DOI: 10.1148/rg.24si045506
RadioGraphics 2004;24:S87-S99
© RSNA, 2004
Pheochromocytoma: An Imaging Chameleon1
Michael A. Blake, FFR(RCSI), FRCR,
Mannudeep K. Kalra, MD, DNB,
Michael M. Maher, MD,
Dushyant V. Sahani, MD,
Ann T. Sweeney, MD,
Peter R. Mueller, MD,
Peter F. Hahn, MD, PhD and
Giles W. Boland, MD
1 From the Department of Radiology, Division of Abdominal Imaging and Intervention, Massachusetts General Hospital, White 270, 55 Fruit St, Boston, MA 02114 (M.A.B., M.K.K., M.M.M., D.V.S., P.R.M., P.F.H., G.W.B.); and the Department of Medicine, Division of Endocrinology, St Elizabeth’s Medical Center, Boston, Mass (A.T.S.). Presented as an education exhibit at the 2003 RSNA scientific assembly. Received February 12, 2004; revision requested March 14 and received April 27; accepted May 5. All authors have no financial relationships to disclose. Address correspondence to M.A.B. (e-mail: mblake2@partners.org).
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Abstract
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Pheochromocytomas are rare catecholamine-secreting tumors with many clinical and imaging manifestations. They may produce overwhelming cardiovascular crises if the diagnosis is not made or if appropriate treatment is delayed. It is thus important to recognize both their characteristic and atypical imaging appearances. Pheochromocytomas are encountered, sometimes unexpectedly, across a range of imaging modalities. They are characteristically solid, hypervascular masses with high signal intensity on T2-weighted magnetic resonance (MR) images. A wide spectrum of imaging appearances is seen, however, and pheochromocytomas may mimic other adrenal lesions, both benign and malignant. They may be dark on T2-weighted MR images, in contrast to their more classic bright T2-weighted appearance. Other atypical features include fatty, hemorrhagic, cystic, and calcific changes. Pheochromocytomas may contain sufficient fat to be mistaken for an adenoma at computed tomography (CT) or MR imaging. They may also demonstrate rapid contrast material washout and be mistaken for an adenoma owing to their deenhancement profile; however, their washout pattern can be inconsistent. The appearance of pheochromocytomas at radionuclide imaging is also unpredictable. These characteristics at CT, MR imaging, and scintigraphy pose diagnostic challenges, since they allow pheochromocytomas to mimic many other adrenal masses. Pheochromocytoma is an important, often clinically occult neoplasm with devastating consequences if overlooked. Radiologists must be aware of the various forms that pheochromocytomas can assume at imaging.
© RSNA, 2004
Index Terms: Adrenal gland, CT, 86.1211 • Adrenal gland, MR, 86.1214 • Adrenal gland, radionuclide studies, 86.1216 • Pheochromocytoma
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Introduction
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Pheochromocytomas are rare catecholamine-secreting tumors derived from chromaffin cells. A major impetus for the noninvasive diagnosis of pheochromocytomas is that any physical contact with these neoplasms can precipitate cardiac arrhythmias and malignant hypertension. Correct preoperative diagnosis is therefore of crucial importance. Untreated pheochromocytomas can produce fatal clinical consequences (1).
More than 90% of pheochromocytomas are located within the adrenal glands, and 98% occur within the abdomen. Extraadrenal pheochromocytomas develop in paraganglionic chromaffin tissue of the sympathetic nervous system. They may occur anywhere from the base of the brain to the urinary bladder and are considered to be paragangliomas. Common locations for extraadrenal pheochromocytomas include the organ of Zuckerkandl (Fig 1), bladder wall, retroperitoneum, heart, mediastinum, and carotid and glomus jugulare bodies. Pheochromocytomas and paragangliomas can have almost any imaging appearance (2–5). They can undergo a variety of forms of pathologic degeneration, which affects their imaging features. This varied and changeable appearance merits the "chameleon" epithet given to this tumor. A chameleon derives its name from the Greek word khamailen, meaning "lion on the ground," and is a reptile characterized by the ability to change color. The imaging diagnosis of pheochromocytoma, although sometimes challenging, should always be considered. In this article, we review the clinical background and pathologic features of pheochromocytomas. We also discuss and illustrate the varied imaging appearances of pheochromocytomas at computed tomography (CT), magnetic resonance (MR) imaging, and radionuclide imaging. In addition, we briefly discuss the features of malignant pheochromocytomas.
