Obstructive hydronephrosis with I-131 MIBG accumulation mimicking huge pheochromocytoma

A case of severe obstructive hydronephrosis, which caused a false- positive I-131 MIBG scan for detection of a pheochromocytoma is reported. A huge area of persistent I-131 MIBG accumulation 72 hours after injection in the left flank region was suspected to be a huge pheochromocytoma. Combined studies with I-131 MIBG, Tc-99m MDP, Tc-99m DMSA, and Tc-99m DTPA as well as furosemide injection disclosed that the abnormal I-131 MIBG accumulation was caused by left obstructive hydronephrosis. The result was finally confirmed by intravenous pyelography and CT studies.