What Does An MIBG Scan Show?
An MIBG scan is an imaging technique in nuclear medicine that involves an injection of a radioactive material known as meta-iodobenzylguanidine, which in short is known as MIBG. The radioactive substance is usually injected in elbow vein.(1)
MIBG is mostly done for the detection of neuroendocrine tumors and it uses a special gamma camera (a type of scanner) to take images to confirm a tumor or neuroendocrine origin. Neuroendocrine tumors are tumors of nerve tissue origin, most commonly including pheochromocytomas (a tumor of the adrenal gland, which is located above and on the surface of kidneys) and neuroblastomas (cancers affecting the nerve tissue). The most common location of neuroblastomas is kidneys, but it can occur elsewhere in the body. So, an MIBG scan may be done for the diagnosis, follow up and metastasis of a known or new tumor.(1)
Paragangliomas originate from extra-adrenal chromaffin cells that produce catecholamines. These are tumors situated along the paravertebral and para-aortic axes and paragangliomas that are situated in the adrenal medulla are known as pheochromocytomas. Most of the pheochromocytomas are adrenal, sporadic, and solitary. Paragangliomas are mostly found intra-abdominally, next to the adrenals in 85% of the cases. The sensitivity and specificity for the detection of pheochromocytomas/paragangliomas with iodine-131-MIBG is 77-90% and 95-100%, respectively; whereas, the sensitivity and specificity for iodine-123-MIBG is 83-100% and 95-100%, respectively. Due to the lower radiation burden and higher imaging quality, the iodine-123-MIBG scan is more commonly used nowadays.(2)
Medullary thyroid cancer originates from thyroid parafollicular C cells. While most of the cases of medullary thyroid cancer are sporadic (75%), the rest being hereditary (25%), which are associated with genetic syndromes such as multiple endocrine neoplasia type 2 (MEN2). Although, the diagnostic utility of MIBG scan in medullary thyroid cancer is limited due to its lower sensitivity of only 30%. The role of MIBG scan in medullary thyroid cancer is limited in patients with MEN2 (as it can detect lesions either in the thyroid or the adrenals) and in patients in whom therapy with iodine-131-MIBG is being considered.(2)
Neuroblastoma is a malignancy that has its origin from the primitive neural crest cells that form the sympathetic nervous system. It is a common tumor found in children and accounts for 8-10% of childhood tumors. Most of the primary tumors are found in the abdomen (65-70%) of which adrenal gland is the most common site. It accounts for 12-15% of childhood cancer deaths. MIBG scan is used to detect primary neuroblastomas and is the preferred test for the detection of metastatic tumors. Iodine-123-MIBG scan has a sensitivity and specificity of 88-93% and 83-92%, respectively. Iodine-123-MIBG is the agent of choice for scintigraphy of pediatric neuroblastoma.(3)
How To Prepare For MIBG Scan?
Since iodine radioisotope interacts with some medications, it is important to discontinue such medications around 3 days prior to the test. These medications include tricyclic antidepressants, antihypertensives, cocaine, sympathomimetics, and nasal decongestants containing pseudoephedrine, phenylpropanolamine, and phenylephrine.(1)
It is important to take potassium iodide tablets or Lugol’s iodine solution prior to the scan to prevent the thyroid gland from absorbing too much of the iodine radioisotope. Since the thyroid gland is more sensitive and susceptible to attracting radioactivity than other body parts that can lead to its malfunction, it is important to consult the dosage and timing of the medication intake. For females, it is also important to tell the staff if they are pregnant or breastfeeding, since the radiation dose may not be suitable for growing fetus. It is important for breastfeeding women to manage to breastfeed and consult a doctor regarding precautions, since after the scan a small amount of radioactivity may be released from the body.(1)
The MIBG scan is usually completed in a course of 2 days. A small side-effect of the scan is a temporary rise in blood pressure in some patients that are monitored and usually resolves within 48 hours of radioisotope injection.(1)