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Figure 1a. Paraganglioma at the organ of Zuckerkandl in a 45-year-old woman. (a, b) Axial T2-weighted (a) and coronal gadolinium-enhanced T1-weighted (b) fat-saturated MR images show an avidly enhancing mass with high signal intensity at the aortic bifurcation (arrow in b). (c) Contrast material-enhanced CT scan demonstrates the mass with high attenuation. Paragangliomas can occur anywhere along the course of the sympathetic chain.
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Figure 1b. Paraganglioma at the organ of Zuckerkandl in a 45-year-old woman. (a, b) Axial T2-weighted (a) and coronal gadolinium-enhanced T1-weighted (b) fat-saturated MR images show an avidly enhancing mass with high signal intensity at the aortic bifurcation (arrow in b). (c) Contrast material-enhanced CT scan demonstrates the mass with high attenuation. Paragangliomas can occur anywhere along the course of the sympathetic chain.
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Figure 1c. Paraganglioma at the organ of Zuckerkandl in a 45-year-old woman. (a, b) Axial T2-weighted (a) and coronal gadolinium-enhanced T1-weighted (b) fat-saturated MR images show an avidly enhancing mass with high signal intensity at the aortic bifurcation (arrow in b). (c) Contrast material-enhanced CT scan demonstrates the mass with high attenuation. Paragangliomas can occur anywhere along the course of the sympathetic chain.
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Clinical Background
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Pheochromocytomas may precipitate life-threatening hypertension or cardiac arrhythmias because of excessive catecholamine secretion. If the diagnosis of pheochromocytoma is overlooked, the consequences can be life threatening; however, if the diagnosis is made, affected patients are potentially curable. Pheochromocytoma is rare and occurs in 0.1%–0.9% of hypertensive individuals. Patients may be completely asymptomatic, with up to 10% of cases being clinically silent (1). Masses in such patients tend to be larger than hyperfunctioning tumors (Fig 2) (6). These clinically silent pheochromocytomas are assumed to have little if any catecholamine secretion. A 1979 study (7) revealed that 50% of cases of pheochromocytoma were diagnosed at autopsy. This study predates the burgeoning increase in the use of imaging during the past 2 decades, and many of these pheochromocytomas are now being encountered in vivo along with many other incidental adrenal tumors at CT (5).
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Figure 2a. Large pheochromocytoma in a 42-year-old woman. (a) Contrast-enhanced CT scan demonstrates a right adrenal mass due to pheochromocytoma (arrow). (b, c) Axial T2-weighted (b) and coronal gadolinium-enhanced T1-weighted (c) MR images demonstrate the large nonfunctioning pheochromocytoma (arrow in c). Clinically silent pheochromocytomas tend to be larger.
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Figure 2b. Large pheochromocytoma in a 42-year-old woman. (a) Contrast-enhanced CT scan demonstrates a right adrenal mass due to pheochromocytoma (arrow). (b, c) Axial T2-weighted (b) and coronal gadolinium-enhanced T1-weighted (c) MR images demonstrate the large nonfunctioning pheochromocytoma (arrow in c). Clinically silent pheochromocytomas tend to be larger.
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Figure 2c. Large pheochromocytoma in a 42-year-old woman. (a) Contrast-enhanced CT scan demonstrates a right adrenal mass due to pheochromocytoma (arrow). (b, c) Axial T2-weighted (b) and coronal gadolinium-enhanced T1-weighted (c) MR images demonstrate the large nonfunctioning pheochromocytoma (arrow in c). Clinically silent pheochromocytomas tend to be larger.
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Despite their usual unilateral and benign manifestation, pheochromocytomas can be bilateral and malignant in 10% of patients (Fig 3). About 10% of pheochromocytomas were thought to be hereditary, but Neumann et al (8) have shown that approximately 25% of patients with sporadic pheochromocytoma and no family history have germ-line mutations of one of four susceptibility genes for pheochromocytoma. These mutations may be seen in isolation (94% of cases) or as part of a syndrome—for example, multiple endocrine neoplasia (MEN) II and III and various neuroectodermal disorders including neurofibromatosis, von Hippel–Lindau syndrome, Sturge-Weber syndrome, and Carney triad.
