S560
Clinical – Head & neck
ESTRO 2026
midline.
modelled based on spread probabilities between LNLs and the primary tumour. The model parameters are inferred from a dataset consisting of 407 patients, treated across three different institutions with detailed clinical and/or pathological LNL status. This results in a model quantifying risk of occult disease, taking clinical diagnosis, T-category and extension over the mid- sagittal plane into account. Results: Fig. 1 compares the model’s predicted prevalence (histograms) to the observed frequency in our dataset (solid lines) for different combinations of involved LNLs, T-category and extension. The model is generally capable of fitting to the data and capturing the dependence of LNL involvement on T-category and midline extension.
Conclusion: The HMM model of oral cavity SCC captures the dependence of lymphatic involvement on relevant risk factors, illustrating the potential of future de-escalated CTV-Ns. Assuming a risk threshold of ~5%, ipsilateral LNL III might not be treated if nodal involvement is limited to LNL I. For patients with a clinically negative contralateral neck, contralateral irradiation might only be required for tumours extending over the mid- sagittal plane. References: 1. Biau (2019) Rad. & Onc.134 1-92. Ludwig (2025) Sci. Rep.15 181523. de Bondt (2007) Eur. J. Radiol.64 266- 272 Keywords: Head and Neck, Lymphatic spread, Occult disease
Fig. 2 shows the model’s predicted risk of occult disease given T-category, extension status and an assumed imaging-based diagnosis of nodal metastases (sensitivity: 81%, specificity: 76% [3]). For ipsilateral disease, T-category and upstream involvement were found to be the most relevant risk factors. The top panel in fig. 2 shows that the predicted risk of occult metastases in ipsilateral LNL III only exceeds ~5% if the upstream level II harbours metastases. For the contralateral neck, extension over the midline was observed to be the most significant risk factor. The bottom panel of fig. 2 illustrates that risk in contralateral LNLs I and II only approaches ~5% for a tumour extending over the
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Proof-of-concept multi-centre delineation study using dual-energy CT for head-and-neck cancer delineation Anh Thu Lê 1 , Nathalie Fournier-Bidoz 2 , Vjona Cifliku 1 , Maria Andersen 3 , Barbara Bachtiary 4 , Adelina
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