Image subtraction analysis with technetium-99M labeled monoclonal antibody and colloid for evaluation of liver lesions. Phantom measurements and patient studies.

The ideal radiolabeled monoclonal antibodies (MoAbs) against cancer antigens are taken up by liver metastases; the background activity of normal liver, however, causes problems for delineation and detectability. In order to study these phenomena, a liver phantom containing hot and cold lesions of different sizes (diameters 5-32 mm) was constructed. It was placed into an elliptic cylindrical container representing a cross section of the abdomen. The specific activities in hot lesions varied from 1.85 to 14.8 MBq/ml, whereas liver phantom and cylinder activities were kept constant during different measurements. Lesions of size 1.3 cm3 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 additional information by a factor 2-9, when the lesion sizes varied from 0.3 to 5.3 cm3 and when the specific activity in the lesions was at least twice as high as in adjacent liver. This subtraction technique was applied in 32 breast and lung cancer patients after injecting about 1,000 MBq 99mTc-labeled anti-CEA MoAb; 24 h after the antibody injection 75 MBq 99mTc-phytate was injected. The phytate + residual MoAb image was subtracted from the original antibody image. Thirteen patients had liver metastases verified by (CT, US), but only four patients had clearly observable abnormal liver uptakes in planar MoAb images. In 9 cases, additional information concerning liver metastases was obtained by subtraction technique. To judge by our phantom measurements the enhanced detectability was not an artefact.