Extraadrenal Pheochromocytoma: I-123 MIBG

Planar image (anterior view) of an I-123 MIBG scan shows intense uptake in the midline of the lower abdomen in a patient with elevated urine metanephrine.

Facts: Pheochromocytoma

• Uncommon tumor arising from pheochromocytes of adrenal medulla, paraganglia near aorta or sympathetic ganglia
• Common in 4th-6th decades of life, men = women
• 10's rule: 10% bilateral, 10% malignant, 10% children, 10% extraadrenal
• Imaging used to localize tumor after laboratory confirmation of elevated urine metanephrine or resting plasma cathecholamine
• I-123 MIBG and/or MRI commonly utilized for localization

I-123 MIBG (meta-iodobenzylguanidine)

• Patient preparation include: suspend all medications that could interfere with MIBG uptake for 5-6 days (i.e., calcium blocker, sympathomimetics, reserpine), use potassium iodine or sodium perchlorate orally 1 day before scanning and continue for 7 days to block thyroid uptake of unbound iodine
• Scan done with planar imaging at 4 and 24 hours after tracer administration (300-370 MBq of I-123 MIBG)
• Diagnosis of pheochromocytoma when 1) adrenal uptake more intense than liver, 2) extra-adrenal focal uptake seen

Imaging Workup Recommendation

Based on a study of 32 patients who underwent CT, MR and MIBG scans (each patient had all three imaging performed for research),

• MRI alone had 93% sensitivity and positive predictive value (PPV)
• MIBG alone had 90% sensitivity and 100% PPV
• MRI + MIBG had 100% sensitivity and PPV

Reference:

Lumachi F, Tregnaghi A, Zucchetta P, et al. Sensitivity and positive predictive value of CT, MRI and I-123 MIBG scintigraphy in localizing pheochromocytomas: a prospective study. Nucl Med Commun 2006;27:583-587.