IMAGE SUBTRACTION ANALYSIS WITH TC-99M LABELED MONOCLONAL-ANTIBODY AND COLLOID FOR EVALUATION OF LIVER-LESIONS - PHANTOM MEASUREMENTS AND PATIENT STUDIES

The ideal radiolabeled monoclonal antibodies (MoAbs) against cancer an tigens are taken up by liver metastases; the background activity of no rmal liver, however, causes problems for delineation and detectability . In order to study these phenomena, a liver phantom containing hot an d cold lesions of different sizes (diameters 5-32 mm) was constructed. It was placed into an elliptic cylindrical container representing a c ross section of the abdomen. The specific activities in hot lesions va ried from 1.85 to 14.8 MBq/ml, whereas liver phantom and cylinder acti vities were kept constant during different measurements. Lesions of si ze 1.3 cm(3) could be detected without any subtractions, if the signal to background ratio was larger than 1.2. Lesions larger than 5 mm in diameter could also be detected using subtraction, which gave addition al information by a factor 2-9, when the lesion sizes varied from 0.3 to 5.3 cm(3) and when the specific activity in the lesions was at leas t twice as high as in adjacent liver. This subtraction technique was a pplied in 32 breast and lung cancer patients after injecting about 1 0 00 MBq Tc-99m-labeled anti-CEA MoAb; 24 h after the antibody injection 75 MBq Tc-99m-phytate was injected. The phytate + residual MoAb image was subtracted from the original antibody image. Thirteen patients ha d liver metastases verified by (CT, US), but only four patients had cl early observable abnormal liver uptakes in planar MoAb images. In 9 ca ses, additional information concerning liver metastases was obtained b y subtraction technique. To judge by our phantom measurements the enha nced detectability was not an artefact.