and organs at risk were contoured on T1 and T2 sequences and dose calculation was performed on the sCT. During treatment, daily six-degree-of-freedom CBCT-to-sCT registrations were performed by radiation therapists (RTTs). Setup shifts were compared to those from an experienced H&N radiation oncologist (RO), using both CBCT-to-sCT and CBCT-to-planning MRI registrations (figure 1). This retrospective study was approved by the Swiss cantonal ethics committee (BASEC-2023- 01508).
Conclusion: Conclusion: Long-term follow-up results suggest that, based on the risk score model(RSM), omission of level IB irradiation is feasible for patients classified as low- risk by the model. References: 1. Zeng, L. et al. Late toxicities after intensity- modulated radiotherapy for nasopharyngeal carcinoma: patient and treatment-related risk factors. Br J Cancer 110, 49–54 (2014).2. Zhu, M.-Y. et al. Risk factors of level Ib lymph node metastasis and clinical outcome of its selectively prophylactic irradiation in nasopharyngeal carcinoma: A real-world study. Radiother Oncol 202, 110620 (2025).3. Meng, Z. et al. The feasibility of level Ib-sparing intensity-modulated radiation therapy in patients with nasopharyngeal carcinoma and high-risk factors classified based on the International Guideline. Radiother Oncol 191, 110027 (2024) Keywords: Nasopharyngeal carcinoma,Level IB,Lymph nodes,IMRT Digital Poster 3224 MR-only radiotherapy planning for head and neck cancer: first clinical implementation and experience Nicolas Martz 1,2 , Panagiotis Balermpas 1 , Bertrand Pouymayou 1 , Matthias Guckenberger 1 , Stephanie Tanadini-Lang 1 , Riccardo Dal Bello 1 1 Radiation oncology department, University Hospital Zürich, Zürich, Switzerland. 2 Radiation oncology department, Institut de Cancérologie de Lorraine, Nancy, France Purpose/Objective MRI-based synthetic CT (sCT) is a promising alternative to conventional planning CT, but its application in head and neck (H&N) radiotherapy remains limited due to complex air, bone, and soft tissues interfaces. This study reports on the first clinical experience with MR-only planning workflow for H&N cancers. Material/Methods Patients receiving definitive or post-operative H&N radiotherapy, without metal or tongue depressor within 5 mm of the planning target volume (PTV), were
Results Between January and August 2025, 20 patients were included, most of whom had oropharyngeal tumors (55%). During treatment, the image registrations performed by the RTTs at the treatment station showed that the differences between the CBCT-to- planning MRI and CBCT-to-sCT registrations, remained within the PTV margin of 3 mm for 97% (58/60) of all fractions. For the remaining two fractions, involving two different patients, isolated shifts of –3.5 mm and +3.4 mm were observed, respectively. All registrations performed by the RO were within clinical tolerance limits. Translational shifts (mm), mean [minimum, maximum], were for RTTs and RO, respectively: lateral = -0.2 [-2.8 – 2.7] and -0.2 [-1.8 – 1.6], longitudinal = – 0.2 [-2.6 – 2.8] and -0.1 [-2 – 2.8], and vertical = 0.3 [-3.5 – 3.4] and 0.4 [-2.1 – 3]. For rotational shifts (degrees), the values were: pitch = -0.1 [-2.3 – 1.4] and 0 [-1.2 – 2.2], couch rotation (yaw) = -0.2 [-2.4 – 1.7] and 0 [-1 – 1], and cranio-caudal axis (roll) = -0.2 [-2.3 – 2.3] and - 0.2 [-2.6 – 0.8] (figure 2).
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