Antibodies to cell surface ligands such as EGFRvIII can be used to therapeutically or diagnostically deliver a radiolabel to tumor cells with high selectivity. The utility of radioconjugated internalizing antibodies is limited, however, by release of the label and its reuptake into normal cells. The invention provides new technology for radioconjugation of internalizing antibodies which reduces the release of label and improves retention of radioconjugated antibodies in lysosomes, where radiation is more effectively and selectively delivered to the nucleus of tumor cells.