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Clinical Observations |
1 All authors: Department of Radiology, University of Pennsylvania School of Medicine, Hospital of the University of Pennsylvania, 3400 Spruce St., Philadelphia, PA 19104.
Received August 25, 2005;
accepted after revision October 25, 2005.
Address correspondence to W. T. Miller, Jr.
Abstract
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CONCLUSION. Paragangliomas of the aortic body and the great vessels have a characteristic imaging appearance. They originate from known sites of the branchiomeric paraganglia such as the aortic body. Knowledge of the location of the aortic body and other paraganglia combined with an appreciation of the how these tumors grow and displace neighboring structures will suggest a potential diagnosis of mediastinal paraganglioma.
Keywords: cancer • chest • CT • paraganglioma
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Paragangliomas of the aortic body are exceptionally rare, making up less than 1% of mediastinal tumors in some series [2]. Unlike pheochromocytomas, they are rarely functional, and clinical presentation is often delayed until compression of nearby structures causes pain or shortness of breath.
The rarity of aortic body paragangliomas is such that there have been no published series that characterize their cross-sectional imaging features. The purpose of our study is to show the unique appearance of these tumors on CT and MRI in four patients with aortic body paragangliomas and in one patient with a subclavian artery paraganglioma. By increasing awareness of the imaging appearance of these tumors, we hope to aid radiologists in making an earlier definitive diagnosis. Early diagnosis is of genuine benefit to the patient because these tumors are usually curable if complete surgical resection can be achieved. The major impediment to cure is invasion of the great vessels, which generally makes complete resection impossible.
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All five patients underwent thoracic CT examination on a variety of scanners, usually from outside institutions, with various imaging protocols. Slice thicknesses ranged from 5 to 10 mm. Iodinated contrast medium was used in all cases. The volume and type of IV contrast media is unknown in four of the five cases. In one case, 100 mL of 300 mg/mL iohexol (Omnipaque, Amersham Health) was used. One patient also underwent 1.5-T MRI (Signa, GE Healthcare) with axial T1-(TR/TE, 600/30) and T2-(2,400/80) weighted images through the mediastinum.
Images were reviewed and evaluated for the following tumor features: location, size, attenuation, enhancement, extrinsic mass effect on adjacent structures, and invasion of adjacent structures. When possible, a review of medical records was also performed to assess for synchronous or metachronous paragangliomas and for related syndromes.
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Smaller masses tended to have uniform soft-tissue attenuation on CT scans or signal intensity on MRI examinations (Figs. 2A, 2B and 3). Larger tumors were more likely to develop areas of lower attenuation thought to represent necrosis (Figs. 1 and 4). Enhancement was generally intense and homogeneous except for the presumed necrotic areas, which enhanced poorly (Figs. 1, 3, and 5). MR images revealed intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images, typical of paragangliomas seen in other locations [1] (Fig. 2A, 2B).
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The branchiomeric paraganglia are associated with arterial vessels and with the cranial nerves of the ontogenic gill arches. The carotid body; aortic body; and coronary, pulmonary, jugulotympanic, subclavian, and laryngeal paraganglia are all included in this category. The intravagal paraganglia are microscopic paraganglia located interior to the perineurium of the peripheral vagus nerve. The aortosympathetic paraganglia are associated with the segmental ganglia of the sympathetic chain, which are distributed bilaterally along the sympathetic chain from the superior cervical ganglion in the neck to the pelvis. The visceral-autonomic paraganglia are a poorly characterized group of paraganglia composed of microscopic cell groups found in the interatrial septum of the heart, the hilum of the liver, the bladder wall, and the mesenteric vessels.
The mediastinal paraganglia are predominantly concentrated in two locations: along the sympathetic chain of the posterior mediastinum, the aortosympathetic paraganglia, and along the great vessels including the aortopulmonary paraganglia, also known as the aortic body, which is part of the branchiomeric paraganglia.
Tumors arising from the aortosympathetic paraganglia will occur in the posterior mediastinum, whereas those arising from the aortopulmonary paraganglia will be intimately associated with the great vessels of the mediastinum.
The aortopulmonary paraganglia are chemoreceptors, which, like the closely related carotid body, sense fluctuations in blood pH, oxygen, and carbon dioxide partial pressure and temperature and influence homeostasis by altering ventilation and heart rate [5, 6]. The carotid body is more anatomically constant and larger than its aortic counterpart, the aortopulmonary paraganglia. The aortopulmonary paraganglia are characteristically found in one of five locations within the thorax (Fig. 6): between the ascending aorta and pulmonary trunk, either anteriorly or posteriorly, adjacent to the aortic root (also known as the coronary paraganglia); associated with the groove between the ductus arteriosus and the pulmonary artery (also known as the pulmonary paraganglia); between the right subclavian and right common carotid arteries; between the left subclavian and left common carotid arteries; or caudad to the left subclavian artery adjacent to the aortic arch. These latter four are also known as subclavian-supraaortic paraganglia [4, 7].
