The efficacy of iodine-123-MIBG as a screening test for pheochromocytoma

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.

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.

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.

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.