The limitations of the dual radionuclide subtraction technique for the external detection of tumours by radioiodine-labelled antibodies.

A dual radionuclide subtraction technique for external detection of tumours has been evaluated to determine the viability of the method for use with radioisotope labelled antibodies. A number of external scintigraphic investigations have been carried out with 131I-labelled antibodies to carcinoembryonic antigen (CEA). The investigations were performed on patients with metastatic disease known to produce CEA. The dual radionuclide subtraction technique was used to account for the blood and tissue background. The 131I-labelled antibodies were found to localise in the metastatic lesions, but the subtraction technique using 99Tcm-labelled HSA and pertechnetate gave ambiguous results, which included the production of artefacts. The ambiguities noted in the clinical results were substantiated by experimental data, which highlight the unreliability of this technique.