Oropharyngeal squamous cell carcinoma (OPSCC) is an increasingly prevalent cancer type in Scotland, with an annual incidence rate of 23.4 per 100,000 of population in 2017 compared to 17.3 per 100,000 in 1993 \[1\]. This is due in part to an increased frequency of human papillomavirus (HPV) -related disease, which accounts for \>70% of cases of OPSCC, and to continuing high rates of smoking. The current standard of care in OPSCC rests on clinical assessment and cross-sectional imaging followed by biopsy with histopathological diagnosis via immunohistochemistry (IHC) and PCR testing of the tumour for HPV. Follow-up is by chemo-radiotherapy (CRT) in most cases, with interval imaging to gauge post-treatment response to therapy and salvage surgery as required \[2\]. HPV+ve OPSCC has a generally good prognosis after treatment with CRT (3-year survival 82%), whilst HPV-ve OPSCC has a much poorer prognosis (3-year survival 57%)\[3\]. Surgical access to post-treatment OPSCC for biopsy can be difficult and even with recent advances in functional imaging there are still significant numbers of patients, at first presentation and at relapse, with indeterminate results \[4\]. The poor prognosis of HPV-ve disease and the major differences in prognosis and management between HPV+ve and -ve disease underline the importance of accurate identification of HPV status in patients with OPSCC and emphasise the need for more reliable markers of residual or recurrent disease.

The analysis of circulating tumour-derived DNA (ctDNA) from patient blood - "liquid biopsy" - represents a minimally invasive approach to cancer diagnosis and management, with the potential to transform clinical care through identification in blood ctDNA of actionable tumour-derived mutations, detection of minimal residual disease and early detection of disease recurrence \[5\]. There has been significant interest in developing liquid biopsy approaches in HPV+ve OPSCC, with several studies including our own indicating that real-time monitoring of HPV levels in plasma cell-free DNA (cfDNA) can accurately determine HPV status, indicate completeness of response to treatment and provide evidence of tumour recurrence earlier than routine surveillance and before symptomatic presentation \[6-8\] (Thomson et al 2020, MedRxiv). In HPV-ve OPSCC, a particular problem in Scotland due to high rates of smoking, no such blood-based markers are available. As well as the opportunities to validate and translate HPV cfDNA as a biomarker for remission and relapse in HPV+ve OPSCC, there is therefore a need to develop new diagnostic assays to assist clinical decision-making steps in the management of HPV-ve disease.

The investigators aim to carry out liquid biopsy studies that will improve diagnostic accuracy and clinical management of Scottish OPSCC patients. In HPV+ve OPSCC, The investigators will expand our existing cohort and prospectively evaluate the clinical utility of cfDNA HPV analysis as a biomarker for routine care. In HPV-ve patients, The investigators will study the development and evolution of HPV-ve disease through combined genomic analyses of tumour DNA and circulating cfDNA, with the aim of identifying blood-based biomarkers and new targets for personalised therapy in this poor prognosis form of OPSCC. Understanding the molecular events surrounding OPSCC development and responsiveness to treatment, and identifying biomarkers for blood-based disease monitoring stand to have a significant impact on patient survival and quality of life of Scottish patients with OPSCC. The work proposed here - built upon a strong foundation of pilot data and collaboration between academia and the NHS - will directly assist clinical care and treatment efficacy for OPSCC patients in Scotland.