Studies at the University of Michigan have shown that 131I-metaiodobenzylguanidine (131I-MIBG) is an effective agent for the diagnosis and localization of pheochromocytomas and paragangliomas. We conducted a study that confirmed and expanded that finding. From January 1983 to March 1984, 48 patients at our institution had 51 131I-MIBG scans during the workup of suspected sporadic or metastatic pheochromocytoma. Scintigrams were obtained after 500 μCi of 131I-MIBG had been administered intravenously. The final diagnosis (true-positive, false-negative, or false-positive result) was made at operation and pathologic examination. A true-negative diagnosis was confirmed by normal plasma and fractionated urinary levels of catecholamines and metabolites and, in most patients, computed tomography (CT). There were 20 true-positive studies (6 pheochromocytomas, 4 paragangliomas, and 10 metastatic or recurrent pheochromocytomas) and 24 true-negative studies. One patient with a suspected recurrent paraganglioma near the bladder had a false-positive 131I-MIBG scan (and also a false-positive CT). Among six patients with false-negative scintigrams (three pheochromocytomas, one paraganglioma, and two metastatic lesions), one also had a false-negative CT. The overall sensitivity of 131I-MIBG scanning was 77%, specificity was 96%, and accuracy was 86%. This test is fairly sensitive in the workup of patients with known or suspected recurrent or metastatic pheochromocytoma. It may also be helpful in the evaluation of suspected sporadic pheochromocytoma when CT findings are normal.
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