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Figure 3a. Malignant pheochromocytoma in a 62-year-old man. (a) Contrast-enhanced CT scan shows a complex left adrenal mass (solid arrows) representing a malignant pheochromocytoma with hepatic metastases (open arrow) and portocaval adenopathy. (b) Pelvic CT scan shows sacral and left iliac bone metastases (arrow). Metastatic spread is the only reliable criterion for differentiating a benign from a malignant pheochromocytoma.
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Figure 3b. Malignant pheochromocytoma in a 62-year-old man. (a) Contrast-enhanced CT scan shows a complex left adrenal mass (solid arrows) representing a malignant pheochromocytoma with hepatic metastases (open arrow) and portocaval adenopathy. (b) Pelvic CT scan shows sacral and left iliac bone metastases (arrow). Metastatic spread is the only reliable criterion for differentiating a benign from a malignant pheochromocytoma.
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When an adrenal mass is found, it is important to determine whether the mass represents a functioning tumor. The adrenal gland may give rise to a number of functioning tumors that produce endocrine disorders (eg, Conn syndrome, Cushing syndrome, virilizing adrenal cortical carcinoma, pheochromocytoma). The most common adrenal tumors are adenomas, which are often nonfunctioning. The diagnosis of pheochromocytoma is dependent on the imaging identification of an appropriately located mass with accompanying clinical and biochemical confirmation. The classic clinical manifestation includes hypertension, which may be episodic or refractory in association with the triad of symptoms of palpitations, headaches, and diaphoresis. Hypertension is a common condition, and pheochromocytoma is present in 0.1%–0.9% of patients with hypertension. Biochemical confirmation of pheochromocytoma is of vital importance and should be obtained before imaging. The diagnostic evaluation should include measurement of plasma metanephrine levels and of 24-hour urinary catecholamine levels. Indiscriminate biopsy of pheochromocytomas may trigger a catastrophic crisis and must be avoided. If percutaneous biopsy is clinically indicated, it should be performed in consultation with endocrine and anesthesia services so that appropriate endocrine blockade and coverage is instituted. A proposed algorithm for the assessment of an adrenal mass and recognition of a pheochromocytoma is provided in Figure 4. The algorithm may be modified according to the amount of initial imaging information known about the adrenal mass.
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Figure 4. Diagram illustrates a proposed algorithm for the assessment of an adrenal mass and recognition of a pheochromocytoma (Pheo). Hx = history, MIBG = metaiodobenzylguanidine, PET = positron emission tomography.
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Pathologic Features
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Pheochromocytomas characteristically demonstrate a nesting (Zellballen) microscopic pattern. This pattern is composed of well-defined clusters of tumor cells containing eosinophilic cytoplasm separated by fibrovascular stroma (Fig 5). A number of gross and microscopic patterns may also be seen. Smaller neoplasms tend to be solid, whereas larger lesions are often hemorrhagic or cystic (Figs 6, 7) (9). Clinically silent pheochromocytomas also tend to be larger (10). Cystic degeneration may be very marked, and sometimes only a few identifiable cells may remain in a thin rim or nodule that serve to disclose the true nature of the lesion. Other reported pathologic features include necrosis, calcification (Fig 7), and fibrosis (9). These gross features of pheochromocytomas have correlative radiologic features (Fig 5). Lipid degeneration has also been described within a pheochromocytoma, with massive accumulation of lipid within the cell cytoplasm (Fig 8) (10).
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Figure 6. Hemorrhagic pheochromocytoma in a 42-year-old man. CT scan shows extensive hemorrhage (highest attenuation value = 100 HU) within and surrounding a left adrenal pheochromocytoma (arrow). Hyperattenuation within a pheochromocytoma may occur as a result of acute hemorrhage.
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Figure 7a. Spectrum of complex pheochromocytomas. (a) Unenhanced CT scan in a 72-year-old woman demonstrates a heterogeneously attenuating mass in the right adrenal gland (arrow). (b) Contrast-enhanced CT scan in an 80-year-old man shows a mass in the right adrenal gland (arrow) with peripheral enhancement and a central area of low attenuation due to necrosis. (c) Unenhanced CT scan in a 41-year-old man shows a mass with cystic and solid components in the left adrenal gland. (d) Contrast-enhanced CT scan in a 59-year-old woman demonstrates a cystic adrenal lesion with a calcified rim (arrow). Pheochromocytomas display a wide variety of appearances at CT.
