A cavity is a gas-filled space that is observed as a lucency or low-attenuated area within pulmonary consolidation, a mass, or a nodule (Fig. 2B) (1). Cavity wall thickness may vary, but the wall is usually relatively thick (34). Some cavitary lesions may appear as thin-walled cavities or cysts at their end-stage presentation (5). Many different diseases present as cavitary lesions. This spectrum of diseases includes acute to chronic infections, chronic systemic diseases, and primary or metastatic malignancies (45).

A cavity is differentiated from a cyst by the presence of a thicker wall and a more irregular shape.

A bulla is an airspace measuring more than 1 cm that is sharply demarcated by a thin wall (1). Radiologically, it appears as a rounded focal lucency or decreased attenuation more than 1 cm in size, and is bounded by a thin, usually almost undetectable wall that is not greater than 1 mm (Fig. 2C). Bullae are usually located in the subpleural lung rather than within the lung parenchyma. Multiple bullae are usually accompanied by adjacent paraseptal and centrilobular emphysema (1).

Bullae can be distinguished from cysts by their almost imperceptible thin-wall, subpleural location, and accompanying adjacent emphysema.

A pneumatocele is a transient, thin-walled, gas-filled lesion, usually caused by pneumonia, trauma, or aspiration of hydrocarbon fluid (1). The mechanism underlying their formation is believed to be a combination of parenchymal necrosis and check valve airway obstruction. Radiologically, a pneumatocele appears as an almost round, thin-walled airspace in the lung (Fig. 2D) (1). Pneumatoceles can be accompanied by adjacent consolidation or ground-glass opacity as a result of recent pneumonia; they may progressively increase in size over the following days or weeks, and then resolve after weeks or months (6).

Pneumatoceles appear transiently following recent pneumonia or trauma, which is an important distinguishing feature from cysts.

Pulmonary emphysema can be classified into three major subtypes based on disease distribution within secondary pulmonary lobules: centrilobular or centriacinar emphysema, panlobular or panacinar emphysema, and paraseptal or distal acinar emphysema (178). Centrilobular emphysema is the most common type of pulmonary emphysema, and is pathologically defined by the presence of destroyed centrilobular alveolar walls and enlarged respiratory bronchioles and associated alveoli (1). Centrilobular emphysema typically appears in focal areas of decreased lung attenuation, usually without visible walls, and it has a predilection for the upper lungs (189). Centrilobular emphysema is usually combined with a central dot which is a central bronchovascular bundle in the secondary pulmonary lobule (Fig. 3B). However, centrilobular emphysema may appear similar to thin-walled cysts. Paraseptal emphysema involves the more distal part of the secondary pulmonary lobule, and usually presents as a single row of elongated, thin-walled, air-filled structures that are distributed within the subpleural lung. Panlobular emphysema involves the destruction of the entire secondary pulmonary lobule, and it involves lung parenchyma more diffusely, especially in the lower lungs (8).

Cysts are larger, fewer in number, lack a centrilobular location, lack a central core vessel, and have more visible walls compared with centrilobular emphysema.

Honeycombing represents destroyed and fibrotic lung tissue containing numerous cystic airspaces with thick fibrous walls, indicative of the late stages of various lung diseases (1). Radiologically, it appears as clustered cystic lesions with 1–3 mm-thick well-defined walls, which are typically 3–10 mm in diameter but may be occasionally larger (Fig. 3C) (110). Honeycombing is characterized by multiple rows of air-filled spaces clustered in the subpleural region, predominantly in the lower lungs. It usually presents as multiple layers, but it can present as a single layer (10). Honeycombing usually accompanies other features of lung fibrosis, such as reticulation and traction bronchiectasis. It is the most distinguishing feature of usual interstitial pneumonia pattern, which is a hallmark radiologic pattern of idiopathic pulmonary fibrosis (10).

Cysts are larger in size, lack a subpleural distribution, do not show associated fibrosis, and are isolated lesions in comparison with honeycombing.

Bronchiectasis is an irreversible, localized, or diffuse bronchial dilatation, usually resulting from chronic infection, proximal airway obstruction, or congenital bronchial abnormalities (1). Bronchiectasis may be classified as cylindric, varicose, or cystic, depending on the appearance of the affected bronchi (1). Cystic bronchiectasis can also be mistaken for cysts when a dilated airway is viewed in cross-section (Fig. 3D).

