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Original Article |
1 Department of Surgery, University of Washington, Box 356410, 1959 N.E. Pacific Street, Seattle, Washington 98195
2 Department of Radiology (Nuclear Medicine), University of Washington, Seattle, Washington 98195
3 Department of Medicine (Endocrinology), University of Washington, Seattle, Washington 98195
Correspondence: Address correspondence and reprint requests to: Gary N. Mann, MD; E-mail: gnmann{at}u.washington.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Methods: Fourteen patients with suspected pheochromocytoma were evaluated by anatomical imaging (computed tomography or magnetic resonance imaging) and [131I]metaiodobenzylguanidine (MIBG) planar imaging. PET imaging was performed by using mHED with dynamic adrenal imaging, followed by a torso survey and FDG with a torso survey. Images were evaluated qualitatively by an experienced observer.
Results: Eight patients had pathology-confirmed pheochromocytoma. Of the other six, two patients had normal adrenal tissue at adrenalectomy, and the other four had subsequent clinical courses inconsistent with a diagnosis of pheochromocytoma. In four of eight patients with pheochromocytoma, MIBG failed to detect one or more sites of pathology-confirmed disease. The mHED-PET detected all sites of confirmed disease, whereas FDG-PET detected all sites of adrenal and abdominal disease, but not bone metastases, in one patient. MIBG and FDG-PET results were all negative in the six patients without pheochromocytoma. One patient with adrenal medullary hyperplasia had a positive mHED-PET scan. PET scanning aided the decision not to operate in three of six patients. The resolution of PET functional imaging was superior to that of MIBG.
Conclusions: PET scanning for pheochromocytoma offers improved quality and resolution over current diagnostic approaches. PET may significantly influence the clinical management of patients with a suspicion of these tumors and warrants further investigation.
Key Words: Pheochromocytoma • Metahydroxyephedrine • Fluorodeoxyglucose • Positron emission tomography • Adrenal surgery • Laparoscopy
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Diagnosis of pheochromocytoma relies on the biochemical demonstration of excessive catecholamines or their metabolites in urine, blood, or both.1 Because treatment is surgical resection, once a clear determination of excessive catecholamine production has been made, localization of the tumor is required. Pheochromocytomas have been defined in surgical dogma by the "rule of tens." Ten percent of tumors have been traditionally believed to be bilateral, extra-adrenal, familial, or malignant. These numbers have recently been disputed.2,3 Nevertheless, they were the motivation for the past surgical approach of an extensive exploratory celiotomy with exploration of both adrenal glands, the para-aortic and para-caval chains, the region of the organ of Zuckerkandl, the base of the bladder, and the pelvis.
With the advent of laparoscopic adrenalectomy, the playing field has been dramatically altered. The laparoscopic approach is a completely directed one, and accurate localization of pheochromocytoma, whether unilateral or bilateral, adrenal or extra-adrenal, has become even more important. Currently, the standard approach is anatomical imaging with computed tomography (CT), magnetic resonance imaging (MRI), or both, coupled with functional imaging by using planar metaiodobenzylguanidine (MIBG) scanning. Single-photon emission computed tomography (SPECT) MIBG scanning has excellent specificity but is still hampered by a lack of resolution, imperfect sensitivity, and subjective poor image quality. Some have argued that in patients with a firm biochemical diagnosis of pheochromocytoma and a unilateral adrenal mass on CT or MRI, a functional study may be unnecessary.4
Positron emission tomography (PET) scanning, usually with [18F]fluorodeoxyglucose (FDG), is becoming a more common imaging modality in oncology and even in the imaging of endocrine tumors. It has shown value in the localization and prognostic determination of thyroid cancer.5 PET scanning allows accurate qualitative functional imaging with excellent image resolution. It also allows quantitation of results. Several groups have investigated its role in the management of pheochromocytoma by using a variety of tracers.6–11 [11C]Metahydroxyephedrine (mHED) is a norepinephrine analogue and was the first positron-emitting probe of the sympathoadrenal system administered in humans.12,13 As such, it holds promise for imaging catechol-secreting tumors such as pheochromocytoma and neuroblastoma.14 FDG-PET has been of varying utility for pheochromocytoma; however, some studies have suggested that malignant pheochromocytomas have enhanced visualization compared with nonmalignant tumors.9 We undertook this study to determine whether PET, by using a combination of FDG and mHED in confirmed or suspected pheochromocytoma, would allow more definitive diagnosis or improved localization. Our hypothesis was that the combination of a specific agent (mHED) and a nonspecific marker associated with more aggressive tumors (FDG) would be able to localize pheochromocytoma with a broad range of biologic characteristics and that the biological characteristics of the tumors would be reflected by the level of uptake of the two tracers.