Radiology Resident Case of the Week
Etiology/Pathophysiology
Round atelectasis is one of many manifestations of asbestos related lung disease. Briefly, asbestos related lung disease can be divided into parenchymal and pleural manifestations. Parenchymal changes include asbestosis proper, which is an interstitial fibrosis. This requires decades of exposure and leads to a restrictive pattern of function testing. Linear opacities predominantly in the lower lobes and subpleural region, eventually leading to honeycombing may be seen on plain film, although high resolution CT (HRCT) is more sensitive. Round atelectasis, described below, is another manifestation. Finally, patients exposed to asbestos are at a dramatically increased risk for bronchogenic carcinoma (5x for nonsmokers and 50x for smokers), which is dose related, often in the bases, and commonly either adeno or squamous cell carcinoma.
Asbestos related pleural disease is most often manifest as pleural plaques, usually (85%) calcified. This is a radiologic hallmark, and they usually appear after a 20-30 year latent period, are asymptomatic, dose related, involve the parietal pleura, and most importantly serve as a marker for other, more serious disease. Diffuse pleural thickening, and benign effusions (exudative) are also seen. Finally, malignant mesothelioma, while uncommon, is highly malignant and rarely seen without asbestos exposure. There is a long latency (>20 yrs), but it is not dose related. Patients often present with pain and metastases are not uncommon; it may be mimicked by diffuse pleural thickening which is a benign condition. Mesothelioma is much less common than bronchogenic carcinoma in patient with asbestos exposure.
Pathology:
Round atelectasis, also known as helical atelectasis, refers to a local area of collapsed lung parenchyma in the periphery which is at least in part surrounded by thickened, invaginated pleura. This creates the appearance of a mass, a so-called pseudotumor.
Exposure to asbestos is usually occupational, and includes shipyard workers, pipefitters, miners, and installers of brake linings and insulation. The major types of silicates causing asbestos related disease are chrysotile, amosite, and crocidolite, with the latter being the most pathogenic.
Miscellaneous
When the characteristic features listed below are present, these lesions should be followed closely, rather than biopsied.
Imaging
Because round atelectasis appears as a mass-like structure in patients who are increased risk for malignancy, four criteria must be met to make the diagnosis. First, the bronchovascular markings leading to the region must be demonstrated, usually seen entering the "hilum" of the mass, the so-called comet tail sign. Second, the mass must be in the periphery of the lung abutting the pleural surface. Third, the pleura overlying the region must be diseased, often thickened or containing plaques. Finally, there must be signs of other asbestos related pleural disease, e.g. calcified pleural plaques. The area of atelectasis is usually densest in the periphery, and occasionally air bronchograms are seen within the center, and subpleural fat may be invaginated into the region of collapse.
Figures 1 and 2 are a PA and Lateral chest film demonstrating an ill defined opacity in the posterior aspect of the right lower lobe (Green arrow). Figures 3 - 5 are contiguous slices from a chest CT showing the characteristic features including the comet tail (Red arrow), thickened overlying pleura (Yellow arrow), and calcified pleural plaques elsewhere (Blue arrow), signs of prior asbestos exposure. Finally, Figure 6 is a slice from a chest CT 5 years prior, showing no interval growth or change.
DDX
Key references
Diagnosis of Diseases of the Chest, 3rd ed. Fraser and Pare, W.B. Saunders, Philadelphia, 1988.
ACR Code
63.749
Keywords:
Round atelectasis; Asbestos
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