The efficacy of iodine-123-MIBG as a screening test for pheochromocytoma.
UNLABELLED: The primary objective of this study was to characterize the effectiveness of 123I-metaiodobenzylguanidine (MIBG) as a screening test for pheochromocytoma in routine clinical practice. An attempt was made to determine why some tumors and some adrenal glands without pheochromocytoma minimally manifest increased uptake. METHODS: Planar images were obtained with a standardized protocol in a diverse group of patients. The intensity of uptake in each adrenal gland was graded on a 0-3-point scale by using the intensity of activity in the liver as a reference. Follow-up data were obtained from both the patients and the referring physicians. A final diagnosis was eventually established in 120 patients who had a total of 238 adrenal glands. RESULTS: There was an intramedullary pheochromocytoma in 24 of the 238 adrenal glands (10.1%). The uptake was very intense (Grade 3) in 21 of them (87.5%). The uptake was only mildly to moderately increased in the other three intra-adrenal tumors. There was no visible uptake in 148 of the 214 (69.2%) adrenals without a pheochromocytoma, but there was mildly to moderately increased activity in 66 (30.8%). There were no other features of the clinical data base which could differentiate between mildly increased uptake in a pheochromocytoma and mildly increased activity in a gland without a tumor, including the 24-hr urinary excretion of catecholamines. CONCLUSIONS: Since every intra- and extra-adrenal tumor was visualized, the findings suggest that 123I-MIBG may be the most sensitive screening test available for diagnosing pheochromocytoma. The test results should be definitive in most patients.