ESTRO 2025 - Abstract Book

S2828

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

2659

Proffered Paper Probabilistic Target Volume for Head and Neck Cancer: A TCP-based Treatment Planning Approach for Elective Nodal Irradiation Kristoffer Moos 1,2,3 , Roman Ludwig 3 , Yoel S Pérez Haas 3 , Muriel Baldinger 3 , Esmée L Looman 3 , Stine S Korreman 1,2 , Jan Unkelbach 3 1 Danish Centre for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark. 2 Department of Clinical Medicine, Aarhus University, Aarhus, Denmark. 3 Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland Purpose/Objective: In radiotherapy for head and neck cancer, elective irradiation is often applied to lymph node levels (LNLs) at risk of harbouring occult metastases, even without clinical evidence of tumor. Although this increases the probability of regional tumour control (TCP), it also raises the risk of normal tissue complication (NTCP). In this study, we propose a TCP-based optimization framework for elective nodal irradiation (ENI) in head and neck cancer, aiming to balance TCP and NTCP. Material/Methods: The proposed framework categorizes LNLs according to the risk of harbouring occult metastasis: high-risk (>10%), receiving 50 Gy with conventional objectives to achieve optimal dose coverage and conformity; low-risk (<2%), excluded from the elective target; and intermediate-risk (2-10%), optimized using TCP objectives. Each voxel within the intermediate risk LNL contributes to the overall TCP through the equation (1-q) + qTCP i (d i ) , where (1-q) represents the probability that the voxel does not harbour occult metastases, and qTCP i (d i ) reflects the likelihood that any existing tumour within the voxel is controlled effectively by the administered dose d i . The overall TCP, for inhomogeneous dose distribution, is then derived by the product of these probabilities across all voxels. The parameter q is based on the overall probability that the LNL harbours occult metastasis, computed by the lymphatic spread model from Ludwig et.al [1]. The parameters of TCP i (d i ) are derived from Okunieff et al. [2]. The framework applies NTCP objectives to manage the radiation-induced risks of xerostomia and dysphagia in adjacent organs-at risk (OARs) [3]. Results: The optimized treatment plans effectively achieved therapeutic dose coverage in regions of the LNL further from the OARs, while reducing doses near relevant OARs. Figure 1 demonstrates this approach for a cN0 patient with 5% risk of occult metastasis in the contralateral level II. The risk of xerostomia reduced from 34.4% to 22.4%, a 12% decrease, with a minimal influence on TCP (99.5% to 98.8%).