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). Suspicion was based on a family history, clinical presentation, or an incidental adrenal mass on anatomical imaging. All patients underwent 24-hour urine collection for catecholamines, metanephrines, and/or vanillylmandelic acid. Several patients also had measurement of plasma metanephrines or catecholamines at the discretion of the treating physician. All patients underwent anatomical imaging with CT, MRI, or both. On the basis of the collective information, they were referred to the nuclear medicine clinic at the University of Washington for functional imaging of pheochromocytoma with [131I]MIBG and PET scan by using FDG and [11C]mHED. Subsequent treatment decisions were generally made in a multidisciplinary fashion with input from University of Washington Medical Center endocrine/oncological surgeons, endocrinologists, and nuclear medicine physicians. Biochemical data and conventional imaging studies (CT, MRI, and MIBG scintigraphy) were used to formulate clinical decisions according to clinical routine. Referring physicians were not blinded to the PET imaging results; however, they were cautioned about the experimental nature of the study and the uncertain significance of the results. The extent to which the PET results were included in the final treatment decision was left to the discretion of the referring physician and the endocrine tumor team.
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Biochemical Studies
At least one 24-hour urine analysis for fractionated catecholamines, metanephrines, or both was performed on each of the subjects before imaging studies. The choice of laboratory for these studies was at the referring physician’s discretion. Ten of the 14 subjects also had plasma metanephrine studies (metanephrine and normetanephrine). When performed, these were performed by the Mayo Medical Laboratories (the normal range for metanephrine is <.50 nmol/L, and for normetanephrine it is <.90 nmol/L). Four patients also had serum catecholamine studies; however, these had a role in the decision to proceed with PET studies in only one subject (patient 12).
Anatomical Imaging
All 14 patients underwent abdominal CT scan, with or without contrast, and/or contrast-enhanced MRI within 3 months of PET imaging. If patients were referred from outside institutions with adequate anatomical images of the adrenal glands, the studies were not repeated. CT scans at our institution were performed by using spiral scanners with slice spacing of 5 mm. All subjects received noncontrast studies, and some subjects in whom diagnoses other than pheochromocytoma were suspected also underwent scans after intravenous contrast injection. In addition to CT, eight subjects underwent MRI, which included T1- and T2-weighted images before contrast and a T1-weighted image after intravenous gadolinium-di-ethylene-triamine-pentaacetic acid contrast, acquired in both the axial and coronal planes. CT and MRI scans underwent prospective clinical interpretation by board-certified radiologists without information from the PET studies. The CT and MRI scans were also used to provide correlative anatomical information in the interpretation of the PET studies.
Radiopharmaceuticals
FDG was made by the Hamacher method.15 The [11C]mHED was made according to the method of Rosenspire et al.13 The [11C]mHED product was purified by semipreparative high-performance liquid chromatography by using an Inertsil ODS-2 C18 column (Metachem, Torrance, CA) 10 x 250 mm, 5-µm particles, eluted with .15 M of phosphate-buffered saline (USP):ethanol (USP) (95:5 v/v). Chemical and radiochemical purity were assessed by using a C18 column (betabasic 2.1 x 150 mm) eluted with .8 mM of formate (pH 7):methanol (95:5 v/v) with radioactivity and mass spectral (ES+) detection. The [11C]mHED was >95% pure with <10 µg of mHED per injection.
MIBG Scintigraphy
MIBG scintigraphy was performed after an injection of .5 mCi of [131I]MIBG, followed by whole-body planar imaging by using a dual-head gamma camera (VG SPECT Tomograph; General Electric Medical Systems, Waukesha, WI) at 24 and 48 hours. Acquisition was performed by using a continuous sweep at 5 cm/min. To prevent thyroid uptake of radioactive iodine and subsequent dysfunction, Lugol’s solution was administered as three drops in water or juice three times daily (or saturated solution of potassium iodide, one drop in water or juice three times a day). This was begun one day before dosing and continued for 15 days afterward. Laxatives were given between the 24-and 48-hour images for bowel clearance.
