A small pilot study in stage IV melanoma patients treated with pembrolizumab (anti-PD-1) suggests FLT-PET-based response assessment in week 6 accurately predicts RECIST-based response in week 12, but its utility as a biomarker to separate pseudo- from truly progressive disease in ICB for HNSCC is unknown. Here, PDCD1 is linked to head and neck squamous cell carcinoma.