Blood and saliva-derived ctDNA is a marker of residual disease after treatment and correlates with recurrence in human papillomavirus-associated head and neck cancer

Background There is an alarming increase in human papillomavirus-associated head and neck cancer (HNC), reaching epidemic levels. While patient prognosis is generally good, off-target treatment effects are associated with decreased quality of life. Thus, non-invasive strategies to predict treatment response and risk of recurrence could help de-escalate treatment. In this study, we tested circulating tumor (ct)DNA in liquid biopsies (blood/saliva) of HPV-positive HNC patients to assess treatment response and disease progression.Methods A total of 235 blood and saliva samples were collected from 60 HPV-positive and 17 HPV-negative HNC patients (control group) before and/or after treatment. Samples were analyzed using ddPCR for HPV16/18/31/33/35/45 and correlated with imaging and pathological examination.Results HPV-ctDNA detection was significantly higher prior to treatment (91%) than after treatment (8.0%) (?(2) p < 0.00001), with high concordance between saliva and blood (93%). In matched samples, all patients positive for ctDNA before treatment showed significant reductions in ctDNA levels post treatment (p < 0.0001). All but one patient with persistent ctDNA after treatment showed residual tumor and subsequent recurrence. Finally, fragmentomic analysis revealed shifts in cell-free DNA fragment size after treatment, suggesting a complementary biomarker for treatment response.Conclusions Blood and saliva were found to be good sources of HPV-ctDNA. The presence of ctDNA strongly correlated with treatment response, demonstrating clinical utility as a non-invasive biomarker to monitor tumor progression in HPV-positive HNC. Liquid biopsy based ctDNA testing could be an effective approach to predict recurrence and stratify patients for de-escalation of treatment, thereby improving quality of life.