PET Imaging
All PET studies were performed by using a two-dimensional acquisition mode (Advance PET tomograph; General Electric Medical Systems). In most instances, [11C]mHED and [18F]FDG PET studies were performed in a single imaging session to provide precise coregistration. In three patients, FDG and mHED PET studies were performed on consecutive days because of scheduling constraints. To clear the urinary background, a Foley catheter was placed before imaging. Intravenous fluids (500 mL of normal saline) were given to ensure proper hydration and urine flow. For the FDG study, in which renal uptake and urinary excretion of tracer cause a more significant background, intravenous furosemide (20 mg) and dilution of the bladder contents with sterile water were used to clear renal and urinary FDG. Patients were encouraged to undergo conscious sedation with oral lorazepam to improve comfort.
The mHED scan consisted of early dynamic imaging over the adrenal glands, followed by a torso survey from the pelvis to the upper neck, covering the potential sites of extra-adrenal pheochromocytoma. On the General Electric Advance tomograph, this typically required five bed positions. The adrenal bed position was identified by reference to the diaphragm on the transmission study. After the positioning scans, short transmission scans of each bed position were acquired by using rotating germanium 68 rod sources. [11C]mHED .2 mCi/kg, up to a maximum of 20 mCi, was infused over 1 minute. The initial 25 minutes consisted of dynamic imaging at the level of the adrenals. After the dynamic imaging, the torso survey was performed, with increasing acquisition times from 7 to 11 minutes per bed position to account for 11C decay. The total patient scan time did not exceed 65 minutes.
After the completion of the mHED study, a 1-minute infusion of 10 mCi of [18F]FDG was administered. Dynamic imaging was again performed over the adrenals. The patient remained on the imaging table to maintain alignment with the mHED scan. Starting at 45 minutes after injection, a torso survey was performed, with 7-minute emission scans per bed position. This was followed by 3-minute postinjection transmission studies per field of view. The total imaging time was 175 minutes.
After correction for scattered and random coincidences, PET projection data were reconstructed by using filtered back-projection and a Hanning filter onto a 128 x 128 x 35 matrices per 15 cm axial field of view. The approximate reconstructed image spatial resolution was 10 mm. For viewing, dynamic data were summed from 15 to 25 minutes for mHED and 30 to 60 minutes for FDG.
Data and Statistical Analysis
MIBG and PET images were analyzed qualitatively; PET images also underwent quantitative analysis. MIBG and PET images were interpreted qualitatively and prospectively by a single observer with extensive experience in MIBG scintigraphy and PET. The interpretation considered clinical and anatomical imaging data, according to clinical routine; however, the interpreter did not have knowledge of subsequent pathology results or the clinical course at the time of image interpretation. Quantitative analysis was emphasized for PET imaging. For each radio-pharmaceutical, the maximal standard uptake value (SUV) for each suspected adrenal focus was measured in each patient by using the standard formula:
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The fraction of pathology-proven pheochromocytoma detected by MIBG scintigraphy and mHED and FDG PET was calculated. The SUVs for adrenal foci proven to be pheochromocytoma versus those without pathology confirmation were compared and assessed for significant differences between groups by using Student’s t-test. P < .05 was considered significant.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). Eight patients were ultimately found to have pheochromocytoma, and six had pheochromocytoma excluded with further testing and follow-up. The high number of patients imaged who were without pheochromocytoma is indicative of the clinical difficulty of arriving at a definitive diagnosis of pheochromocytoma and of the challenging population referred to by our study. Hypertension was found in nine patients, only six of whom had pheochromocytoma. Two patients had incidental adrenal masses noted on CT scans performed for symptoms unrelated to pheochromocytoma. Patient 3 underwent abdominal CT for nephrolithiasis, and his pheochromocytoma was discovered as a large asymptomatic left adrenal mass. Patient 11 was found to have pheochromocytoma after evaluation for unrelated thoracic chest pain led to a chest CT, which included cuts through the adrenal glands. Two of the three patients with familial pheochromocytoma had bilateral tumors (one with von Hippel–Lindau syndrome [patient 1] and 1 with multiple endocrine neoplasia 2A [patient 14]). One patient had multifocal pheochromocytoma at presentation (patient 2), and two patients with unilateral sporadic pheochromocytoma have recurrent disease and were scanned as part of their metastatic work-up (patients 8 and 13).
