Generally, cavities have walls that are at least 5 mm thick, while cysts have walls that are 4 mm or less, and often less than 2 mm.The distinction between cysts and cavities is important because the thicker the wall is, the more likely it is to be cancer. Thus, cystic lesions are unlikely to be cancer, while cavitary lesions are often caused by cancer. In a study from 1980 that used chest X-rays to evaluate 65 cases of solitary lung cavities, 0% percent of cavities with walls 1 mm or less were malignant (that is, cancerous), versus 8% of cavities with walls 4 mm or less, 49% of cavities with walls 5 to 15 mm, and 95% of cavities with walls 15 mm or greater. However, a 2007 study that used CT to evaluate lung cavities showed no relationship between wall thickness and the likelihood of malignancy. It did show that malignant cavities are more likely than benign cavities to have an irregular internal wall (49% vs 26%) and have an indentation of the outer wall of the cavity (54% vs 29%).Areas of emphysema are abnormal, air-filled spaces that usually do not have visible walls, and bullae are very thin walled (<1 mm). Cystic bronchiectasis is irreversible bronchial dilation, which is permanent widening of the bronchioles (small airways) in the lung. It can be distinguished on imaging by a lack of bronchial tapering, meaning that the bronchioles do not get narrower as they travel further into the lung.
Congenital
Congenital lung cavities, or lung cavities present at birth, include bronchogenic cysts, congenital pulmonary airway malformation, and pulmonary sequestration. These congenital lesions are the most common cause of lung cavities in infants, children, and young adults. Bronchogenic cysts are due to abnormal budding of the bronchial tree. About 70% are found in the mediastinum, which is the central part of the chest where the heart is. Another 15 to 20% are intrapulmonary (within the lung), usually in the lower lobes. Congenital pulmonary airway malformation, formerly called congenital cystic adenomatoid malformation, is a benign tumor the results in the formation of single or multiple cysts. Pulmonary sequestration refers to abnormal lung tissue that gets its blood supply from the systemic circulation instead of the pulmonary circulation, like the rest of the lung. This lung tissue is also not connected to the trachea. See also
Focal lung pneumatosis, article comparing lung blebs, bullae, cysts, and cavities


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