This study aims to evaluate the role of early response assessment by 18F-FDG PET/CT in patients with recurrent and/or metastatic (R/M) head and neck squamous cell carcinoma with PD-L1 CPS >1 treated with pembrolizumab monotherapy, in accordance with routine clinical practice.

Eligible patients will receive two administrations of Pembrolizumab as monotherapy.

An 18F-FDG PET/CT scan will be performed at baseline (T0; no more than 30 days before the first drug administration) and after two cycles (T1, at 6 weeks).

Patients will be considered responders if they achieve a complete response (defined as complete resolution of FDG uptake within the target lesions) or a partial response (i.e., >=30% decrease in the target tumor FDG SULpeak).

After the first two administrations of Pembrolizumab, as well as in case of progressive disease (i.e., >=30% increase in the target tumor FDG SULpeak or advent of new 18FDG-avid lesions), any treatment modification will be determined by the physician according to routine clinical practice

Inclusion Criteria:

• Histological diagnosis of recurrent and/or metastatic HNSCC, not amenable for curative locoregional treatment (either surgery or radiotherapy), and amenable for first line systemic treatment
• CPS PD-L1 >=1

Exclusion Criteria:

• Any controindication to immune checkpoint inhibitors

recurrent and/or metastatic HNSCC candidate for ICI-based first-line treatment