(Radiology. 1999;213:553-554.)
© RSNA, 1999
The Comet Tail Sign1
Vince A. Partap, MD
From the Department of Diagnostic Radiology, Royal Victoria Hospital, 687 Pine Ave West, Montreal, Quebec H3A 1A1, Canada. Received April 7, 1998; revision requested June 29; revision received August 19; accepted December 28. Address reprint requests to the author (e-mail: vparta@po-box.mcgill.ca).
Index terms: Asbestosis, 60.773 • Lung, collapse, 60.749 • Lung, CT, 60.1211 • Lung, diseases, 60.749 • Signs in imaging
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APPEARANCE
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The comet tail sign is a finding that can be seen on conventional tomographic and computed tomographic (CT) scans of the chest (1–4). It consists of a curvilinear opacity that extends from a subpleural "mass" toward the ipsilateral hilum (Figs 1, 2).
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Figure 1. Comet tail sign in a 62-year-old man. Conventional tomographic scan of the chest in a lateral projection shows a large subpleural mass (arrowhead) in the right lower lobe of the lung. A curvilinear opacity (arrow), the comet tail sign, arises from the inferior pole of the mass and courses toward the hilum.
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Figure 2a. Round atelectasis in an asymptomatic 66-year-old man with a history of asbestos exposure. (a) Axial 2-mm, nonenhanced CT scan of the chest demonstrates a mass (short arrow) in the right lower lobe of the lung. Bronchovascular bundles (long arrow) converge into the mass in a curvilinear fashion. Pleural thickening (arrowhead) is present. The major fissure (open arrow) is displaced posteriorly, signifying volume loss in the right lower lobe. (b) Sequential 2-mm, nonenhanced CT scans demonstrate the comet tail sign. Bronchovascular bundles (solid arrow) appear to be pulled into the mass in a curvilinear fashion. Pleural thickening (arrowhead) is present. The major fissure (open arrow) is displaced posteriorly owing to the volume loss in the right lower lobe.
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Figure 2b. Round atelectasis in an asymptomatic 66-year-old man with a history of asbestos exposure. (a) Axial 2-mm, nonenhanced CT scan of the chest demonstrates a mass (short arrow) in the right lower lobe of the lung. Bronchovascular bundles (long arrow) converge into the mass in a curvilinear fashion. Pleural thickening (arrowhead) is present. The major fissure (open arrow) is displaced posteriorly, signifying volume loss in the right lower lobe. (b) Sequential 2-mm, nonenhanced CT scans demonstrate the comet tail sign. Bronchovascular bundles (solid arrow) appear to be pulled into the mass in a curvilinear fashion. Pleural thickening (arrowhead) is present. The major fissure (open arrow) is displaced posteriorly owing to the volume loss in the right lower lobe.
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EXPLANATION
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The comet tail sign is produced by the distortion of vessels and bronchi that lead to an adjacent area of round atelectasis, which is the mass. The bronchovascular bundles appear to be pulled into the mass and resemble a comet tail (Fig 2).
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DISCUSSION
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Round atelectasis is an unusual form of lung collapse that is adjacent to the pleural surface and may simulate a pulmonary neoplasm. It has been described by Loeschke (5). Blesovsky (6) discussed it in detail and called it "the folded lung." It was further delineated by Hanke and Kretzschmar (4), who coined the term "round atelectasis." Since then, a variety of terms have been used to describe this condition, including "Blesovsky syndrome" (7), "atelectatic pseudotumor" (8), and "shrinking pleuritis with atelectasis" (2).
The mechanism of round atelectasis remains controversial. However, two theories predominate, one of which was postulated by Hanke and Kretzschmar (4): An underlying pleural effusion causes local atelectasis in the adjacent lung. A cleft or infolding of the visceral pleura will form if the rate of pleural fluid formation exceeds that of alveolar air absorption. This causes the lung to tilt on the cleft. The lung then curls on itself in a concentric fashion. Fibrous adhesions that suspend the atelectatic segment and usually tilt the lung cranially develop. As the effusion resorbs, aerated lung fills in the space between the area of round atelectasis. Organization of the fibrinous exudate and fibrous contraction lead to additional distortion of the lung parenchyma.
An alternative theory postulated by Schneider et al (1) and expanded on by Dernevik and colleagues (2) suggests that the event underlying round atelectasis is a local pleuritis that is caused by irritants such as asbestos. In the event of a benign asbestos-related pleural effusion, the pleura contracts and thickens. The underlying lung shrinks, and atelectasis develops in a round configuration. This theory, which is supported in various pathology studies (5), is now favored, but the multifactorial etiology of round atelectasis suggests that both mechanisms probably operate in different patients (9,10).
