Study to Evaluate Immunological Response to PD-1 Inhibition in Squamous Cell Carcinoma of the Head and Neck (SCCHN)

"* Baseline: Blood sampling, tumor biopsy, \[18F\]F-AraG PET/CT scan within two weeks prior to standard of care anti-PD-1 therapeutic dose.~* Anti PD-1 per standard of care~* Blood sampling and tumor biopsy within 2-3 weeks after first anti-PD-1 SOC dose.~* \[18F\]F-AraG PET/CT scan 6 - 12 weeks post first anti-PD-1 dose."

"* Baseline: Blood sampling, tumor biopsy, \[18F\]F-AraG PET/CT scan within two weeks prior to treatment.~* Anti PD-1, single dose~* Blood sampling, tumor biopsy and \[18F}F-AraG PET/CT scan within 2-3 weeks after single dose of anti-PD-1 treatment. For patients having surgical resection, biopsy will be immediately prior to resection or from sample of resection."