S648
Clinical – Head & neck
ESTRO 2026
irradiation, RNI), or omitted (involved node irradiation, INI). Dose distribution to organs at risk and corresponding changes in NTCPs were analyzed. Results: Most common primary tumor site was the oropharynx (n = 14), followed by the larynx (n = 5), hypopharynx (n = 5), and oral cavity (n = 2). Thirteen patients had unilateral, nine bilateral, and four no lymph node metastases. The median total planning target volume was 540 cm3 (IQR 395 cm3) with ENI, 418 cm3 (IQR 359 cm3) with RNI and 173 cm3 (IQR 188 cm3) with INI.Both de-escalation strategies significantly decreased modeled toxicity. The mean risk of dysphagia was reduced by 5.9 pp with RNI (95% CI: 2.3-9.6 pp; padj = 0.005) and 11.0 pp with INI (95% CI: 6.9-15.2 pp; padj < 0.001) compared with a mean risk of 28.9% with ENI. The modeled mean risk of tube feeding dependence of 6.8% with ENI was lowered by 2.8 pp with RNI (95% CI: 1.2-4.5 pp; padj = 0.005) and 4.2 pp with INI (95% CI: 2.5-6.0 pp; padj < 0.001). INI additionally reduced the mean risk of xerostomia by 8.8 pp (95% CI: 5.5-12.2 pp; padj < 0.001), whereas RNI showed a non-significant decrease of 2.8 pp (95% CI: - 0.0-5.7 pp; padj = 0.054) compared to a mean risk of 40.7% with ENI. The largest NTCP reductions were observed in patients with hypopharyngeal or laryngeal cancer. Conclusion: PET-guided de-escalation of elective nodal irradiation significantly reduced modeled toxicity promising to improve the tolerability of radiotherapy in head and neck cancer. Keywords: de-escalation, elective neck irradiation Digital Poster 4100 Machine learning based models for prediction of tumor recurrence in patients undergoing radiotherapy for Carcinoma of Nasopharynx Amit Bahl 1 , Ranjit Singh 1 , Jaimanti Bakshi 2 , J Selvaraj 3 , Roshan Verma 2 , Satyawati Mohindra 2 1 Radiotherapy and Oncology, PGIMER, Chandigarh, India. 2 Otolaryngology, PGIMER, Chandigarh, India. 3 Radiation Oncology, Austin Health, Melbourne, Australia Purpose/Objective: Treatment outcomes in carcinoma nasopharynx have improved over the years due to improvements in radiotherapy and chemotherapy. Still a significant proportion of patients experience recurrent disease. A prediction model for recurrences can help stratify and improve management decisions. Machine learning models can be useful in view of the multitude of parameters involved in the development of recurrences. In the present study a comparision was
group, 638 comparison group). After PSM, 304 patients remained in each group. With a median follow-up of 98 months (interquartile range: 80–122), 5-year regional recurrence-free survival was comparable between the modified and comparison groups (94.41% vs. 92.43%, P=0.348), with no significant difference in long-term survival. The modified group received significantly lower doses to organs-at-risk (P<0.001) and had a markedly lower late post- radiotherapy dysphagia incidence (13.2% vs. 23.7%, P<0.001). For the lymph nodes spared from irradiation, the mean target spillage dose was 42.26 Gy, with no recurrence observed in the follow-up. Conclusion: Compared with standard radiotherapy, our selective level III target volume optimization protocol provides comparable survival outcomes while significantly reducing radiotherapy-related toxicity, particularly late dysphagia. Keywords: nasopharyngeal carcinoma Digital Poster 4088 FDG-PET-Guided De-escalation of Elective Radiotherapy Reduces Modeled Toxicity Risk in Head and Neck Cancer Katharina Dvornikovich 1 , Florian Stritzke 1 , Hin Lau 1 , Thomas Tessonnier 1 , David Neugebauer 1 , Philipp Schröter 1 , Nils Netzer 1 , Lukas Bauer 1 , Rubens Thölken 2 , Erik Winter 3 , Semi Harrabi 1 , Andrea Mairani 1 , Uwe Haberkorn 3 , Jürgen Debus 1 , Thomas Held 1 1 Department of Radiation Oncology, Heidelberg University Hospital, Heidelberg, Germany. 2 Department of Otorhinolaryngology, Head and Neck Surgery, Heidelberg, Germany. 3 Department of Nuclear Medicine, University Hospital Heidelberg, Heidelberg, Germany Purpose/Objective: Radiotherapy for head and neck cancer often impairs quality of life due to swallowing-related toxicity. This study evaluated two de-escalation strategies using PET-guided target definition and reduced elective nodal irradiation, aiming to lower dose to sensitive organs at risk while preserving oncologic safety. Material/Methods: Twenty-six patients with HNSCC treated between 2021 and 2024 who underwent pre-treatment PET-CT imaging were included in this study. For each patient, the clinical standard plan (standard elective node irradiation, ENI; 70/63/56 Gy, SIB) was compared to two de-escalated approaches. High- and intermediate- risk areas were maintained, while elective volumes were either limited to 2 cm cranially and caudally of the GTV with elective dose reduced from 56 Gy to 45 Gy using a sequential boost (reduced elective node
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