PRF1 and neoplasm: —Cytotoxic CD8-positive T-cells play an essential role in antitumor immune responses through the release of effector molecules including cytokines, perforin, and granzyme B.22 In addition, increased CD8-positive T-cell infiltration has been associated with positive treatment responses and improved prognosis in a series of preclinical studies.23,24 So far, the first data indicate the usefulness of CD8-targeting immuno-PET for tracking immune responses in several tumor entities.25–27