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Figure 7b. Spectrum of complex pheochromocytomas. (a) Unenhanced CT scan in a 72-year-old woman demonstrates a heterogeneously attenuating mass in the right adrenal gland (arrow). (b) Contrast-enhanced CT scan in an 80-year-old man shows a mass in the right adrenal gland (arrow) with peripheral enhancement and a central area of low attenuation due to necrosis. (c) Unenhanced CT scan in a 41-year-old man shows a mass with cystic and solid components in the left adrenal gland. (d) Contrast-enhanced CT scan in a 59-year-old woman demonstrates a cystic adrenal lesion with a calcified rim (arrow). Pheochromocytomas display a wide variety of appearances at CT.
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Figure 7c. Spectrum of complex pheochromocytomas. (a) Unenhanced CT scan in a 72-year-old woman demonstrates a heterogeneously attenuating mass in the right adrenal gland (arrow). (b) Contrast-enhanced CT scan in an 80-year-old man shows a mass in the right adrenal gland (arrow) with peripheral enhancement and a central area of low attenuation due to necrosis. (c) Unenhanced CT scan in a 41-year-old man shows a mass with cystic and solid components in the left adrenal gland. (d) Contrast-enhanced CT scan in a 59-year-old woman demonstrates a cystic adrenal lesion with a calcified rim (arrow). Pheochromocytomas display a wide variety of appearances at CT.
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Figure 7d. Spectrum of complex pheochromocytomas. (a) Unenhanced CT scan in a 72-year-old woman demonstrates a heterogeneously attenuating mass in the right adrenal gland (arrow). (b) Contrast-enhanced CT scan in an 80-year-old man shows a mass in the right adrenal gland (arrow) with peripheral enhancement and a central area of low attenuation due to necrosis. (c) Unenhanced CT scan in a 41-year-old man shows a mass with cystic and solid components in the left adrenal gland. (d) Contrast-enhanced CT scan in a 59-year-old woman demonstrates a cystic adrenal lesion with a calcified rim (arrow). Pheochromocytomas display a wide variety of appearances at CT.
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Figure 8a. Pheochromocytoma containing fat in a 49-year-old woman with refractory hypertension. (a) Unenhanced CT scan shows a low-attenuation (8 HU) adrenal mass (arrow). (b) Photomicrograph (original magnification, x300; hematoxylin-eosin stain) shows that the cells contain bubbly fat. Pheochromocytomas can infrequently mimic an adenoma at unenhanced CT.
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Figure 8b. Pheochromocytoma containing fat in a 49-year-old woman with refractory hypertension. (a) Unenhanced CT scan shows a low-attenuation (8 HU) adrenal mass (arrow). (b) Photomicrograph (original magnification, x300; hematoxylin-eosin stain) shows that the cells contain bubbly fat. Pheochromocytomas can infrequently mimic an adenoma at unenhanced CT.
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Radiologic Characteristics
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Pheochromocytomas may be imaged with a variety of modalities, including CT, MR imaging, radionuclide imaging, and, rarely, ultrasonography. We will focus our discussion on the imaging features of pheochromocytomas at CT, MR imaging, and radionuclide imaging.
Computed Tomography
Lesion Attenuation.—
Pheochromocytomas may be homogeneous or heterogeneous, solid or cystic complex masses or may show calcification (Fig 7). Smaller tumors tend to have a more uniform attenuation. Most incidental adrenal lesions are detected at CT, with characterization of the lesions being predominantly dependent on attenuation parameters (1). CT attenuation values have proved particularly valuable for characterizing adrenal adenoma, the most common adrenal tumor. Adenomas can often be differentiated from other masses owing to intracellular fat, which causes attenuation to decrease to less than 10 HU (11,12). Most pheochromocytomas have an attenuation higher than 10 HU; rarely do they contain sufficient intracellular fat to have an attenuation of less than 10 HU (Fig 8) (13). Still, some pheochromocytomas could be incorrectly categorized as adenomas (12). Pheochromocytomas and paragangliomas with macroscopic fat may also be seen (Fig 9). Conversely, some pheochromocytomas may demonstrate very high attenuation due to hemorrhage (Fig 6).
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Figure 9. Paraganglioma containing fat in a 39-year-old woman. Contrast-enhanced CT scan shows a left paraaortic paraganglioma (solid arrow) that contains macroscopic fat-attenuation components (open arrow). The mass also contains highly vascular components posteriorly. Extraadrenal paragangliomas share many of the imaging features of adrenal pheochromocytomas.
