MIBG selectively accumulates in tissues that store catecholamines in neurosecretory granules. The uptake of MIBG into a tissue is proportional to the concentration of neurosecretory granules.34 Scintig-raphy using radioactive iodine (123I)-MIBG or 131I-MIBG is useful for imaging an occult pheochromo-cytoma, paraganglioma, or neuroblastoma or for confirming that a certain mass is a neuroendocrine tumor (Figure 9-4).35 It is also useful for screening patients for metastases. MIBG uptake does occur in apparently nonfunctioning pheochromocytomas. The isotope that is preferable for precise imaging is 123I-MIBG because 123I has a more useful photon flux, with lower-energy gamma emissions than 131I, allowing for clearer images and single-photon emission computed tomography (SPECT). 123I-MIBG SPECT scanning is more sensitive than 123I-MIBG planar imaging for imaging small metastases.36 However, most centers use 131I-MIBG because it has a longer half-life than 123I-MIBG and is also less expensive.37 The overall sensitivity of 123I-MIBG for pheochromocytomas is about 85%; it is more sensitive for unilateral, adrenal, benign, capsular-inva-sive, and sporadic pheochromocytomas. Scanning with 123I-MIBG is less sensitive for bilateral, malignant, extra-adrenal, noninvasive, and pheochromo-cytomas related to MEN types IIA and IIB.38
To block the thyroid's uptake of free 123I or 131I, Lugol's solution (potassium iodide [KI], 5 drops orally three times daily) is given before the injection and daily for 7 days afterward. The 123I-MIBG is given intravenously, and gamma camera scanning may be performed between 1 and 3 days afterward.
False-negative scans occur in about 15% of both benign and malignant pheochromocytomas. False-negative scans are more common in patients who, within 6 weeks, have taken tricyclic class drugs, for example, antidepressants or cylcobenzaprine (Flex-eril). Other drugs can cause false-negative scans if taken within 2 weeks: amphetamines, cocaine, phenylpropanolamine hydrochloride, haloperidol, phenothiazines, thiothixene, reserpine, nasal decon-gestants, and diet pills. Labetalol causes some decreased uptake, but the scan can still be done with reasonable sensitivity (Table 9-3).
False-positive scans do occur. Uptake in the normal adrenal medulla and renal pelvis and bladder is commonly seen on day 139; the scan is then repeated on days 3 and 5. Uptake in the salivary glands is the rule. Some uptake of 123I-MIBG by the heart and liver is common. Skin contaminated by urine can also cause a false-positive scan.
When a pheochromocytoma is suspected on clinical and biochemical grounds, the patient can be imaged with full abdominal CT scanning, from the diaphragm through the pelvis. Thin sections are obtained through the adrenals (Figure 9-5). Glucagon should not be used because it may provoke a hypertensive crisis. Hypertension should be treated prior to CT scanning because intravenous contrast can also cause hypertensive crisis. If no mass is dis-
Table 9-3. FACTORS INHIBITING MIBG UPTAKE BY PHEOCHROMOCYTOMA
Inhibitors of type I catecholamine uptake: cocaine, tricyclic drugs, labetalol (2-6 wk) Stimulants of catecholamine discharge: reserpine (2 wk) Displacement of catecholamines from intracellular stores and competition with uptake of MIBG: all amphetamines; nasal decongestants—oral or nasal (2 wk) Others: phenothiazines, haloperidol, thiothixene (2 wk)
MIBG = metaiodobenzylguanidine.
covered, either a 123I-MIBG scan may be obtained and/or the CT scan may be extended into the chest and thoracic spine in search of a paraganglioma. The overwhelming majority of pheochromocytomas are greater than 2 cm in diameter, well within the resolution of the CT scan. The overall sensitivity of CT scanning for an adrenal pheochromocytoma is about 90% and over 95% for pheochromocytomas over 0.5 cm diameter.40 However, it is less sensitive for the detection of extra-adrenal paragangliomas, metastases, and recurrent small tumors in the adrenal bed.
MRI has the advantage of not requiring intravenous iodinated contrast, thereby minimizing the risk of hypertensive crisis. The lack of radiation makes it the localizing procedure of choice during pregnancy. MRI can visualize and confirm metastases to bone suspected on MIBG imaging. It can help determine whether an adrenal mass is a pheochromocytoma when biochemical studies are inconclusive. The T2-weighted signal is usually (75%) hyperintense to the liver (Figure 9-6). However, some adrenal adenomas may have the same appearance, so the MRI scan lacks true specificity. MRI of the abdomen has a sensitivity of about 95% for adrenal pheochromocytomas over 0.5 cm diameter.40 However, MRI is less sensitive for the detection of extra-adrenal paragan-gliomas, metastatic disease, and recurrent small tumors in the adrenal surgical bed.
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