Round atelectasis is not a rare entity. It has been described in patients between the ages of 20 and 92 years, with 80% being men and 65% having a history of asbestos exposure (11). Conditions such as congestive heart failure, pulmonary infarct, Dressler syndrome, parapneumonic effusion, tuberculous effusion, and nonspecific pleurisy can precede its formation. Round atelectasis is almost always asymptomatic and is detected on chest radiographs obtained for other reasons. A history of asbestos exposure is frequently (in 70% of cases) present (2,11).
The radiographic features of round atelectasis are characteristic. It is seen on conventional chest radiographs as a round or oval subpleural opacity 2.5–8.0 cm in diameter (11). It forms acute angles, with the pleura indicating its parenchymal location, and it is usually separated from the diaphragm by interposed lung. Adjacent pleural thickening is an essential finding. Round atelectasis usually manifests as a single lesion, although multiple lesions are seen occasionally (3). It is commonly found in the lower lung lobes, either posteriorly or posteromedially; however, the upper lobes and particularly the middle lobe and the lingula, can be involved (3). There is volume loss of the affected lobe, and air bronchograms may be present within the mass. Pericardial or pleural tenting also may be seen (11).
The characteristic feature of round atelectasis is the comet tail sign. As the lung collapses, the vessels and bronchi that lead to the mass are pulled into the region. As they reach the mass, they diverge and arch around the undersurface to merge with the inferior pole of the mass. This characteristic distortion is well demonstrated at conventional tomography. CT findings confirm the conventional tomographic findings and demonstrate the comet tail sign (3,12). Crowded and converging bronchovascular bundles are seen entering the mass from all sides, giving the appearance of crow's feet or a talon sign (12). Overlying pleural thickening is invariably seen and well demonstrated at CT (Fig 2). Although homogeneous enhancement occurs with the intravenous administration of contrast material, it cannot be used as a differentiating sign because it also occurs with some carcinomas. The appearance of round atelectasis at magnetic resonance imaging is described as a lesion with a signal intensity similar to that of the liver on T1-weighted images, with bronchovascular bundles curving into the mass (13).
Conditions to rule out in the main differential diagnosis include bronchogenic carcinoma. No specific treatment is needed for round atelectasis. It is usually a stable or very slow growing process and will occasionally disappear spontaneously (11). If round atelectasis is suspected, then CT should be performed. If a confident diagnosis is made, then follow-up with conventional radiography is indicated. Fine-needle aspiration biopsy is indicated in cases that are equivocal (11). If the results of fine-needle aspiration biopsy are negative and doubt persists, then excisional biopsy is recommended (11).
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Acknowledgments
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I thank John Kosiuk, MD, and Robert Hanson, MD, for their assistance in editing.
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Footnotes
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A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
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References
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Schneider HJ, Felson B, Gonzalez LL. Rounded atelectasis. AJR 1980; 134:225-232.[Abstract]
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Dernevik L, Gatzinsky P, Hultzman E, et al. Shrinking pleuritis with atelectasis. Thorax 1982; 37:252-258.[Abstract/Free Full Text]
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Carvalho PM, Carr D. Computed tomography of folded lung. Clin Radiol 1990; 41:86-91.[Medline]
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Hanke R, Kretzschmar R. Round atelectasis. Semin Roentgenol 1980; 15:174-182.[Medline]
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Menzies R, Fraser R. Round atelectasis: pathologic and pathogenic features. Am J Surg Pathol 1987; 11:674-681.[Medline]
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Blesovsky A. The folded lung. Br J Dis Chest 1966; 60:19-22.[Medline]
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Payne CR, Jaques P, Kerr IH. Lung folding simulating peripheral pulmonary neoplasm (Blesovsky's syndrome). Thorax 1980; 35:936-940.[Abstract/Free Full Text]
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Stark P. Rounded atelectasis: another pulmonary pseudotumor. Am Rev Respir Dis 1982; 125:248-250.[Medline]
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Stephenson N, Price J. CT appearances of rounded atelectasis. Australas Radiol 1992; 36:308-312.[Medline]
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Hillerdale G. Rounded atelectasis: clinical experience with 74 patients. Chest 1989; 95:836-841.[Abstract/Free Full Text]
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Szydlowski G, Cohn H, Steiner R, Edie R. Rounded atelectasis: a pulmonary psuedotumor. Ann Thorac Surg 1992; 53:817-821.[Abstract]
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Matthews JI, Grabowski WS, Blatt ES, Bush BA, Aldarondo S, Sullivan CJ. Rounded atelectasis: a new criterion for benignancy. South Med J 1986; 79:767-770.[Medline]
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Verschakelen JA, Demaerel P, Coolen J, et al. Rounded atelectasis of the lung: MR appearance. AJR 1989; 152:965-966.[Free Full Text]
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