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Enhancement and Washout Characteristics.—
Pheochromocytomas typically enhance avidly but can be heterogeneous or show regions of no enhancement due to cystic changes (Fig 7). When early intense enhancement occurs, it reflects the capillary-rich framework of the tumor. Studies of washout profiles of adrenal masses have also shown successful differentiation of adenomas from nonadenomas (14–17). Adenomas have a relative washout rate of more than 40% when delayed images are obtained 15 minutes after contrast material administration. The washout rate in malignant lesions is less than 40%. With use of absolute percentage washout, which takes the precontrast attenuation value into account, the cut-off threshold value, again with a 15-minute delay, is 60% (15). However, pheochromocytomas, in typical inconsistent fashion, can demonstrate different and variable washout patterns and may again, therefore, be confused with either adenomas or metastases (13,15,17). This inconsistent contrast material washout may be due to varied pathologic degeneration that produces abnormal capillary networks, which alter both enhancement and washout. It should be remembered, however, that there are some theoretical concerns about administering iodinated contrast material to a patient with pheochromocytoma. Despite reassuring reports of studies performed with low-osmolar nonionic contrast agents (18,19), current accepted practice would generally avoid the use of iodinated contrast material in a patient known to have a pheochromocytoma.
Evidence of Fat.—
A further potential cause of diagnostic confusion at CT is the presence of macroscopic fat in adrenal lesions. Macroscopic fat (<–30 HU) within an adrenal mass is characteristic of a benign myelolipoma. However, the presence of fat within a large soft-tissue mass is not absolutely specific for myelolipoma. Pheochromocytomas may contain extensive lipid degeneration that can be mistaken for neoplasms of the adrenal cortex at pathologic examination (11). Some extraadrenal paragangliomas are known to stain positively for fat (Fig 9) (20).
MR Imaging
The most common MR imaging appearance of pheochromocytoma is a mass with low signal intensity at T1-weighted imaging and with high signal intensity at T2-weighted imaging (Fig 10). Pheochromocytomas commonly enhance avidly at T1-weighted imaging after administration of gadolinium-based contrast material (21,22). In general, pheochromocytomas appear still more hyperintense on fat-suppressed T2-weighted images due to a signal intensity rescaling effect, which reflects the reduced signal intensity of background fat. The appearance can vary, however, with many low-signal-intensity pheochromocytomas encountered at T2-weighted imaging (Fig 11) (22). In addition, as with CT, pheochromocytomas can also occasionally be wrongly characterized as adenomas on opposed-phase MR images because the diagnosis of adenomas at MR imaging is also dependent on their intracellular fat content. There is indeed considerable overlap between the MR imaging appearance of pheochromocytoma and that of other adrenal lesions. Approximately 65% of pheochromocytomas are correctly identified, whereas 35% are misclassified as malignant lesions or benign adenomas because of atypical low signal intensity on T2-weighted MR images (22). A pheochromocytoma cannot be excluded on the basis of a lack of high signal intensity at T2-weighted MR imaging. Conversely, nonpheochromocytomas, including some metastases, may be wrongly classified as pheochromocytomas because of very high lesion signal intensity (23). The varied pathologic degenerative processes described earlier again have their own MR imaging counterparts, with fat and hemorrhage, when present, producing high signal intensity at T1-weighted imaging (Figs 6, 12). Punctate signal voids representing tumor vessels in paragangliomas create a salt-and-pepper pattern characteristically seen on T1- and T2-weighted images.
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Figure 10a. Pheochromocytoma in a 42-year-old man. Axial fat-saturated T1-weighted (a) and T2-weighted (b) MR images show a left adrenal mass (arrow in a) representing a pheochromocytoma. The mass has heterogeneous high signal intensity on the T2-weighted image. Approximately 65% of pheochromocytomas have high signal intensity at T2-weighted MR imaging.
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Figure 10b. Pheochromocytoma in a 42-year-old man. Axial fat-saturated T1-weighted (a) and T2-weighted (b) MR images show a left adrenal mass (arrow in a) representing a pheochromocytoma. The mass has heterogeneous high signal intensity on the T2-weighted image. Approximately 65% of pheochromocytomas have high signal intensity at T2-weighted MR imaging.
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Figure 11. Pheochromocytoma in a 37-year-old woman. T2-weighted MR image shows a low-signal-intensity mass (arrow). Although most pheochromocytomas have high signal intensity on T2-weighted MR images, low-signal-intensity pheochromocytomas are sufficiently common to make this finding unreliable for differentiating pheochromocytoma from other adrenal or retroperitoneal masses.
