MIBG Scan
The MIBG scan uses meta iodobenzylguanidine, an analogue of noradrenaline, labelled with iodine-123 (or iodine-131). It is taken up by sympathetic nerve endings and by tumours of neural crest origin through the noradrenaline transporter, and stored in vesicles.
The scan finds neuroblastoma, phaeochromocytoma and paraganglioma, and measures sympathetic nerve function in the heart.
The adult dose of iodine-123 MIBG is 200 to 400 MBq intravenously, injected slowly over a few minutes. Children receive a weight based dose with a minimum of about 80 MBq.
Iodine-131 MIBG (20 to 40 MBq) is used for diagnosis only when iodine-123 is not available, and in higher doses for treatment.
The thyroid is protected with potassium iodide or perchlorate, starting the day before and continuing for 1 to 2 days. Whole body planar images and SPECT-CT are taken at 24 hours (also at 4 and 48 hours if needed).
The effective dose is about 3 to 5 mSv for iodine-123 and much higher for iodine-131.
Neuroblastoma is the commonest extracranial solid tumour in children, with MIBG avid disease in over 90%. Phaeochromocytoma and paraganglioma are rare (about 1 to 2 in 100 000 each year), and are important because of hypertension and hereditary syndromes.
MIBG has a sensitivity of 85 to 90% and a specificity of 95 to 100% for phaeochromocytoma, and a sensitivity of 85 to 95% in neuroblastoma.
Indications.
- Neuroblastoma: staging, response to treatment, residual disease, and selection for iodine-131 MIBG treatment.
- Phaeochromocytoma and paraganglioma: confirmation, localisation, search for multiple and metastatic tumours, and selection for treatment.
- Hereditary syndromes (multiple endocrine neoplasia, neurofibromatosis, von Hippel-Lindau disease) for surveillance.
- Carcinoid and medullary thyroid cancer when other imaging is negative (limited role).
- Heart failure: cardiac sympathetic innervation for prognosis and risk of arrhythmia.
- Parkinsonian syndromes and dementia: low cardiac uptake supports Parkinson disease and dementia with Lewy bodies, while normal uptake favours multiple system atrophy and progressive supranuclear palsy.
- Evaluation of cardiac denervation after transplant and in diabetic autonomic neuropathy.
Contraindications and precautions.
- Pregnancy: contraindicated. Breast feeding: interrupt for several days or stop, because of iodine excretion in milk.
- Thyroid blocking is essential, and allergy to iodine must be checked.
- Many drugs block uptake, and they are stopped before the scan if it is safe: labetalol, tricyclic antidepressants, sympathomimetics (decongestants such as pseudoephedrine and phenylephrine), reserpine, some calcium channel blockers, cocaine, opioids such as tramadol, and some antipsychotics. The interval is at least 5 half lives, usually 1 to 2 weeks, and the local list is used.
- Alpha and beta blockade for phaeochromocytoma is not stopped without medical advice. The drug list is reviewed with the clinician.
- Renal failure lowers clearance and increases background.
- Children need sedation only rarely.
- A bladder catheter may be used in small children to reduce the dose and the activity.
- Radiation safety advice is given for several days after iodine-131.
Preparation and technique.
The patient takes thyroid blocking for the stated period. The tracer is given over 1 to 5 minutes, with monitoring of blood pressure and heart rate in suspected phaeochromocytoma, as rapid injection can cause a hypertensive reaction.
Images are taken at 24 hours with a gamma camera and a medium energy collimator, with whole body views and SPECT-CT of the chest, abdomen and pelvis. Delayed images at 48 hours help to separate tumour from bowel activity.
For cardiac imaging, planar chest images are taken at 15 to 20 minutes (early) and at 3 to 4 hours (late).
The heart-to-mediastinum ratio is measured with regions on the heart and upper mediastinum, and the washout rate between early and late images is calculated. Reference values are specific to the camera, collimator and software.
