Salivary cell-free DNA in head and neck cancer: emerging clinical applications and future perspectives.

Head and neck cancer (HNC) diagnosis and surveillance still rely primarily on clinical examination, imaging, and tissue biopsy, with no established non-invasive biomarkers for routine early detection or disease monitoring. This limitation underscores the need for alternative strategies that can facilitate timely diagnosis and longitudinal assessment. Salivary cell-free DNA (scfDNA) has emerged as a promising liquid biopsy biomarker owing to its close proximity to primary HNC lesions and its suitability for non-invasive, repeatable sampling. Detection and characterization of scfDNA enable the identification of tumour-associated molecular alterations, including somatic mutations, aberrant DNA methylation, copy number alterations, viral DNA, and fragmentation signatures, providing valuable insights into tumour biology and disease dynamics. These molecular features have demonstrated considerable potential for early detection, prognostication, therapeutic monitoring, and post-treatment surveillance in head and neck cancer. However, current data are derived largely from exploratory studies with heterogeneous methodologies, small validation cohorts, and limited standardization of analytical workflows. Nevertheless, advances in sample processing, extraction methods, high-sensitivity sequencing technologies, and advanced bioinformatic approaches are enhancing assay accuracy and reproducibility. This review summarizes the current evidence on scfDNA in HNC by delineating its biological basis, evaluating the robustness and translational relevance of available clinical studies, and examining the analytical, technical, and implementation challenges that must be addressed to facilitate its integration into evidence-based clinical practice.