Bronchiectasis is usually distinguished from true cysts by careful examination of contiguous CT images. Findings such as tubular rather than spherical dimensions, branching patterns, associated bronchial wall thickening, centrilobular densities, and air-trapping are helpful for the diagnosis of bronchiectasis.

A solitary cyst or several small lung cysts can be found incidentally on CT scans (Fig. 4). One study reported that cysts were seen in 25% of subjects over 75 years of age, but were not seen in subjects under 55 years of age (11). Another study found lung cysts in 7.6% of subjects on chest CT. The prevalence of multiple lung cysts was 0.9% among all participants. Although pulmonary cysts are mostly solitary, they can occur sporadically or in multiple. Cysts are not observed in participants younger than 40 years of age, and their prevalence increases with age. Cysts are most likely to appear solitarily in the lower peripheral lungs and remain unchanged or slightly increase in size over time. Cysts are not associated with impairment in measures of spirometry, cigarette smoking, or emphysema. Therefore, incidental lung cysts could be a part of the normal aging process (12). However, a single cyst may also be a remnant of a previous infection or trauma (13).

It is important to differentiate incidental lung cysts from other cystic lung diseases. Solitary or several small thinwalled lung cysts incidentally found on CT in the old age group may be a part of the normal aging process. Thus, old age, smaller number, and a lack of association with other radiologic findings are useful for the differentiation of incidental lung cysts from cystic lung diseases.

Lymphangioleiomyomatosis (LAM) is a rare disease that predominantly affects the lung parenchyma of women of childbearing age (181920). LAM can occur sporadically, but is more common in patients with tuberous sclerosis complex (TSC-LAM) (202122). TSC-LAM is an autosomal-dominant neurocutaneous syndrome characterized by hamartoma formation in multiple organs (21). Pathologically, typical findings are nodules or small clusters of smooth muscle cells (LAM cells) near cystic lesions and around terminal bronchioles, alveolar walls, pulmonary vessels, and the lymphatics (2023). Cysts in the lung parenchyma may be the result of terminal bronchiole obstruction by LAM cells with associated air-trapping, which is thought to cause progressive dilatation of the distal airspaces leading to cyst formation and/or from degradation of the lung parenchyma due to an imbalance between proteases and protease inhibitors (20). Lymphatic obstruction may result in chylous pleural effusion, chylous ascites, or both. Spontaneous or recurrent pneumothorax is the presenting finding in up to 50% of LAM patients (22).

Radiologically, lung cysts have been seen in 100% of patients with LAM reported to date (19). Cysts are typically numerous, thin-walled, well-circumscribed, and diffusely and symmetrically distributed throughout the lungs without lobar predominance. Cysts are typically round or ovoid and are usually 2–10 mm in diameter, but can occasionally be as large as 30 mm. The lung parenchyma between cysts is typically normal (Fig. 7) (192022). LAM can appear with small nodules, reticulation, or ground-glass opacity (1922); however, these presentations are rare. Other extrapulmonary findings, such as pneumothorax, chylothorax, chylous ascites, bony sclerotic lesion, and renal tumor can be seen on chest CT (192021).

Birt-Hogg-Dubé syndrome (BHD) is an uncommon, autosomal-dominant, multiorgan systemic condition characterized clinically by fibrofolliculomas, pulmonary cysts, and renal neoplasms (24252627). On CT, more than 80% of adult patients with BHD have multiple lung cysts, and lung parenchyma except for multiple lung cysts generally appears normal (224). The presence of lung cysts is significantly associated with spontaneous pneumothorax (2). The histological features of BHD cysts are generally indistinguishable from those of emphysema (2). All patients with pulmonary manifestations of BHD have characteristic skin and/or renal lesions or a family history. Therefore, the presence of spontaneous pneumothorax in a young adult, with a family history of pneumothorax, skin lesions, or renal tumors, should indicate the possibility of BHD (2).

Radiologically, multiple thin-walled lung cysts are predominantly seen in lower, peripheral lung zones and along the mediastinum (Fig. 8) (26). These cysts are surrounded by normal lung parenchyma. The shape and size of cysts are variable; they can be round, oval, lentiform, lobulated, or irregularly shaped, and generally have perceptible thin walls. Large cysts, particularly those in the lower lungs, have a lobulated, multiseptated appearance (26). Predominant distribution of cysts in the lower medial lung zone is a characteristic finding. Cysts abutting or including the proximal portions of lower pulmonary arteries or veins may also exist (28).

