Reporting a Case of False-positive Radio-iodinated Metaiodobenzylguanidine (123I-MIBG) Uptake in Undifferentiated Adrenal Malignant Tumor

123I-Metaiodobenzylguanidine (MIBG) scintigraphy is a widely used functional imaging tool with a high degree of sensitivity and specificity in diagnosis of pheochromocytoma. However, rare cases of false positive reactions have been reported. A 67-year-old male patient was admitted with epigastric pain. Abdominal computed tomography (CT) revealed a heterogeneous left adrenal mass 6 cm in diameter; following hormone testing, 123I-MIBG scintigraphy was performed to determine the presence of pheochromocytoma, which confirmed eccentric uptake by a large left adrenal gland mass. Chest CT and PET-CT confirmed metastatic lymphadenopathy; therefore, endobronchial ultrasound transbronchial needle aspiration was performed. Metastatic carcinoma of unknown origin was suspected from a lymph node biopsy and surgical resection was performed for definitive diagnosis and correction of excess hormonal secretion. A final diagnosis of undifferentiated adrenal malignant tumor was rendered, instead of histologically malignant pheochromocytoma, despite the uptake of MIBG demonstrated by scintigraphy. When the degree of MIBG uptake is low-grade in relationship to a large tumor mass, then a false-positive result should be entertained, particularly in the absence of typical clinical and biochemical features of pheochromocytoma.