Pre-treatment circulating tumour DNA (ctDNA) to predict survival and aid decision-making in head and neck cancer patients: 5-year outcome analysis.

e18017 Background: HPV-associated (HPV+) head and neck squamous cell carcinoma (HNSCC) has a favourable prognosis compared to HPV-independent disease (HPV-), forming the basis for de-escalation trials. However, 25% of HPV+ patients experience recurrence, with no biomarker to identify this high-risk subset. In Stage IV disease (HPV+ and HPV-), deciding which patients should have radical treatment remains challenging, particularly in borderline curative cases where morbidity is balanced against chance of cure. This study investigated whether pre-treatment somatic variants could identify high-risk patients within favourable subgroups, and predict suitability for curative treatment in advanced stage disease. Methods: This prospective observational study recruited 41 HNSCC patients undergoing curative surgery or chemoradiotherapy. HPV status was determined by immunohistochemistry/in situ hybridisation. Blood samples were taken pre-treatment for ctDNA extraction. Somatic mutations were detected using a next-generation sequencing panel targeting 17 genes. Libraries were sequenced on Illumina MiSeq or NextSeq platforms. Pre-treatment somatic variants were profiled using ClinVar/COSMIC databases and classified as benign, variants of unknown significance or pathogenic. Kaplan-Meier survival analysis and Cox regression were performed using Python 3.13 with univariate analysis to identify variables associated with disease-free survival (DFS) at 5 years. Log rank tests compared survival and Firth’s penalisation was used in complete separation. Results: Thirty-five patients remained in the study, and 25 were HPV+. Five-year DFS was 66% with disease-specific survival of 77%. Thirty-one somatic variants were identified across 13 genes. HPV- patients had worse DFS vs HPV+ patients (HR 3.9, 95CI:1.3-11.2, p=0.0124). Within HPV+ subgroup, those with pre-treatment circulating variants had a worse 5-year DFS compared to HPV+ with no variants (HR 9.7, 95CI:1-80), p=0.04). Within HPV-, pre-treatment pathogenic variants resulted in worse DFS compared to HPV- and non-pathogenic variants (HR 8.2, 95CI:0.95-70, p=0.055). Notably, TP53 variants were associated with relapse ≤28months. Stage IV disease had worse DFS vs Stage I-III (HR 5.3, 95CI:2-15, p=0.002). Furthermore, Stage IV with a circulating variant had worse DFS than Stage IV with no variants (log rank p=0.028). Stage I-III with circulating variants had worse DFS than those without (HR 10.2, 95CI:1-85, p=0.032). Conclusions: Pre-treatment somatic variants are a potential prognostication tool and with further validation, could be used in decision-making. Within HPV+ and disease-stage subgroups, they identified patients with worse DFS who may be suitable for additional monitoring, treatment escalation or palliation. Funding: ORACLE Cancer Trust, Oxford NIHR Biomedical Research Centre.