A radioimmunoconjugate composed of MPDL3280A, a human, Fc-optimized, monoclonal antibody directed against programmed cell death-1 ligand 1 (PD-L1) and labeled with the radioisotope zirconium Zr 89, with potential use for assessing PD-L1-expressing tumor cells using positron emission tomography (PET). Upon administration of zirconium Zr 89-labeled anti-PD-L1 monoclonal antibody MPDL3280A, the antibody moiety targets and binds to PD-L1 and is internalized. Following tumor cell uptake, the radioisotope moiety can be visualized using PET. This may result in both the quantification of PD-L1-expressing tumor cells and an assessment of the expected response to treatment with MPDL3280A as well as selection of patients that would respond to MPDL3280A. PD-L1 is overexpressed on many human cancer cell types; PD-L1 binding to programmed cell death 1 (PD-1) on T-cells suppresses the immune system and results in increased immune evasion.