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5. The thoracic CT shows numerous thin-walled cysts representing bronchiectasis
The thoracic CT shows numerous thin-walled cystic structures; these account for the curvilinear opacities detected at chest radiography. Once recognized, the differential considerations for cystic pulmonary lesions include the numerous causes of cystic pulmonary diseases 9such as Langerhans cell histiocytosis, lymphangioleiomyomatosis, lymphocytic interstitial pneumonia, hypersensitivity pneumonitis, and Birt-Hogg-Dubé syndrome, among others), emphysema, air trapping, cystic pulmonary metastases, and bronchiectasis. Centrilobular emphysema can be distinguished from cystic lung disorders and bronchiectasis by the presence of a “dot” within the center of the low attenuation area, which represents the centrilobular artery (Figure 4).
Figure 4. Axial thoracic CT images displayed in lung windows shows numerous, variably-sized thin-walled cystic lesions in a somewhat clustered distribution bilaterally. Note the close relationship of the cystic lesions to adjacent pulmonary arteries (arrowheads); this relationship is consistent with the normal bronchoarterial relationship and identifies the cystic lesions as dilated bronchi. The trachea and mainstem bronchi show a mildly saccular appearance also.
Additionally, centrilobular emphysema lacks a definable wall, whereas bronchiectasis and cystic lung disorders have definable, often thin, walls (Figure 5).
Figure 5. Various cystic pulmonary disorders. (A) Centrilobular emphysema. Note the lack of a definable wall surrounding the cystic lesions and the presence of the central “dot” within the area of low attenuation (arrowheads), representing the centrilobular artery. Occasionally this dot is not readily visible or may appear eccentrically located; the key to identification rests with reviewing multiple regions to identify the characteristic relationship. (B) Langerhans cell histiocytosis. The cysts in this disorder are typically distributed in the upper lobes and often display “bizarre” shapes (arrowheads), and associated pulmonary abnormalities, typically pulmonary nodules, are common. Patients are usually smokers. (C) Lymphangioleiomyomatosis. The cysts in this disorder are more uniform in appearance, with normal intervening lung, and are distributed throughout the upper, mid, and lower lungs evenly, and no internal opacity is present. Spontaneous pneumothorax may occur. Patients are typically women of childbearing age or patients with tuberous sclerosis. (D) Birt-Hogg-Dubé syndrome. The cysts in this disorder (arrowheads) are more basally distributed and may be associated with spontaneous pneumothorax (*); affected patients often have renal and skin tumors as well. (E) Lymphocytic interstitial pneumonia. The cysts in this disorder vary in size and number, but may be associated with infiltrative pulmonary findings, such as ground-glass opacity, nodules, and interlobular septal thickening. The cysts typically do not form clusters. Differentiation from other cystic pulmonary disorders, particularly lymphangioleiomyomatosis, can be difficult. (F and G) Lobular low attenuation due to small airway obstruction and air trapping. The low attenuation areas in patients with small airway obstruction producing lobular low attenuation typically do not show walls, and the low attenuation regions have a polygonal shape, reflecting the shape of the affected secondary pulmonary lobule (arrowheads). (H and I) Cystic pulmonary metastases. Typically cystic pulmonary malignancies, whether primary or secondary, show some degree of wall thickness and irregularity. Occasionally cystic metastatic disease may appear uniformly thin-walled (arrowheads, H), as in this patient with metastatic testicular carcinoma. Often the clinical history will clarify the situation; the cystic appearance typically develops following treatment (occasionally some tumors, such as metastatic angiosarcomas, may appear as thin-walled cysts even before therapy). However, serial scanning, or review of previous imaging (I), will usually resolve the issue. In this case, the previous imaging (I) shows solid metastatic nodules (arrowheads, I) where the cysts (arrowheads, H) subsequently developed. All the cysts on the follow up imaging in this patient could be correlated with metastatic nodules on the pre-treatment imaging.
Cystic pulmonary disorders may be distinguished from one another, with variable success, using patient demographics, the shape of the cysts, the presence / absence of associated pulmonary nodules and other pulmonary findings, the distribution of the cysts, and behavior of the lesions over time (Figures 5B-E). Lobular low attenuation due to small airway obstruction-induced air trapping typically produces polygonal-shaped areas of low attenuation which lack true walls; the polygonal shape reflects the shape of the affected secondary pulmonary lobule (Figure 5F and G). Cystic pulmonary metastatic disease usually results in cystic lesions that show some degree of wall thickness and irregularity, allowing them to be distinguished from other cystic pulmonary disorders. However, occasionally, the walls of cystic pulmonary metastases may show a uniformly thin-wall, closely simulating cystic pulmonary disease (Figure 5H). In this setting, clinical history and the behavior of the cystic lesions over time usually clarifies the situation (Figure 5I). Bronchiectasis results in cystic lesions of variable size and wall thickness, but often the cysts have a clustered distribution, rather than the more dispersed appearance typical of other cystic pulmonary disorders. Furthermore, because bronchi and pulmonary arteries course adjacent to one another through the lung parenchyma, in the case of bronchiectasis, the cystic lesions are typically closely associated with an adjacent solid opacity, creating the appearance of a cyst with a “dot” along the cyst wall, as may be seen in this case (Figure 3).
What additional scan finding suggests the etiology of the bronchiectasis in this patient? (Click on correct answer to proceed to the next panel)