Diffusion-weighted MRI and apparent diffusion coefficient in head and neck malignancies: practical utility for diagnosis, triage and post-treatment surveillance

Diffusion-weighted imaging (DWI) with apparent diffusion coefficient (ADC) mapping is widely available on modern magnetic resonance imaging platforms and is increasingly embedded in head and neck oncological pathways. The technique can refine lesion characterisation, support nodal assessment, and strengthen post-treatment surveillance when it is applied to specific clinical questions. In this narrative review, we synthesise practical applications of DWI/ADC across major head and neck malignancies, including lymphoma, squamous cell carcinoma (SCC), sinonasal tumours, and salivary gland neoplasms, with emphasis on common diagnostic pitfalls and the limits of cross-study threshold transferability. We also summarise evidence relevant to treatment monitoring, adaptive radiotherapy strategies, and the technical repeatability needed for longitudinal ADC interpretation. Across the literature, markedly low ADC values favour lymphoma, whereas substantial overlap constrains benign-malignant separation in salivary and sinonasal lesions. In the post-treatment setting, meta-analytic evidence supports lower ADC values in recurrent disease than in treatment-related change, with a commonly cited recurrence threshold near 1.10 x 10-3 mm2/s. Overall, DWI is best treated as a probability-modifying biomarker that complements morphology, endoscopy, pathology, and clinical trajectory rather than replacing them.