In planning the localization of a pheochromocytoma, one should consider whether the tumor is sporadic or familial and whether the patient is an adult or child. Most pheochro-mocytomas are sporadic (85%), and 15% are familial and occur in patients with multiple endocrine neoplasia (MEN) type 2, neurofibromatosis, or Sturge-Weber syndrome. Although 85% of the tumors are unilateral, bilateral pheochromocytomas are found in 50% of familial cases. About 15% of pheochromocytomas are malignant and are usually distinguishable clinically only by the absence or presence of local invasion and metastases. Tumors in extra-adrenal locations, which are more likely to be malignant, are found in only 15% of adults, but in children the incidence rises to about 25%. Fewer than 2% of tumors are found in the mediastinum, neck, or head.

Localization Studies

Because pheochromocytomas have usually attained a considerable size before being discovered, CT28,29 and Tl- and T2-weighted MRI scans reliably detect pheochromocytomas, with an accuracy of almost 100%.8'26 After peroral contrast preparation of the bowel, an abdominal CT scan from the diaphragm distal to the aortic bifurcation is performed as the initial imaging procedure. Because tumors may extend superiorly and inferiorly from an otherwise normal-appearing adrenal gland, contiguous thin sectioning (0.5- to 1.0-cm slices) is recommended.26 Pheochromocytomas show a wide range of morphologic patterns; most pheochromocytomas are rounded masses with homogeneous densities similar to or less than that of liver tissue, and they may occasionally show a hemorrhagic, cystic ("Swiss cheese") appearance or calcified lesions (Fig. 66-6). Despite this heterogeneity, diagnosis of malignancy is unreliable unless local invasion or metastases are apparent.

The diagnostic capability of CT scanning may be increased with contrast enhancement, but there is a small and unpredictable risk of precipitating a hypertensive crisis, making a-adrenergic blockade before invasive localization studies essential.28 Contrast enhancement of the tumors is irregular; the periphery of these tumors is often more intensely enhanced. Plain CT is highly accurate (-95%) for intraglandular lesions, but it is less useful in identifying extra-adrenal lesions and lesions in MEN patients, for which it is only 60% to 80% sensitive. CT scanning is even less accurate in evaluating patients with metastatic or recurrent disease (sensitivity of about 60%).8-26-28 Contrast enhancement is essential to provide acceptable sensitivity for detecting extra-adrenal pheochromocytomas, especially those in the neck and mediastinum.30

CT scanning in combination with MIBG (MIBG-CT) appears to be the procedure of choice for identifying extra-adrenal, ectopic tumor locations. MIBG is selectively taken up by the adrenal medulla and by pheochromocytomas and has been especially useful in evaluating ectopic disease and malignant pheochromocytomas (Fig. 66-7). It is the recommended procedure for patients with recurrent or metastatic disease.31 Some experts recommend MIBG scanning as the initial diagnostic procedure for every patient suspected of

Swiss Cheese Lesions

FIGURE 66-6. Contrast-enhanced CT scan of a patient with pheochromocytoma manifesting as a rounded mass with density similar to or slightly less than that of liver tissue. The tumor has several cystic lesions, giving it a "Swiss cheese" appearance. Despite this heterogeneity, a diagnosis of malignancy is unlikely.

FIGURE 66-6. Contrast-enhanced CT scan of a patient with pheochromocytoma manifesting as a rounded mass with density similar to or slightly less than that of liver tissue. The tumor has several cystic lesions, giving it a "Swiss cheese" appearance. Despite this heterogeneity, a diagnosis of malignancy is unlikely.

Pheochromocytoma Patients
FIGURE 66-7. Metaiodobenzylguanidine scanning confirms a left adrenal pheochromocytoma, identified as a mark on top of the left of the two symmetrically depicted kidneys, together with an ectopically located second tumor at the level of the aortic bifurcation (marker).

having pheochromocytoma because this modality detects most tumors.32 34 For diagnostic use, a tracer dose of 0.5 mCi is given, imaging the adrenals in only a few normal control subjects. For localizing primary, metastatic, and recurrent disease, this approach has a false-negative rate of 11.4% and a false-positive rate of 1.8%.35 MIBG has been used for radioablative treatment of unresectable or metastatic pheochromocytomas. Although there have been some good responses, overall, it has been disappointing.36

Positron emission tomography has been used after administration of 2-fluorine-18-fluoro-2-deoxy-D-glucose (FDG). The method was found promising for localization of pheochromocytomas in the 10% of patients with false-negative MIBG scintigrams.37

Pheochromocytomas are localized and characterized by MRI. The approach is at least equivalent to CT for localization of adrenal pheochromocytomas that are larger than 2 cm.7 Tumors tend to be hyperintense on T2-weighted pulse sequences.38 Because of the consistency of hyperintensity, recurrent tumor, metastatic disease, and extra-adrenal pheochromocytomas are readily identified, and in this instance MRI is more accurate and sensitive than CT. Moreover, owing to the marked hyperintensity of functioning pheochromocytomas on T2-weighted pulse sequences, MRI permits differentiation of pheochromocytomas from nonfunctioning adrenal neoplasms. Tumors of the urinary bladder or the paracardiac region, which are difficult to evaluate by CT, are clearly recognized by MRI. In one study, T2-weighted MRI and MIBG-CT scans were nearly equivalent for localizing and staging adrenal pheochromocytomas.39'40 Tl-weighted MRI

sequences using gadolinium-DTPA enhancement enable determination of vascular invasion by these tumors.41 However, because the spatial resolution of MRI is inferior to that of CT, routine scanning should be done with CT. Moreover, CT has greater sensitivity in detecting liver metastases than MRI.

Other Imaging Modalities

In the rare event that these noninvasive techniques fail, arteriography may be useful. However, patients must be treated with a-adrenergic blockade (e.g., phenoxybenzamine) to avoid hypertensive crisis. |3-Blockade should be used in a patient with tachyarrhythmias whose tumor secretes epinephrine rather than norepinephrine. Arteriography should include the superior, middle, and inferior adrenal arteries and may be enhanced by subtraction techniques, especially in the 15% of tumors displaying no or only moderate hyper-vascularization. Selective venography and selective venous sampling of veins in the abdomen, pelvis, and chest may very occasionally be useful for diagnosing small lesions, especially intraglandular lesions or tumors at ectopic sites. A positive result allows the physician to narrow the specific region for further anatomic imaging. However, these techniques require great care and may be dangerous (Fig. 66-8).

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