CD274 and neoplasm: Concerning PET, one approach to potentially overcome this problem is the development of novel PET radiotracers specifically targeting the cytotoxic CD8+ T cells [17] or the PD-1 or PD-L1 pathways [34,35] in order to assess the T-cell content and the PD-1 and/or PD-L1 expression in tumor lesions, respectively.