FAP and cancer: FAP may have immunosuppressive effects in the tumor microenvironment (TME) [8, 10–13], which could be enhanced by the upregulation of programmed death-ligand 1 (PD-L1) expression on cancer cells in response to radiation [14] or exposure to interferon-g (IFNγ) secreted by T cells [15], which can suppress T cell activation and as a consequence attenuate RLT efficacy.