Since BHD and LAM share some clinical manifestations, such as lung cysts with pneumothorax, renal tumors, and occasionally skin lesions, differential diagnosis of these two diseases can be problematic, especially in female patients. Quantitative analysis of lung cysts for differentiation of these two diseases yields significantly different findings, and the independent parameters for differentiating the two diseases are a family history of pneumothorax, zonal predominance of cysts, diffusing capacity, and size of the cysts (25). Lung cysts in BHD are predominantly distributed in the lower peripheral lung zones and along the mediastinum, generally larger in size and fewer in number, in comparison with lung cysts in LAM.

Pulmonary Langerhans cell histiocytosis (PLCH) is a rare interstitial lung disease that typically affects young adults and is associated with cigarette smoking (2022). PLCH is characterized by peribronchiolar infiltration by Langerhans and inflammatory cells and formation of granulomas, leading to stellate interstitial nodules (2229). These stellate nodules can later cavitate, resulting in bronchial dilatation and formation of thin- and thick-walled cysts and cavities. These cysts may coalesce, forming bizarre shapes. Pathologically, temporal heterogeneity is typically present. Early interstitial infiltration of Langerhans cell histiocytosis cells is followed by nodules with variable numbers of lymphocytes and eosinophils. As the disease progresses, early cellular nodules are replaced by fibroblastic proliferation in a centripetal fashion, eventually leaving fibrotic scars surrounded by enlarged airspaces (2229).

Radiologically, nodules are the predominant features in the early stages, while cysts tend to develop later. In the early stages, small (1–10 mm) irregularly shaped nodules appear in a bilateral symmetric pattern with upper-middle lung dominance and a spared lung base and costophrenic angle. As the disease progresses, cystic degeneration appears as round or oval to bizarre with thin-wall and associated nodules (Fig. 9) (202229). Cysts usually measure < 10 mm in diameter but may be as large as 20 mm. They may manifest as round or ovoid cystic spaces or exhibit bizarre configurations with twolobed, clover leaf, septated, and branching morphologies that resemble bronchiectasis. Cyst walls may be thin and barely perceptible or may appear thick, irregular, and nodular, depending on the degree of nodule cavitation. Different cyst appearances may coexist. Bizarre-shaped cysts associated with nodules predominantly in the upper lung are a key imaging discriminator for PLCH in a young smoker.

Cystic lung metastasis is most frequently seen in patients with angiosarcoma or squamous cell carcinoma mainly in the head and neck (2030). In a study of lung metastasis from angiosarcoma, nodules were seen in 85% and multiple cysts were seen in 55% of patients (30). There are four possible mechanisms for cyst formation. First, excavation of solid nodular lesions can induce cyst formation. Second, infiltration of malignant cells into the walls of pre-existing bullous lung tissue can result in cyst formation. Third, infiltration of tumor cells into the walls of air sacs and the ball-valve effect can cause cystic distension. Last, tumor cell proliferation can form blood-filled cystic spaces (31).

Radiologically, multiple solid nodules and multiple thinwalled cysts, often admixed with hemorrhagic change, are common features of metastatic angiosarcoma (31). Cystic metastasis from angiosarcomas shows variability in the walls, air-fluid levels, and vessels or bronchi penetrating the cysts (Fig. 10) (30). As with other lung metastases, cystic metastasis tends to show different sizes and a basilar predominance (32). Pneumothorax is a potential complication of cystic metastasis.

A patient's previous malignancy history is critical for diagnosis. If new lung cysts are detected in patients with a known malignancy, cystic metastasis should be considered, and adequate tissue confirmation is required for diagnosis.

Amyloidosis is a rare disease characterized by extracellular deposition of abnormal insoluble proteins (3334). Characteristically, the amyloid deposit shows apple-green birefringence with Congo red stain and under polarized light (35). The thorax can be affected, and pulmonary involvement of amyloidosis is relatively rare (35). Pulmonary amyloidosis can present with cystic lung disease, and amyloid-associated cystic lung disease is rare. Amyloid-associated cystic lung disease can occur with Sjögren's syndrome and mucosa-associated lymphoid tissue lymphoma (34). Narrowing of the airways from surrounding inflammation or amyloid protein deposits may lead to the development of a check valve phenomenon. Alternatively, an ischemic process with amyloid deposition causing capillary disruption may lead to alveolar wall destruction and cyst formation (222).