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Figure 12a. Large pheochromocytoma with central necrosis in a 66-year-old man. Unenhanced T1-weighted (a), T2-weighted (b), and coronal gadolinium-enhanced T1-weighted (c) MR images show a large right adrenal mass with peripheral marked enhancement and central necrosis (arrows in a and c). Smaller pheochromocytomas tend to be solid.
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Figure 12b. Large pheochromocytoma with central necrosis in a 66-year-old man. Unenhanced T1-weighted (a), T2-weighted (b), and coronal gadolinium-enhanced T1-weighted (c) MR images show a large right adrenal mass with peripheral marked enhancement and central necrosis (arrows in a and c). Smaller pheochromocytomas tend to be solid.
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Figure 12c. Large pheochromocytoma with central necrosis in a 66-year-old man. Unenhanced T1-weighted (a), T2-weighted (b), and coronal gadolinium-enhanced T1-weighted (c) MR images show a large right adrenal mass with peripheral marked enhancement and central necrosis (arrows in a and c). Smaller pheochromocytomas tend to be solid.
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Radionuclide Imaging
A number of agents have been used in the radionuclide imaging of pheochromocytomas. Iodine-131 or 123I metaiodobenzylguanidine (MIBG) scintigraphy is often useful when clinically suspected pheochromocytoma cannot be localized, to confirm that a mass is a pheochromocytoma, or to exclude metastatic disease (Fig 13). MIBG has almost 100% specificity but limited sensitivity (24). Somatostatin receptor scintigraphy performed with octreotide, an analogue of somatostatin, provides another method for localizing pheochromocytoma (Fig 14). However, a conventional dose of octreotide enables localization of pheochromocytoma in less than 30% of cases (25).
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Figure 13a. Paraganglioma in a 54-year-old man. (a) CT scan shows a mass with a focus of calcification near the splenic hilum (arrow) representing a paraganglioma. (b) MIBG scintigram shows corresponding avid uptake (arrow). MIBG scintigraphy is often useful for confirming that a mass is a pheochromocytoma. MIBG has almost 100% specificity but limited sensitivity.
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Figure 13b. Paraganglioma in a 54-year-old man. (a) CT scan shows a mass with a focus of calcification near the splenic hilum (arrow) representing a paraganglioma. (b) MIBG scintigram shows corresponding avid uptake (arrow). MIBG scintigraphy is often useful for confirming that a mass is a pheochromocytoma. MIBG has almost 100% specificity but limited sensitivity.
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Figure 14a. MEN in a 44-year-old woman. (a) T2-weighted MR image shows a right adrenal pheochromocytoma with high signal intensity (arrow). (b) T1-weighted MR image shows a pancreatic gastrinoma (arrow). (c) Axial scintigram obtained with octreotide shows increased uptake in both the right adrenal gland and pancreas that corresponds to sites of pheochromocytoma (solid arrow) and gastrinoma (open arrow), respectively. Pheochromocytomas are seen in a number of syndromes, including MEN II and III.
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Figure 14b. MEN in a 44-year-old woman. (a) T2-weighted MR image shows a right adrenal pheochromocytoma with high signal intensity (arrow). (b) T1-weighted MR image shows a pancreatic gastrinoma (arrow). (c) Axial scintigram obtained with octreotide shows increased uptake in both the right adrenal gland and pancreas that corresponds to sites of pheochromocytoma (solid arrow) and gastrinoma (open arrow), respectively. Pheochromocytomas are seen in a number of syndromes, including MEN II and III.
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Figure 14c. MEN in a 44-year-old woman. (a) T2-weighted MR image shows a right adrenal pheochromocytoma with high signal intensity (arrow). (b) T1-weighted MR image shows a pancreatic gastrinoma (arrow). (c) Axial scintigram obtained with octreotide shows increased uptake in both the right adrenal gland and pancreas that corresponds to sites of pheochromocytoma (solid arrow) and gastrinoma (open arrow), respectively. Pheochromocytomas are seen in a number of syndromes, including MEN II and III.
