18F-FDG-PET/CT-based treatment planning for definitive (chemo)radiotherapy in patients with head and neck squamous cell carcinoma improves regional control and survival

•FDG-PET/CT-based nodal target volume definition improves regional control and survival.•No isolated recurrences in the electively treated neck occurred with FDG-PET/CT-based nodal target volume definition.•FDG-PET/CT-based nodal target volume definition causes target volume transformation.•FDG-PET/CT can guide gradient dose prescription with de-escalation of the elective dose. Multimodality imaging including 18F-FDG-PET has improved the detection threshold of nodal metastases in head and neck squamous cell carcinoma (HNSCC). The aim of this retrospective analysis is to investigate the impact of FDG-PET/CT-based nodal target volume definition (FDG-PET/CT-based NTV) on radiotherapy outcomes, compared to conventional CT-based nodal target volume definition (CT-based NTV). Six-hundred-thirty-three patients treated for HNSCC with definitive (chemo)radiotherapy using IMRT/VMAT techniques between 2008 and 2017 were analyzed. FDG-PET/CT-based NTV was performed in 46% of the patients. The median follow-up was 31 months. Diagnostic imaging depicting the regional recurrence was co-registered with the initial CT-scan to reconstruct the exact site of the recurrence. Multivariate Cox regression analysis was performed to identify variables associated with radiotherapy outcome. FDG-PET/CT-based NTV improved control of disease in the CTVelective-nodal (HR: 0.33, p = 0.026), overall regional control (HR: 0.62, p = 0.027) and overall survival (HR: 0.71, p = 0.033) compared to CT-based NTV. The risk for recurrence in the CTVelective-nodal was increased in case of synchronous local recurrence of the primary tumor (HR: 12.4, p