The diffuse cystic lung diseases (DCLDs) are a diverse set of disorders that share a common radiologic phenotype of thin-walled parenchymal lucencies.1 Although often grouped together with other interstitial lung diseases (ILDs), the underlying pathobiology, differential diagnosis, mode of presentation, and natural history of disease progression in DCLDs are sufficiently distinct to warrant classification of DCLDs as a separate independent entity.2 The subsequent sections of this chapter will provide an overview of the classification and differential diagnosis of DCLDs; detail the clinical, radiologic, and histopathologic features of various DCLDs; and culminate with an integrated approach toward the appropriate diagnosis and management of patients presenting with DCLD.
DEFINITION OF PULMONARY CYST
A pulmonary cyst is defined radiologically as a thin-walled (<2 mm wall thickness) parenchymal lucency interfaced with normal pulmonary parenchyma.3 Table 61-1 lists the other parenchymal lesions that can mimic cysts and need to be carefully distinguished from cysts. Pathologically, cysts are defined as dilated spaces within the lung tissue that have an epithelial lining. Lung cysts may contain fluid or gas. Dilated spaces without an epithelial lining are referred to as pseudocysts and must be distinguished from true epithelial-lined lung cysts. Some lesions designated as cysts clinically and radiologically may not represent true cysts pathologically; the most common cyst mimics are bronchiectasis, honeycomb remodeling, pulmonary interstitial emphysema, centrilobular emphysema, α1-antitrypsin deficiency, and cavitary lesions.
TABLE 61-1Radiologic Definition of Pulmonary Cysts and Other Parenchymal Lucencies That Can Mimic Cysts ||Download (.pdf) TABLE 61-1 Radiologic Definition of Pulmonary Cysts and Other Parenchymal Lucencies That Can Mimic Cysts
|Cyst ||Thin walled (<2 mm), spherical parenchymal lucency interfaced with normal lung |
|Cavity ||Gas-filled space within pulmonary consolidation, mass, or nodule, typically thick walled (>2 mm) and more irregularly shaped than cysts |
|Bulla ||Spherical focal lucency, ≥1 cm in diameter, bounded by a thin wall (<1 mm); usually accompanied by emphysematous changes in the adjacent lung |
|Bleb ||Cystic air space bounded by a thin wall adjacent to the visceral pleura, typically <1 cm in size. |
|Emphysema ||Permanently enlarged airspaces distal to the terminal bronchiole with destruction of alveolar walls; usually without visible walls |
|Pneumatocele ||Approximately round, thin-walled, air-filled space in the lung; most frequently caused by infections, trauma, or aspiration of hydrocarbon fluid and usually transient |
|Honeycombing ||Clustered cystic air spaces, typically 3–10 mm in diameter, with 1- to 3-mm wall thickness, that are often stacked on top of each other in layers; usually subpleural and associated with other features of pulmonary fibrosis such as traction bronchiectasis and reticulations |
|Bronchiectasis ||Localized or diffuse bronchial dilatation, usually resulting from chronic infection, proximal airway obstruction, or congenital bronchial abnormality |
CLASSIFICATION OF DIFFUSE CYSTIC LUNG DISEASES