Pheochromocytoma Workup

The Endocrine Society, the American Association for Clinical Chemistry, and the European Society of Endocrinology have released clinical practice guidelines for the diagnosis and management of pheochromocytoma and paraganglioma (jointly referred to as PPGL). ^{[34, 35]}

• Biochemical testing via measurement of plasma free metanephrines ^[36] or urinary fractionated metanephrines should be performed in patients suspected of having PPGL.
• Patients with a known germ-line mutation that predisposes to PPGL should undergo periodic biochemical testing.
• Computed tomography (CT), rather than magnetic resonance imaging (MRI), is recommended as the first-line imaging technique.
• Blood pressure, heart rate, and glucose levels should be monitored immediately after surgery.
• Patients with PPGLs should participate in shared decision-making for genetic testing.

Catecholamines produced by pheochromocytomas are metabolized within chromaffin cells. Norepinephrine is metabolized to normetanephrine and epinephrine is metabolized to metanephrine. Because this process occurs within the tumor, independently of catecholamine release, pheochromocytomas are best diagnosed by measurement of these metabolites rather than by measurement of the parent catecholamines. ^[37]

Guidelines from the North American NeuroEndocrine Tumor Society (NANETS) recommend biochemical testing for pheochromocytoma in the following cases ^[37] :

• Symptomatic patients
• Patients with an adrenal incidentaloma
• Patients who have a hereditary risk for developing a pheochromocytoma or paraganglioma (extra-adrenal pheochromocytoma)

The choice of diagnostic test should be based on the clinical suspicion of a pheochromocytoma. Plasma metanephrine testing has the highest sensitivity (96%) for detecting a pheochromocytoma, but it has a lower specificity (85%). ^[38]  In comparison, a 24-hour urinary collection for catecholamines and metanephrines has a sensitivity of 87.5% and a specificity of 99.7%. ^[39]

General laboratory features of pheochromocytoma include the following:

• Hyperglycemia
• Hypercalcemia
• Erythrocytosis

Imaging studies should be performed only after biochemical studies have confirmed the diagnosis of pheochromocytoma. Computed tomography (CT) scanning or magnetic resonance imaging (MRI) can be used for detection of the disorder. Scintigraphy may be used when these techniques fail to localize the tumor. Positron emission tomography (PET) scanning has shown promising results as an imaging modality for pheochromocytoma.

High-risk patients, including those who have a genetic syndrome that predisposes them to pheochromocytoma (eg, multiple endocrine neoplasia [MEN] types 2A or 2B, von Hippel-Lindau [VHL] disease, neurofibromatosis, a prior history of a pheochromocytoma, a family history of a pheochromocytoma), should be screened with plasma metanephrine testing. In these scenarios, a higher-sensitivity test that lacks specificity is justified. ^[40]

A fractionated plasma free metanephrine level may be measured in a standard venipuncture sample, drawn about 15-20 minutes after intravenous catheter insertion. Positioning of the patient for the venipuncture is controversial. Although some experts advocate having the patient seated, NANETS guidelines recommend drawing the sample with the patient in a supine position, as tests in seated patients have a higher false-positive rate. ^[37]

Perform a 24-hour urine collection for creatinine, total catecholamines, vanillylmandelic acid, and metanephrines. Measure creatinine in all collections of urine to ensure adequacy of the collection. The collection container should be dark and acidified and should be kept cold to avoid degradation of the catecholamines. Optimally, collect urine during or immediately after a crisis.

Some authors have reported good experience with evaluating epinephrine and norepinephrine separately (in part to confirm the total catecholamine level and in part to determine whether levels reflect the high norepinephrine-to-epinephrine ratio expected). Separate measurement of metanephrine and normetanephrine, to confirm the total metanephrine level and the normetanephrine-to-metanephrine ratio, has also proved useful.

Although dopamine is a major catecholamine, measurement of dopamine levels in 24-hour urine is not useful, because most urinary dopamine is derived from renal extraction.

Factors affecting test results

Major physical stress may interfere with the assay and cause false elevations of metanephrines and normetanephrines. Ethanol and multiple prescription drugs, including the following, may also cause such results:

• Tricyclic antidepressants
• Phenoxybenzamine
• Levodopa
• Beta blockers ^[41]
• Labetalol
• Amphetamines
• Buspirone
• Methyldopa
• Chlorpromazine

Other compounds decrease 24-hour urine levels of metanephrines. These include methyltyrosine, which inhibits tyrosine hydroxylase, the rate-limiting enzyme in catecholamine synthesis, and methylglucamine, which is present in radiocontrast media.

Provocative testing was used in the past to confirm or exclude pheochromocytoma. However, such testing can cause dangerous hypertensive episodes. In addition provocative testing with glucagon has been shown to have less than 50% sensitivity. ^[42]

Suppression tests using phentolamine and clonidine can also be used for diagnostic purposes. NANETS guidelines recommend clonidine suppression testing when plasma metanephrine values are less than 4-fold above the upper reference limit. ^[37]

Chromogranin A is an acidic monomeric protein that is stored with and secreted with catecholamines. Plasma levels of chromogranin A reportedly are 83% sensitive and 96% specific for identifying a pheochromocytoma. Chromogranin A levels are sometimes used to detect recurrent pheochromocytomas.

