Comparing FDG PET/CT and FDG PET/MRI in the diagnostic accuracy of detecting local recurrence 12 weeks after the end of CRT in head and neck squamous cell carcinoma patients. Forty patients aged more than 18 years who have a histologically confirmed HNSCC and have received chemoradiation therapy will be recruited for the study. The patients will be scanned with both PET/CT and PET/MRI 12 weeks after the end of CRT.

Since recurrence of HNSCC occurs in up to 50 percent of cases in the first two years following treatments and the recurrent diseases are less responsive to new treatments, detecting local/regional recurrence at an earlier time is crucial in order to provide salvage treatments. The primary objective of this prospective study is to compare the diagnostic accuracy of FDG PET/CT to FDG PET/MRI in the detection of local recurrence 12 weeks after the end of chemoradiation treatment in HNSCC patients. The hypothesis is that PET/MRI might be able to diagnose locoregional recurrence earlier and more accurately which would lead to a better outcome for the patient. If it is proven correct, it would be possible to optimize the use of PET/MRI imaging on selected patients who are likely to benefit clearly from the use of that modality.

Forty patients aged more than 18 years who have a histologically confirmed HNSCC and have received chemoradiation therapy will be recruited for the study. The patients will be scanned with both FDG-PET/CT and FDG-PET/MRI 12 weeks after the end of CRT.

All study participants will be consented, and the estimated time for completing the PET scans of 40 patients will take 12 to 15 months. The results will be reported in international peer-reviewed high-impact journals.

Inclusion Criteria:

• Age 18 years or older
• Language spoken: Finnish or Swedish
• Diagnosis: Histologically confirmed HNSCC
• Chemoradiotherapy treatment with curative intent to the primary tumor and/or neck LN metastasis that has ended 12+- 4 weeks before the PET scan.
• Clinical stage: Stage I-II at least 20 patients, Stage III-IV at least 20 patients
• Mental status: Patients must be able to understand the meaning of the study
• Informed consent: The patient has to have signed the appropriate Ethical Committee (EC) approved informed consent documents in the presence of the designated staff

Exclusion Criteria:

• Uncontrolled serious infection
• Contraindications for MRI (cardiac pacemaker, intracranial clips etc., allergic reaction to contrast agent)
• Severe claustrophobia