Radiologically, lung cysts are commonly numerous (more than 10), are often peribronchovascular or subpleural, and are frequently associated with nodular lesions that are often calcified (34). Cysts tend to be multiple, round or lobulated, small to moderate in size, and thin-walled (34). Other associated findings include interlobular septal thickening, honeycombing, ground-glass opacity, circumferential thickening of the tracheal wall, and lymphadenopathy (34).

Pneumocystis jirovecii pneumonia (PCP) is a fungal infection that has a strong association with immunocompromised conditions such as human immunodeficiency virus (HIV) infection (93236).

Radiologically, ground-glass opacity indicating acute pneumonia is the dominant feature of this condition (36). The pattern of these opacities is often bilateral, multifocal, and mainly symmetrical, and distributed in the central portions of the lungs. Opacities can predominate in the upper lung zones in patients receiving prophylactic therapy for this infection. Thin-walled cysts are now recognized as a relatively common manifestation of this infection and are reported in as many as one-third of all patients (3637). Cysts are usually variable in size, shape, and wall thickness, and are usually multiple and bilateral, subpleurally or intraparenchymally located, and are predominantly in the upper lung zone (Fig. 11) (93637). PCP is associated with an increased incidence of spontaneous pneumothorax, which is believed to occur in association with ruptured subpleural cysts (37).

Desquamative interstitial pneumonia (DIP) is characterized by the accumulation of numerous pigmented macrophages within most of the distal airspace of the lung (38). According to the international multidisciplinary classification of idiopathic interstitial pneumonias, DIP is classified as a smoking-related interstitial lung disease (39). Up to 90% of DIP patients have a smoking history, but other conditions besides smoking, such as occupational exposure to certain inhaled toxins, drugs, viral illnesses, and autoimmune diseases, can also cause DIP (38).

Radiologically, DIP presents with patchy ground-glass opacity with a predilection for the basal and peripheral lungs. Reticular density may also be present. Small (usually < 2 cm), well-defined, thin-walled cysts are seen within ground-glass opacity, which is an important finding in DIP (Fig. 12) (134041). Honeycombing is also possible, but uncommon (42). Cysts in DIP have imperceptible walls, and they are mostly discrete but occasionally can be clustered and are surrounded by ground-glass opacity. Cysts in DIP are believed to represent dilated bronchioles and alveolar ducts, and in the later stages of DIP, cysts may also represent early centrilobular emphysema or honeycombing (22). The presence of small cysts admixed within groundglass opacity is a unique feature of DIP. This feature is reported in approximately one-third of DIP patients (38).

Lymphoid interstitial pneumonia (LIP) is an uncommon benign polyclonal lymphoproliferative disease (2043). Idiopathic LIP is categorized as a rare idiopathic interstitial pneumonia (39). Most cases of LIP are associated with various underlying disorders, including HIV infection, connective tissue diseases such as Sjögren's syndrome, Hashimoto's thyroiditis, and systemic lupus erythematosus (2039). Pathologically, diffuse interstitial infiltration of small polyclonal lymphocytes and plasma cells is seen (2022). Cysts result from ischemia due to vascular obstruction, postobstructive alveolar dilatation, or compression of bronchioles by lymphoid tissue leading to a check valve mechanism (20).

Radiologically, a combination of ground-glass opacity, poorly defined centrilobular nodules, small subpleural nodules, interlobular septal thickening, thickening of the bronchovascular bundles, and scattered cysts in lower lungs are seen (Fig. 13) (2044). Lung cysts are present in up to 68% of patients; they are usually fewer in number but are distributed diffusely in both lungs, although they are often subpleural and peribronchovascular (44). Cysts in LIP are generally < 3 cm in diameter, are variable in shape, and may be the sole manifestation of this disease. In most cases, follow-up CT reveals the resolution of groundglass opacity, and cysts are the only residual finding in more chronic cases. Lymphadenopathy can be present, but pleural effusion or airspace consolidation are extremely rare (43). The diagnosis of LIP should be considered in patients with lung cysts and immunological abnormalities. The appearance of a few scattered cysts in a patient with Sjögren's syndrome is very likely to indicate LIP.