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Paragangliomas of the aortopulmonary paraganglia are exceptionally rare. They are outnumbered by carotid body paragangliomas by a factor of 15 and make up less than 1% of mediastinal tumors [2]. A few isolated case reports have been published. The largest series of aortic body paragangliomas, including four patients, dates from 1978 and contains no cross-sectional imaging [7]. A literature review of cases of aortopulmonary paraganglia in 1978 recorded a total of 36 cases in the English-language literature [5, 7].
Despite the rarity of paragangliomas, our study suggests that a suspicion of aortopulmonary paraganglia can be based on the crosssectional imaging appearance and location of the mass. Because the organ of origin occupies specific locations adjacent to the great vessels, these tumors characteristically occur in these locations. In our series, in four of five patients this was most often the anterior or posterior groove between the aortic arch and the pulmonary artery, the typical location of the aortic body. In the fifth patient, the tumor arose from another variably present paraganglion along the surface of the subclavian artery near the origin of the common carotid artery. The intimate association of the tumor to the great vessels is explained by the presence of the paraganglia in the vessel wall because it is ideally situated to monitor pH and partial gas pressures. These paraganglia are typically located in the branch points of vessels, and, therefore, it appears as though these tumors will typically fill the groove between vessels.
As in other reports of paragangliomas in other locations, the tumors in our study typically avidly enhance after administration of IV contrast media [1] (Figs. 1, 3, and 5). With increasing size, the tumor generally becomes increasingly heterogeneous with central areas of decreased attenuation, presumably due to central necrosis (Figs. 1 and 4).
Our single case with an MRI examination showed the tumor to be of intermediate signal intensity on T1-weighted sequences and of very high signal intensity on T2-weighted sequences relative to skeletal muscle (Fig. 2A, 2B). This is consistent with MRI findings for other paragangliomas, especially of the carotid body [1]. Paragangliomas have also been reported to display a salt-and-pepper appearance, referring to foci of hemorrhage (salt) and vessel flow voids (pepper), although this was not seen in our case.
Aortic body paragangliomas are accompanied by other synchronous paragangliomas in about 10% of cases [5]. Rarely, paragangliomas can occur as part of the syndrome known as Carney's triad in which they are associated with pulmonary chondroma and gastrointestinal stromal tumor [8], as seen in patient 3 of our series. Paragangliomas can also occur as part of a familial disorder [6].
In our series, one patient presented with hypertension and diaphoresis secondary to elevated catecholamine levels. However, in most cases aortic body paragangliomas do not secrete catecholamines in clinically relevant quantities [3]. Instead, clinical presentation is usually due to a complication of local mass effect such as pain, cough, dysphagia, hoarseness, or dyspnea, as seen in three of our five patients. In 50% of cases, the tumor is discovered incidentally on an imaging study performed for an unrelated indication. The most common age of onset is 45-50 years, although there are reports of diagnoses at all ages, including childhood [5].
There are no definite cellular characteristics that reliably differentiate malignant from benign paragangliomas. The presence of metastases is the only universally agreed marker for malignancy, which raises the clinical conundrum of distinguishing metastases from synchronous primary paragangliomas. The distinction is made on the basis of location, with metastases, which occur in approximately 15% of cases, arising from a location lacking paraganglia, whereas synchronous primary tumors will always arise from a location known to contain paraganglia. Cure is achievable by complete surgical resection in the nonmetastatic patient. However, in many cases, invasion of the great vessels prevents complete surgical resection, with a resultant mortality rate of approximately 40% [7].
In summary, we have shown that, although aortopulmonary paragangliomas are rare tumors, the diagnosis is suggested by visualization of a mass originating from the known site of an aortopulmonary paraganglion, most commonly in the groove between the aorta and the main pulmonary artery and extending out to splay, encase, or invade neighboring great vessels and associated structures. These masses typically display avid and uniform contrast enhancement when small, as with paragangliomas located elsewhere in the body, and heterogeneous enhancement when large, presumably due to necrosis. Timely definitive diagnosis may enhance the chances for cure by increasing the chances of success for complete surgical resection before invasion of the great vessels or heart renders the tumor inoperable.
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