MIBG Scan

INDICATION

The MIBG scan is a useful, qualitative, method of locating the site of a phaeochromocytoma.
It should not be undertaken without biochemical evidence for a tumour being present (24 hour urine VMA, circulating catecholamines, clonidine or pentolinium test), and should be backed up by ultrasound, CT scanning, and, where indicated, venous sampling. It is particularly useful for extra-adrenal and metastatic or residual phaeochromocytoma.

CONTRAINDICATIONS

No absolute contraindications except pregnancy or its possibility, allergy to iodine.
Caution in any patient with any drug allergies.
Many drugs may interfere with the study - nuclear scanning have a list. These include tricyclic antidepressants, SSRIs, calcium channel antagonists, catecholamine receptor agonists and antagonists, phenothiazines, butyrophenones (e.g. haloperidol etc.), guanethidine and reserpine.

SIDE EFFECTS

None.

PREPARATION

Liase with nuclear medicine at least 1 week in advance of planned scan.
Avoid IV phenoxybenzamine, although p.o. phenoxybenzamine is OK.
Thyroid uptake should be blocked by potassium iodide 60 mg bd for 48 hours beforehand and for 5 days afterwards.

METHOD

Metaiodobenzylguanidine is injected intravenously (this molecule is similar to noradrenaline, transported in similar fashion and stored in catecholamine vesicles). The patient is scanned twice at 24 and 48 hours.

INTERPRETATION

Spots of increased uptake on scanning are the tumours. Most common sites: adrenals, organ of Zuckerkandl (usually pelvis). May be multiple.

SENSITIVITY AND SPECIFICITY

In patients with a proven phaeochromocytoma but uncertain site the scan has 90-96% sensitivity and 98-99% specificity (Shapiro et al., Cardiology 1985; 72: suppl. 1, 137-142).

REFERENCES

Sisson J.C. et al., New Engl. J. Med. 305: 12-17 (1981).