Is MIBG scan still needed?

MIBG is a chemical that has similar structure to that of catecholamines. Radioiodine-labeled MIBG is taken up by both normal adrenal medulla and pheo. Because pheo is usually much larger than the adrenal medulla, if one exists, it tends to light up with MIBG much more than normal adrenal medulla.

When MIBG scan began to be used clinically in the 1980s, CT and MRI were not yet available in clinical care. Initial clinical studies indeed showed that the MIBG scan was a great test, boasting a sensitivity of 90% (meaning that if there is a pheo, the chance that it will be shown is 90%) and a specificity of 95-100% (meaning that if you don’t have a pheo, the chance that no signal will be shown is 95-100%). The very high specificity was especially appealing to those clinicians who had little experience on pheo. Pheo is a disease nobody wants to miss. So, the reasoning goes, if MIBG scan result is negative, pheo can be ruled out.

Since the 1980s, better biochemical tests have become available and CT and MRI household names. Of course the new tests and CT and MRI have their own problems but every patient suspected to have pheo get those. Should the patient also get an MIBG scan? Will the MIB scan provide new information on whether the patient has pheo in the first place, or if she/he has pheo, how many and where they are?

In this month, two papers with opposite views appear respectively in Journal of Clinical Endocrinology and Metabolism and Endocrine Practice, the former being the world’s leading endocrinology journal, the latter a smaller journal with a readership of mostly American practicing endocrinologists. The first paper is an analysis of 15 clinical studies on MIBG using iodine 123. The conclusion is that MIBG scan has a sensitivity of 94% and specificity of 92%. The authors of this paper believe that their paper “supports the continued utility of I-123 MIBG imaging in the diagnosis and management of patients with pheochromocytoma.”

The second paper is a retrospective clinical study of MIBG scan (both I-131 and I-123) in a large US hospital in the last 14 years. There are quite a few interesting findings in this paper. First, MIBG scan is not really indicated in 76% of patients (in other words, three quarters of modern-day MIBG scans are wasted). Second, the sensitivity and specificity are both only about 70%. Lastly, the most important finding is that the MIBG scan does not provide any new information after biochemical testing and CT or MRI. Even worse, about 35% of the MIBG scans provide misleading information (i.e. showing adrenal uptake but patient does have pheo or no uptake but patient has pheo). The authors of this paper conclude that MIBG scan “is not necessary for most patients in modern practice.”

Why do these two papers have opposite conclusions on the MIBG scan? Like everything else, the devil is in the details. The 15 studies analyzed in the first paper are all well-controlled clinical studies performed by expert clinicians. They use very stringent and clearly defined criteria to call the MIBG scan result positive or negative. As matter of fact, borderline adrenal uptake is considered a negative result. In the second paper, the MIBG scan is ordered by clinicians with various experience on pheo. Borderline adrenal uptake is considered positive result. The authors’ rationale is that most clinicians and patients treat any adrenal uptake as alarming, which I do agree with. If borderline uptake is considered negative, the sensitivity and specificity increase to about 90%. Even that, the MIBG scan still does not provide useful new information. The first paper unfortunately does not address the utility of MIBG scan after biochemical testing and CT or MRI. Besides the second paper, a couple of other papers in the last few years also fail to show that MIBG scan has any clinical utility.

What do I do? My own experience is that MIBG scan seldom helps. I do order it in the following patients who I already diagnose as having pheo: young patients (<50-year-old), patients with family history or mutations, patients with previous pheo, and patients with multiple pheo, patients who have adrenal masses on both sides, and patients in whom CT/MRI does not find the pheo. Those patients are rare, though.

Dr. Pheo