Eliminating the high-risk CTV1 margin in DAHANCA oropharyngeal radiotherapy: Dosimetric impact on dysphagia and organ-at-risk doses.

Background and purpose: Radiotherapy for head and neck cancer must balance tumour control with late toxicities such as dysphagia and xerostomia. Recent retrospective studies suggest that the margin from the gross tumour volume (GTV) to the high-dose clinical target volume (CTV1) may not be critical for local control, while larger irradiated volumes increase the risk of toxicity. The study quantifies potential reductions in dose to organs at risk (OARs) and predicted dysphagia risk when the standard 5 mm GTV-to-CTV1 margin is eliminated in oropharyngeal cancer. Patient/material and methods: Retrospectively 30 oropharyngeal cancer patients treated consecutively during 2023 according to the DAHANCA guidelines (5 mm GTV-to-CTV1 margin) were selected. For each patient, a standard plan and a modified experimental plan (CTV1 = GTV, and CTV2 reduced by 5 mm accordingly) were generated using Pinnacle3 Auto-Planning. All plans met the DAHANCA target coverage and OAR dose constraints. Dose-volume data for relevant OARs were extracted and compared in MATLAB. Normal tissue complication probability (NTCP) model for dysphagia was applied.

Results: Margin elimination reduced high-dose CTV volumes by 70%, yielding significant dose reductions to multiple OARs. Mean doses to the upper/middle pharyngeal constrictors decreased by around 4-5 Gy (p < 0.001) and to the contralateral submandibular gland by ~5 Gy (p < 0.001). These dosimetric gains correspond to an estimated median ΔNTCP of 6.0% of late grade ≥ 2 dysphagia. Target coverage and conformity were maintained in all plans.

Interpretation: Omitting the high-risk CTV margin can substantially reduce the dose to dysphagia--associated OAR without compromising target coverage. This approach shows promise for improving patient-reported swallowing outcomes and warrants clinical evaluation.