Localization

Computed Tomography

Whereas 3 decades ago bolus nephrotomography or angiography might have provided the mainstay of localization, three excellent localization modalities are currently available. Soon after the development of CT scanning, the adrenal was noted to be exceptionally well depicted,19 and CT is now considered the most reliable, efficient, precise, and widely available localization technique (Fig. 71-2). Because 90% of tumors are located in the adrenal glands, a high-quality CT scan is likely to identify virtually all of these tumors as well as image the normal contralateral gland. However, because it is a purely anatomic representation, additional extra-adrenal pheochromocytomas may be overlooked. Therefore, patients should be routinely scanned from the diaphragm to at least below the bifurcation of the aorta. In addition, the radiologist must be reminded to avoid use of glucagon during the examination as this may precipitate a paroxysmal attack.

Magnetic Resonance Imaging

MRI not only defines the anatomy but on T2-weighted images, pheochromocytomas and paragangliomas frequently show a characteristic and nearly unique high-intensity signal (Fig. 71-3). Owing to this special imaging distinction, which was 100% effective in the series by Doppman and

FIGURE 71-3. A, T2-weighted MRI image of paraganglioma located between aorta and inferior vena cava, which it compresses. The bright white appearance of the tumor is typical of pheochromocytoma or paraganglioma when imaged by T2-weighted MRI. B, Gross appearance of paraganglioma showing somewhat gelatinous cut surface with some central necrosis.

FIGURE 71-3. A, T2-weighted MRI image of paraganglioma located between aorta and inferior vena cava, which it compresses. The bright white appearance of the tumor is typical of pheochromocytoma or paraganglioma when imaged by T2-weighted MRI. B, Gross appearance of paraganglioma showing somewhat gelatinous cut surface with some central necrosis.

colleagues,20 MRI provides both anatomic and physiologic imaging capabilities, in contrast to CT scanning. Other advantages that may cause MRI to challenge CT scanning as the optimal localization test include its lack of radiation exposure, the clear definition of surrounding vascular structures, and the lack of interference from preexisting metal clips. The disadvantages at present include relative lack of availability, claustrophobia that some patients feel during the examination, cost, and the anatomic detail, which is not quite as precise as that shown by current CT technology. In fact, MRI can miss small pheochromocytomas, as depicted in the scan in Figure 71-4. Both CT and MRI provide excellent images of the liver and periaortic lymph nodes, both possible sites of metastatic disease.

Metaiodobenzylguanidine

Using either 131I- or 123I-tagged metaiodobenzylguanidine (MIBG) nuclear medicine scintigraphy, abnormal adrenergic

FIGURE 71-4. A, MRI scan demonstrated the characteristic bright white appearance of a pheochromocytoma on the right (short arrow) but was not interpreted to show the small tumor on the left (area just beyond the long, angled arrow). B, Gross cut specimens of the right (top) and left (bottom, less than 1 cm in size).

FIGURE 71-4. A, MRI scan demonstrated the characteristic bright white appearance of a pheochromocytoma on the right (short arrow) but was not interpreted to show the small tumor on the left (area just beyond the long, angled arrow). B, Gross cut specimens of the right (top) and left (bottom, less than 1 cm in size).

tissue can be demonstrated. MIBG is taken up and concentrated within adrenergic vesicles in pheochromocytomas, paragangliomas, and their metastases with 80% to 90% sensitivity.2122 MIBG scans are most valuable to image or search for bilateral tumors such as in multiple endocrine neoplasia (MEN) type 2 syndromes or to identify multiple tumors (Figs. 71-5 and 71-6). The advantage of physiologic localization is unfortunately somewhat offset by the added complexity of the examination. To prevent its ablation, the thyroid must be blocked by oral iodine consumption before and after administration of the radioactive iodine. Repeated scans may be required for up to 72 hours to obtain optimal images, and localization is not precise, usually requiring correlation with anatomic detail provided by either CT or MRI scans (Fig. 71-7). However, mediastinal and intracardiac tumors23 have been localized by this method as well as bone metastases missed by conventional bone scans.24 These three examinations complement each other and should be used for their specific advantages as indicated by the clinical situation. In a large series of 315 patients from the University of Michigan, where this test was developed, 123I did not reveal unsuspected metastatic or bilateral disease in any of the 48 patients with a unilateral pheochromocytoma.25 It was concluded that in this setting, the addition of 123I MIBG scintigraphy was unnecessary. This contradicts a report from the National Institutes of Health26 that supports

FIGURE 71-5. A, Metaiodobenzylguanidine scan, anterior view, demonstrating bilateral adrenal pheochromocytomas in a patient with multiple endocrine neoplasia (MEN) type 2A syndrome. B, Of importance, in the photograph of the gross appearance of the bilateral tumors is the multinodular appearance of the right adrenal, characteristic of MEN syndrome.

the addition of MIBG scintigraphy as a minimum confirmatory test (and to exclude malignancy) in all patients with an adrenal pheochromocytoma.

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