The goal of this observational longitudinal study is to learn about circulating tumor Human Papilloma Virus-DNA (ctHPV-DNA) as a biomarker for HPV positive oropharyngeal cancer and cancer of unknown primary of the head and neck. The main questions it aims to answer are:

• Can ctHPV-DNA be used for treatment evaluation in HPV positive oropharyngeal cancer and cancer of unknown primary of the head and neck?
• Can circulating HPV-DNA be used as a biomarker for recurrent disease during surveillance?

Participants will be asked to leave plasma samples at diagnose, at the end of treatment and at every clinical follow-up.

The patients are there own controls.

In the multicenter study of CIRCOS, Circulating biomarkers in oropharyngeal cancer, patients with oropharyngeal cancer or cancer of unknown primary of the head and neck are consecutively included. Plasma samples are collected at diagnosis, at the end of treatment and during surveillance after treatment.

At diagnose participants will fill in informed consent and a form regarding known risk factors for cancer. Tissue from the tumor will be analyzed for HPV genotype with a multiplex q-PCR. Information about p16 will be collected from medical records.

ctHPV-DNA are short DNA fragments that leaks into the blood stream from tumor cells during apoptosis and necrosis. In the study, ctHPV-DNA will be extracted from blood plasma. Levels of ctHPV-DNA (copies/mL) will be measured using digital droplet PCR (ddPCR) with genotype specific assays (based on the result of q-PCR at diagnose) used in singleplex (SAGA diagnostics). A negative sample after treatment will be defined as a good molecular response for evaluation after treatment. Two consecutive, positive samples during surveillance will be defined as molecular recurrence. If a molecular recurrence is seen patients will be contacted and offered an extra clinical control at an Ear nose and throat department.

If a patient is HPV negative in tissue, the tissue will be analyzed with whole genome sequencing. If a mutation is found, a personalized ddPCR-kit will be used for plasma.

All patients will be followed for five years.

Inclusion Criteria:

• Oropharyngeal cancer
• Cancer of unknown primary in the head and neck

Exclusion Criteria:

• Previous treatment of cancer in the oropharynx.
• Previous treatment of unknown primary tumor.
• Remote metastases
• Patients unwilling or unable to comply with the study protocol and follow-up schedule

Study participants are recruited from multidisciplinary conferences of head and neck cancer at Örebro University hospital - Region Örebro County, Västmanland's hospital - Region Västmanland and Karolinska University hospital - Region Stockholm. Örebro University hospital is a tertiary referral hospital of three regions in central Sweden (Region Värmland, Region Sörmland and Region Dalarna) and therefore patients from these regions will also be included in this study. All patients with OPSCC at any of the study centers will be asked to participate in CIRCOS. All study participants will sign an informed written consent upon inclusion