Abdominal CT scanning has an accuracy of 85-95% for detecting adrenal masses with a spatial resolution of 1 cm or greater but is less accurate for lesions smaller than 1 cm. Differentiating an adenoma from a pheochromocytoma is more difficult using CT scanning. While most pheochromocytomas have CT attenuation of greater than 10 Hounsfield units (HU), they rarely contain sufficient intracellular fat to have an attenuation of less than 10 HU. ^[43]  In fact, a retrospective study by Buitenwerf et al found that out of 222 pheochromocytomas, only one had an attenuation value at or below 10 HU on unenhanced CT scanning, indicating a sensitivity of 99.6% for the 10 HU threshold. ^[44]

However, most pheochromocytomas, although hypervascular, have variable enhancement loss that may, in some cases, be similar to that of adrenal metastases but in others may be similar to enhancement loss of adrenal adenomas. ^[45]  Therefore, in patients in whom pheochromocytomas are strongly suspected, adrenal pheochromocytomas cannot be entirely excluded from the list of differential diagnoses of adrenal neoplasms with an attenuation value of less than 10 HU and a washout of greater than 60% on delayed scanning.

Although it has been thought that the use of intravenous contrast poses a risk of inducing hypertensive crisis in patients with pheochromocytomas, a controlled, prospective study in patients receiving low-osmolar CT-scan contrast ^[46]  and a retrospective review in patients who received nonionic contrast ^[47]  concluded that this use of intravenous contrast is safe, even in patients not receiving alpha or beta blockers. A CT scan of a paraganglioma appears below.

Abdominal computed tomography (CT) scan demonstrating left suprarenal mass of soft-tissue attenuation representing a paraganglioma.

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MRI is preferred for detection of pheochromocytoma in children and in pregnant or lactating women. MRI has a reported sensitivity of up to 100% in detecting adrenal pheochromocytomas, does not require contrast, and does not expose the patient to ionizing radiation. MRI is also superior to CT scanning for detecting extra-adrenal pheochromocytomas.

In approximately 70% of cases, pheochromocytomas appear hyperintense on T2-weighted images (as demonstrated in the image below), because of their high water content. ^[48]

Axial, T2-weighted magnetic resonance imaging (MRI) scan showing large left suprarenal mass of high signal intensity on a T2-weighted image. The mass is a pheochromocytoma.

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Initial studies have suggested that MR spectroscopy can be used to distinguish pheochromocytomas from other adrenal masses. ^{[49, 50]}  Specifically, a resonance signature of 6.8 ppm appears to be unique to pheochromocytomas; the signature apparently is attributable to the catecholamines and catecholamine metabolites present in pheochromocytomas. ^[50]

A scan with iodine-123 (¹²³I)–labeled metaiodobenzylguanidine (MIBG) is reserved for cases in which a pheochromocytoma is confirmed biochemically but CT scanning or MRI does not show a tumor. MIBG is a substrate for the norepinephrine transporter and concentrates within adrenal or extra-adrenal pheochromocytomas. MIBG scanning is frequently used in cases of familial pheochromocytoma syndromes, recurrent pheochromocytoma, or malignant pheochromocytoma.

Estimates of the sensitivity and specificity of ¹²³I-MIBG vary widely. Reported sensitivity ranges from 53-94% and specificity ranges from 82-92%. ^{[51, 52, 53]  123}I has a short half-life and is expensive.

A somatostatin receptor analog, indium-111 (¹¹¹In) pentetreotide, is less sensitive than MIBG. However, it may be used to visualize pheochromocytomas that do not concentrate MIBG.

PET Scanning

PET scanning with ¹⁸F-fluorodeoxyglucose (18F-FDG), which is selectively concentrated as part of the abnormal metabolism of many neoplasms, has been demonstrated to detect occult pheochromocytomas. Pheochromocytomas usually show increased uptake on PET scanning, as do adrenal metastases. In SDHB-related metastatic paraganglioma, 18F-FDG-PET has a sensitivity approaching 100%. ^[54]

The most impressive results to date have been with 6-[18F]-fluorodopamine (FDOPA) PET scanning and carbon-11 hydroxyephedrine (¹¹C-HED) PET scanning. Studies suggest that scans performed with these radioisotopes are extremely useful in the detection and localization of pheochromocytomas. Further study results with these agents are eagerly awaited.

