In-111 monoclonal antibody versus Ga-67 citrate and Tc-99m SC subtraction in a patient with malignant melanoma

Gallium-67 is routinely used for follow-up of patients with malignant melanoma. However, its nonspecificity for melanoma and its high rate of false-positive results have always been a matter of concern. The authors describe a patient who encountered serious problems with the use of gallium. Because gallium is taken up well by the liver and by melanoma, results of gallium scintigraphy of the liver may appear normal even if there is metastatic disease. In this patient, results of gallium scintigraphy of the liver were negative for metastasis but revealed extrahepatic foci detected by the monoclonal antibody. Computed tomography showed areas of attenuation, revealing only a few intrahepatic tumors and no extrahepatic disease. Tc-99m SC revealed intrahepatic metastases, but no extra-hepatic metastases were seen. A monoclonal antibody (ZME-018) scintigram did reveal hepatic metastases along with probable small, extrahepatic, metastatic foci. Overall hepatic uptake of the monoclonal antibody was relatively low. An image subtraction algorithm was devised whereby the sulfur colloid image was subtracted from the gallium scintigram. The resultant image revealed both the intrahepatic and extrahepatic metastases seen on the ZME-018 images. It is likely that in the past many hepatic metastases have been missed because Tc-99m SC images have not been routinely used as part of melanoma management protocols. The uptake of the ZME-018 by the tumor was significantly higher than that of the normal liver, suggesting that ZME-018 labeled with the appropriate emitter may be an effective specific therapeutic tool in selected patients.