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Several reports have described the use of positron-emitting imaging agents to help visualize pheochromocytoma (26,27). Imperfect sensitivity and specificity remain problems with most of these agents. FDG (2-[fluorine 18]fluoro-2-deoxy-D-glucose) positron emission tomography (PET) is the most commonly used technique in clinical practice, and pheochromocytomas usually show increased uptake at PET, as do adrenal metastases (Fig 15). To our knowledge, the most impressive results to date have been with 6-[18F] fluorodopamine PET and carbon-11 hydroxyephedrine PET (27,28). Results of these studies suggest that PET performed with both of these radioisotopes is extremely useful in the detection and localization of pheochromocytoma. Further studies with these agents are eagerly awaited.
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Figure 15a. Pheochromocytoma in a 50-year-old man. Coronal (a) and axial (b) FDG PET scans show increased uptake (arrow) due to a right adrenal pheochromocytoma. Pheochromocytomas and metastases usually show increased uptake at FDG PET. (c) Corresponding CT scan shows the right adrenal pheochromocytoma (arrow).
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Figure 15b. Pheochromocytoma in a 50-year-old man. Coronal (a) and axial (b) FDG PET scans show increased uptake (arrow) due to a right adrenal pheochromocytoma. Pheochromocytomas and metastases usually show increased uptake at FDG PET. (c) Corresponding CT scan shows the right adrenal pheochromocytoma (arrow).
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Figure 15c. Pheochromocytoma in a 50-year-old man. Coronal (a) and axial (b) FDG PET scans show increased uptake (arrow) due to a right adrenal pheochromocytoma. Pheochromocytomas and metastases usually show increased uptake at FDG PET. (c) Corresponding CT scan shows the right adrenal pheochromocytoma (arrow).
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Malignant Pheochromocytomas
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Ten percent of pheochromocytomas are malignant (26). Metastatic spread is the only reliable criterion for the diagnosis of malignant pheochromocytoma (Figs 3, 16). Tumor size, mitotic rate, and vascular or capsular invasion are not sufficiently discriminating features to enable the differentiation of benign from malignant tumors. The pathologic distinction is not crucial, however, because all pheochromocytomas are generally removed because of their potentially "malignant" functional effects on the systemic circulation.
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Figure 16a. Malignant paraaortic paraganglioma in a 32-year-old woman. Consecutive axial images from a contrast-enhanced CT study show a large left paraaortic paraganglioma (arrows) with central necrosis and invasion of the contiguous aortic wall. Ten percent of pheochromocytomas are malignant.
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Figure 16b. Malignant paraaortic paraganglioma in a 32-year-old woman. Consecutive axial images from a contrast-enhanced CT study show a large left paraaortic paraganglioma (arrows) with central necrosis and invasion of the contiguous aortic wall. Ten percent of pheochromocytomas are malignant.
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Conclusions
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Pheochromocytomas are often clinically silent, and the diagnosis can sometimes be first made at imaging. Although an incidental adrenal nodule is unlikely to represent a pheochromocytoma, additional endocrine investigation is warranted if there is any clinical suspicion for this tumor. As with every imaging interpretation, radiologists should be cognizant of the individual patient’s clinical history and, in particular, of his or her blood pressure status. If an apparently asymptomatic mass has characteristic features of pheochromocytoma, endocrine assessment would be appropriate, since the clinical manifestation can sometimes be subtle. The imaging appearances of pheochromocytomas and paragangliomas are varied with all imaging modalities, primarily due to the multiple pathologic processes that they may undergo. Pheochromocytomas are usually solid when small but may appear as fatty, cystic, or calcified adrenal masses. They can be of sufficiently low attenuation at CT to be mistaken for an adenoma and may also mimic lipid-rich adenomas on unenhanced and washout studies. Despite the fact that most pheochromocytomas have high signal intensity on T2-weighted MR images, more than 30% have low signal intensity on these images. Although pheochromocytomas usually show increased uptake on FDG PET scans, results at conventional radionuclide imaging can also be inconsistent. However, new PET radioisotopes may allow more consistently accurate tests in the future, and these tests may soon play a larger role in the imaging evaluation of both incidental adrenal masses and suspected pheochromocytomas. Pheochromocytoma currently remains a true imaging chameleon, and its diagnosis should always be considered in the imaging evaluation of a patient with an adrenal mass, hypertension, or characteristic symptoms.
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Footnotes
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Abbreviations: FDG = 2-[fluorine 18]fluoro-2-deoxy-D-glucose,
MEN = multiple endocrine neoplasia,
MIBG = metaiodobenzylguanidine
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References
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Abstract
Introduction
