A radioimmunoconjugate composed of a nanobody directed against programmed cell death-1 ligand 1 (PD-L1; cluster of differentiation 274; CD274) and labeled with the beta-emitting radioisotope lutetium Lu 177, with potential imaging activity of PD-L1 tumor antigen expression during single-photon emission computerized tomography/computerized tomography (SPECT/CT) and with potential antineoplastic activity. Upon administration of lutetium Lu 177 anti-PD-L1 nanobody RAD204, the RAD204 moiety targets and binds to the PD-L1 expressed on certain tumor cells. Upon uptake and imaging, the radioisotope moiety can be visualized and the extent of PD-L1 expression can be assessed. Also, this radioconjugate specifically delivers a cytotoxic dose of beta radiation to PD-L1-expressing tumor cells. PD-L1, a transmembrane protein, is overexpressed on the surface of many cancer cell types.