In naïve mice with B16-F10 melanoma tumors that received PD-L1 and Cytotoxic T lymphocyte-associated protein 4 (CTLA-4 or CD152) directed ICI therapy, high tracer uptake was observed in the tumor upon irradiation, suggesting that PD-1 PET can be used for ICI treatment monitoring. Here, PDCD1 is linked to melanoma.