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Four of the six patients without pheochromocytoma had definite or borderline increases of their urinary catecholamines, metabolites, or both, with 24-hour metanephrines 1.1- to 1.8-fold the ULN and normetanephrines 1.9- to 5.0-fold the ULN. The remaining two had normal levels. Plasma metanephrines were obtained in three of six patients. Metanephrines were normal in all, whereas normetanephrine ranged from 1.28 to 3.71 nmol/L.
Clinical Outcomes
Six patients with nonmetastatic pheochromocytoma were imaged per protocol before adrenalectomy. Patient 1 had bilateral tumors found only on PET and underwent bilateral laparoscopic adrenalectomy (Fig. 2A–C). Patient 5 had negative MIBG but positive PET and underwent a unilateral laparoscopic adrenalectomy. The remaining four patients had unilateral sporadic pheochromocytoma, with concordant anatomical and functional imaging.
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In the two patients imaged for suspected recurrent pheochromocytoma after prior resection of adrenal pheochromocytoma, PET imaging proved to be accurate and useful in their clinical management. Patient 8 was found to have bone metastases that were imaged most conspicuously with mHED PET. Her metastases were not visible by CT or MIBG scan. Bony disease was confirmed by bone marrow aspiration. Subsequent confirmation of disease progression by PET resulted in a change of chemotherapy regimen (Fig. 3A–C). Patient 13 had a persistent pulmonary nodule by CT after chemotherapy for metastatic pheochromocytoma. PET confirmed active disease, and the patient was ultimately referred for bilateral pulmonary metastasectomy on the basis of his metastatic survey.
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, which shows uptake in a patient with right adrenal pheochromocytoma (patient 5) compared with blood clearance (aorta) and adjacent tissues (kidney and liver). Uptake in the pheochromocytoma was rapid and relatively constant over the 25 minutes of dynamic imaging. The uptake in the pheochromocytoma was higher than in the surrounding tissues and distinct from the adjacent kidney by approximately 15 minutes after injection, thus indicating the optimal torso survey scan start time.
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. SUVs (mean) for FDG-PET scans for pheochromocytoma and normal adrenals were 6.1 (range, 2.1–13.3) and 3.4 (range, 2.8–4.2), respectively. SUVs (mean) for mHED-PET scans for pheochromocytoma and normal adrenal glands were 20.7 (range, 5.3–65.0) and 8.0 (range, 7.2–8.5), respectively. The level of uptake of FDG and mHED was considerably higher for pheochromocytoma versus no pheochromocytoma. For both tracers, differences in SUV for pheochromocytoma versus normal adrenal were of borderline statistical significance (P = .05), despite small numbers and a wide range in pheochromocytoma uptake. It is important to note that the distinction between pheochromocytoma and normal adrenal was somewhat better for mHED than for FDG. All pheochromocytomas except one had an mHED SUV of 
9, even for lesions <1 cm. The sole exception was a large, necrotic pheochromocytoma with progressive loss of mHED over the course of imaging.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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We found all patients with biopsy-proven pheochromocytoma to have uptake on mHED-PET scanning. Pheochromocytoma was more accurately detected with our PET imaging protocol compared with standard anatomical imaging and [131I]MIBG. Bilateral and extra-adrenal disease were detected in several patients by PET but not by anatomical imaging or MIBG scintigraphy. This had important beneficial clinical consequences for these patients. Even in the one case that we scored as a false positive on mHED-PET, final pathologic analysis revealed adrenal medullary hyperplasia that potentially contributed to the increased mHED uptake. This is an as-yet unproven interpretation.
There are limitations to this study. Patient numbers were small and, as such, are insufficient to calculate diagnostic sensitivity and specificity. Our numbers are, however, comparable to those in other studies that have evaluated PET scanning of pheochromocytoma by using several radiopharmaceuticals. This study looked at all patients undergoing PET scanning for suspected pheochromocytoma, including some who did not, in fact, have a pheochromocytoma. As noted in our study, even some patients without a pheochromocytoma had increases in their catecholamines or metabolites, thus making its exclusion difficult at times. Forty-three percent (6 of 14) of patients in this study did not have pheochromocytoma at the time of imaging. An important finding was that PET imaging was very useful in helping to exclude pheochromocytoma in most of these patients, thus avoiding surgery or potentially morbid therapy. Patient follow-up is limited (<3 years); therefore, rates of recurrence or subsequent pheochromocytoma development may be an underestimation. All patients tolerated PET scanning well.
