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History: Woman with normal CNS workup has unexplained headaches and sweating

Which choice best characterizes the salient finding, if one if visible?

A Subacute aortic dissection.

B Mesenteric mass.

C Pancreatic mass.

D Adrenal mass.

E Images are within normal limits.

Which ONE of the following does NOT take up MIBG?

a. Neuroblastoma.

b. Carcinoid.

c. Medullary thyroid carcinoma.

d. Paraganglioma.

e. All of the above take up MIBG.

Images from a I123 MIBG scan are shown. Which choice is the MOST LIKELY diagnosis?

a. Adrenal adenoma

b. Adrenal carcinoma

c. Pheochromocytoma.

d. Neuroblastoma.

e. Myelolipoma.

Which ONE of the following does NOT take up MIBG?

a. Neuroblastoma.

b. Carcinoid.

c. Medullary thyroid carcinoma.

d. Paraganglioma.

e. All of the above take up MIBG.

Findings: CT Abdomen shows right adrenal nodule with HU less than 20 on non contrast CT. Nuclear medicine I123 MIBG (24 hour) scan with SPECT demonstrates focal uptake in the right adrenal gland.

Additional clinical history: Patient was also noted to be hypertensive

Differential Diagnosis

• MIBG Uptake
• Pheochromocytoma
• Neuroblastoma
• Carcinoid
• Medullary thyroid carcinoma
• Paraganglioma
• Adrenal MIBG Uptake
• Pheochromocytoma
• Neuroblastoma
• Adrenal Nodule
• Adenoma
• Adrenal carcinoma
• Metastasis
• Hematoma
• Myelolipoma

Discussion

Pheochromocytoma is a catecholamine secreting tumor which may arise in the adrenal gland (90%) or outside the adrenal gland known as paraganglioma. Pheochromocytoma accounts for 0.05-0.2% of hypertension. Typically, symptoms are intermittent. Pheochromocytoma may occur with MEN 2a, 2b, neurofibromatosis, and Von Hippel Lindau. Laboratory tests include plasma metanephrine (96% sensitive, 85% specific) and 24 hour urine catecholamine and metanephrine (87.5% sensitive, 99.7% specific). Imaging studies include CT, MRI, and nuclear medicine. Treatment includes medication alpha and beta receptor blockade and surgical resection.

Radiologic Overview:

CT or MRI may detect adrenal or extra adrenal nodules but are nonspecific for pheochromocytoma. Nuclear medicine imaging can be considered for extra adrenal pheochromocytoma detection or small adrenal nodules.

I-123 metaiodobenzylguanidine, MIBG, is used for pheochromocytoma and neuroblastoma imaging. It is a precursor of norepinephrine and is taken up selectively by the adrenal medulla, the sympathetic autonomic nervous system, and tumors derived from these tissues. Other tumors, such as carcinoid carcinoma and medullary thyroid carcinoma may be visualized, but with relatively low sensitivity; octreotide imaging may be superior for these tumors. In111 octreotide is less sensitive for pheochromocytoma detection.

There is expected normal activity in the spleen, heart, salivary glands and liver. Urinary bladder activity can sometimes be seen due to free radioiodine.

MIBG can be labeled with either I-131 or I-123. Although it is more expensive, I-123 MIBG is the preferred radiopharmaceutical because of much better image quality. Radiation dose to the thyroid gland should be minimized by blocking of thyroidal I-131 uptake with potassium iodide or Lugol's solution. Technetium pertechnetate may be used to block thyroid uptake if using I123 MIBG.

Many drugs interfere with uptake of MIBG, particularly tricyclic antidepressants, sympathomimetics (e.g., pseudoephedrine), and certain antihypertensives (labetalol, reserpine, and calcium channel blockers). An important indicator of an acceptable MIBG uptake is the presence of cardiac activity.