A pulmonary cyst is an aberrant pulmonary parenchymal area that does not contain lung but is filled with air and/or fluid, can be congenital or acquired in origin, and often has limited contact with neighboring normal lung and a wall thickness of more than 1 mm. Pulmonary cysts are a common finding on thoracic computed tomography. Occasionally, lung cysts are discovered inadvertently during thoracic imaging investigations and, although infrequent, may be misdiagnosed as a result of typical age-related pulmonary alterations. There are several origins of cysts within the lung parenchyma, and pulmonary cysts must be separated from other causes of localized lucent pulmonary lesions such as emphysema, bronchiectasis, air trapping, and cavitary lung disease. Diffuse pulmonary cystic diseases are a distinct subset of pulmonary cystic diseases that include Langerhans cell histiocytosis, lymphangioleiomyomatosis, Birt-Hogg-Dubé syndrome, and lymphocytic interstitial pneumonia, among other less common entities, such as asthma and bronchiolitis obliterans, and even amyloidosis or light-chain deposition disease.

The clinical history and presentation, as well as the imaging appearances of the cysts, allow a specific diagnosis. However, there is overlap in the imaging appearances of several causes of diffuse cystic pulmonary disease, so tissue sampling is frequently required to establish the correct diagnosis.