Two of the major issues with novel imaging agents are the availability of the agent and the generaliz-ability of the results. [11C]mHED has a short half-life (20 minutes) and requires an on-site cyclotron, which limits its widespread use. There are several centers in the United States with such a capacity, and regionalized imaging of these tumors is possible. More important is the proof of concept that PET imaging with a combination of radiopharmaceuticals may improve detection of pheochromocytoma and possibly help to predict its behavior. Although the half-life of 11C limits its more generalized application, its use does allow single-session imaging with other 18F-based radiotracers, such as FDG, because of their longer half-lives. Conceivably, this dual study may allow improved detection and localization in difficult cases, although this would admittedly be the exception. What is compelling is the potential use of combined agents in helping to predict behavior and outcome in a fashion analogous to what is seen in thyroid cancer.5 Because the malignant phenotype of pheochromocytoma is predicated on its biology, i.e., recurrence or metastasis, and not its histological characteristics, a secondary goal is to ultimately see whether the combination of tracers will allow us to better predict the behavior of pheochromocytoma. The use of two 18F-based tracers would necessitate multiple imaging days, thus obviating one of the benefits of dual tracer imaging with 11C and 18F radioisotopes. An adequate test of this hypothesis requires larger patient numbers, with more long-term follow-up, and active further study of this is ongoing. Only one study has evaluated the use of FDG-PET in imaging benign and malignant pheochromocytoma.9 Even though more malignant pheochromocytomas than benign pheochromocytomas were detected, quantitative SUVs between the two biologically different tumors were similar.
Two other small series have reported the use of [11C]mHED in imaging of pheochromocytoma. In an initial series, using an earlier PET tomograph with relatively thick image slices, Shulkin et al.10 were able to localize pheochromocytoma in 9 of 10 patients who were imaged. Compared with [131I]MIBG, PET scanning detected more lesions with greater contrast and in a much more timely fashion. The single false-negative result was in a patient whose tumor secreted dopamine and little norepinephrine or epinephrine, in keeping with mHED’s mechanism of action, which reflects catecholamine uptake and storage, rather than synthesis. In a more contemporary series with 19 patients, Trampal et al.11 found equally impressive results with detecting pheochromocytoma by using [11C]mHED. Sensitivity was found to be 92%, specificity was 100%, and accuracy was 95%. The single patient with a false-negative study had a tumor that secreted both norepinephrine and epinephrine, unlike the patient in the Shulkin study. Our study had no false negatives and only a single false positive (in the patient with adrenal medullary hyperplasia).
Other PET radiotracers have been used to image and localize pheochromocytoma, most of them labeled with 18F. Pacak et al.7 demonstrated a high positive and negative predictive value of PET scanning of pheochromocytoma with [18F]fluorodopamine, noting positive scans in all 17 patients with pheochromocytoma and negative scans in 9 of 11 patients without pheochromocytoma. A follow-up study by the same authors of 16 patients with meta-static pheochromocytoma confirmed the superiority of [18F]fluorodopamine to [131I]MIBG; that study found disease in more patients and in more sites.19 In a series of 14 patients, Hoegerle et al.6 found that the novel radiotracer [18F]dihydroxyphenylalanine had a higher sensitivity than [123I]MIBG for detecting pheochromocytoma (100% vs. 71%). These results await confirmation in a larger series and more widespread availability of these radiotracers.
There are many advantages of PET scanning over [131I]MIBG. PET exposes patients to lower radiation doses than [131I]MIBG. Because PET uses no iodine, there are no adverse effects on the thyroid, and thyroid blocking is not required. Patient throughput is significantly faster with PET, because scans can be obtained immediately or within a few hours of administration of the analogue. [131I]MIBG requires delayed scans at 24, 48, and sometimes 72 hours. Finally, and importantly, PET scanning is superior in its spatial resolution, as opposed to the planar imaging of [131I]MIBG. Improved-quality images with [123I]MIBG are possible,20 but image resolution for SPECT is inferior to that for PET, and image quantification, which was important in our study, is not accurate with SPECT.
In conclusion, this study demonstrates the feasibility and usefulness of FDG-PET and especially mHED-PET scanning in the high-quality efficient detection of pheochromocytoma. Results suggest that PET imaging may play a crucial role in surgical approach and outcome. In certain patients, these techniques may also prove invaluable in excluding the diagnosis of pheochromocytoma and, hence, avoiding unnecessary surgery. PET scanning with specific tracers for the localization of pheochromocytoma may well become the future functional imaging modality of choice.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Received for publication April 25, 2005. Accepted for publication August 11, 2005.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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