FDOPA

FDOPA, an amino acid precursor, is preferentially stored by neuroendocrine tumors, in which it has been shown to accumulate more readily than does FDG or somatostatin analogues. In a study of 30 consecutive patients presenting to a tertiary care center with suspected pheochromocytoma or paraganglioma, Fottner et al estimated the sensitivity and specificity of FDOPA-PET scanning to be 98% and 100%, respectively. Sensitivities were 94% for adrenal and extra-adrenal abdominal lesions and 100% for thoracic/cervical lesions. ^[52]

In a comparison of FDOPA-PET scanning combined with CT scanning versus FDOPA-PET or CT scanning alone in images from 25 consecutive patients with suspected pheochromocytoma, 19 lesions were detected by all 3 modalities. All 19 were identified positively as pheochromocytoma by PET/CT and PET scanning, whereas CT scanning produced one false-negative finding. PET/CT and CT scanning also definitively localized all lesions, but PET scanning alone definitively localized only 15 lesions. Overall, the authors estimated the sensitivity and specificity of FDOPA-PET/CT scanning to be 100% and 88%, respectively. ^[55]

HED

HED is a catecholamine substrate analogue that is transported into sympathetic neurons by the norepinephrine transporter and stored in vesicles. Yamamoto et al reported 91% sensitivity and 100% specificity for pheochromocytoma by HED-PET scanning. HED accumulation, as measured by maximum standardized uptake value (SUV_max), was higher in metastases and in the presence of sympathetic symptoms and correlated significantly with biochemical findings (plasma normetanephrine, urinary norepinephrine). ^[56]

Once the diagnosis of pheochromocytoma is made, additional studies to rule out a familial syndrome may be indicated. Testing for every possible gene would be inappropriate and expensive; however, the following information can suggest which genetic tests to select ^[37] :

• Biochemical profile of catecholamine secretion
• Age of the patient
• Site of the primary tumor
• Family history

Profiles of plasma catecholamine metabolites in patients with hereditary pheochromocytoma that can serve as a guide to genotype testing include the following ^{[10, 37]} :

• Increased metanephrine (epinephrine metabolite): All patients with MEN 2 and neurofibromatosis type 1 (NF1)
• Increased normetanephrine only (norepinephrine metabolite): Most patients with VHL
• Increased methoxytyramine (dopamine metabolite): 70% of patients with SDHB and SDHD mutations; increased normetanephrine may also be present

Eisenhofer et al found that the combination of increased normetanephrine and metanephrine differentiated patients with NF1 and MEN 2 from those with VHL, SDHB mutations, and SDHD mutations in 99% of cases. ^[10]

In MEN 2, VHL, and NF1, pheochromocytomas are almost always in the adrenal gland, whereas SDHB -related tumors are found in extra-adrenal sites. Patients with MEN 2, VHL, and NF1 often have a positive family history, but currently only 10% of patients with SDHB mutations have a positive family history for pheochromocytoma or paraganglioma. ^[40]

Perform screening for mutations in the ret proto-oncogene in any patient with a familial syndrome or to distinguish a sporadic pheochromocytoma from a familial pheochromocytoma. ^[16]  Mutation analysis involves amplification of sequences, including exons 10, 11, 13, 14, and 16 of the ret proto-oncogene from the patient's genomic deoxyribonucleic acid (DNA), followed by sequence analysis.

Particular attention is given to specific sequences for the codons known to be hot spots for mutations causing the MEN 2A and 2B syndromes. Over 95% of cases of MEN 2A and 85% of cases of familial medullary thyroid cancer are associated with mutations affecting 1 of 5 codons located in exons 10 (codon 609, 611, 618, and 620) and 11 (codon 634). Over 95% of individuals with MEN 2B have a germline mutation in codon 918 of exon 16.

In patients with MEN 2A, also obtain a serum intact parathyroid hormone level and a simultaneous serum calcium level to rule out primary hyperparathyroidism (which occurs in MEN 2A). Obtain a serum calcitonin level as well. Some investigators advocate a pentagastrin infusion test; however, genetic screening tests for the ret proto-oncogene may eliminate the need for this provocative test.

In patients with VHL disease, obtain a consultation with an ophthalmologist to rule out retinal angiomas, and consider brain MRI to exclude cerebellar hemangioblastomas. Obtain a CT scan of the kidneys and pancreas to rule out cysts.

Venous Sampling and Arteriography

Because of the high sensitivity of MRI and CT scanning, procedures are rarely indicated for localization of pheochromocytomas. Selective venous sampling is seldom performed to localize pheochromocytomas but has occasionally been used to detect extra-adrenal pheochromocytomas that were not found at surgery.

In most cases, this procedure is not helpful in detecting extra-adrenal tumors, because catecholamine levels have marked variability. An exception to this rule, however, occurs if the norepinephrine concentration is greater than the epinephrine concentration in the venous effluent. Because the primary catecholamine produced and stored in the adrenal gland is epinephrine, a ratio of norepinephrine to epinephrine that is greater than 1 suggests a pheochromocytoma.

Arteriography is rarely indicated. It provides little additional information compared with MRI or CT scanning, and the direct use of intra-arterial contrast may induce a hypertensive crisis.

Pheochromocytomas vary from 2 g to 3 kg but, on average, weigh 100 g (a healthy adrenal gland weighs 4-6 g). These tumors are well encapsulated, highly vascular, and appear reddish brown on cut section.

Histologically, the tumor cells are arranged in balls and clusters separated by endothelium-lined spaces; this classic pattern characteristic of pheochromocytoma is termed zellballen. The cells vary in size and shape and have finely granular basophilic or eosinophilic cytoplasm. The nuclei are round or oval with prominent nucleoli. Nuclear gigantism and hyperchromasia are common and do not portend prognosis.