Cystic Lung Diseases: Algorithmic Approach

Abstract

Cysts are commonly seen on CT scans of the lungs, and diagnosis can be challenging. Clinical and radiographic features combined with a multidisciplinary approach may help differentiate among various disease entities, allowing correct diagnosis. It is important to distinguish cysts from cavities because they each have distinct etiologies and associated clinical disorders. Conditions such as emphysema, and cystic bronchiectasis may also mimic cystic disease. A simplified classification of cysts is proposed. Cysts can occur in greater profusion in the subpleural areas, when they typically represent paraseptal emphysema, bullae, or honeycombing. Cysts that are present in the lung parenchyma but away from subpleural areas may be present without any other abnormalities on high-resolution CT scans. These are further categorized into solitary or multifocal/diffuse cysts. Solitary cysts may be incidentally discovered and may be an age related phenomenon or may be a remnant of prior trauma or infection. Multifocal/diffuse cysts can occur with lymphoid interstitial pneumonia, Birt-Hogg-Dubé syndrome, tracheobronchial papillomatosis, or primary and metastatic cancers. Multifocal/diffuse cysts may be associated with nodules (lymphoid interstitial pneumonia, light-chain deposition disease, amyloidosis, and Langerhans cell histiocytosis) or with ground-glass opacities (Pneumocystis jirovecii pneumonia and desquamative interstitial pneumonia). Using the results of the high-resolution CT scans as a starting point, and incorporating the patient's clinical history, physical examination, and laboratory findings, is likely to narrow the differential diagnosis of cystic lesions considerably.

Keywords: cystic lung disease; diffuse lung disease; focal lung luciencies; lung cysts; pulmonary cysts.

Figure 1

A, True cysts: Round parenchymal lucencies surrounded by distinct wall < 2 mm in thickness, exhibiting a well-defined interface with normal lung tissue. Cysts may be seen in otherwise normal individuals but may occur in association with other lung diseases. B, Cavity. This is a gas-filled space, seen as a lucency or low-attenuation area, with a wall thickness > 4 mm. It is seen within pulmonary consolidation, a mass, or nodule.

Figure 2

Tracheobronchial papillomatosis. Note that the thin-walled cysts may compress portions of the lung, giving the spurious impression of asymmetric wall thickening.

Figure 3

Traumatic pneumatocele. Thin-walled, gas-filled space in the lungs, usually transient, that can be caused by acute pneumonia, trauma, or aspiration of hydrocarbon fluids. In this illustration, there are adjacent areas of increased parenchymal density from lung contusion.

Figure 4

A five-step approach to managing cystic lung disease.

Figure 5

A-B, Minimum intensity projection (MINiP) imaging. Sequential coronal images through the posterior aspect of the lungs show extensive cystic bronchiectasis within the right apex and the entire left lower lobe. With MINiP imaging, only those voxels with the lowest density in any row are projected in two dimensions, effectively eliminating all high-density structures, including most pulmonary vessels. Note that there is also evidence of mosaic lung attenuation identifiable as alternating geographical areas of low-density and higher lung density, indicative of extensive air-trapping in this case due to associated obstructive small airways disease. These changes are seen to especially good advantage using the MINiP technique.

Figure 6

Detailed algorithmic approach to cystic lung disease. It is important to exclude mimics of cystic lung disease before proceeding with the actual algorithm. BHD = Birt Hogg Dube syndrome; DIP = desquamative interstitial pneumonia; HRCT = high-resolution CT scan; LAM = lymphangioleiomyomatosis; LCDD = light-chain deposition disease; LIP = lymphoid interstitial pneumonia; PJP = pneumocystis jirovecii pneumonia; PLCH = pulmonary Langerhans cell histiocytosis.

Figure 8

Centrilobular emphysema. These lucencies demonstrate a predilection for the upper lung zones. Centrilobular lucencies are seen that lack distinct walls and measure up to 1 cm in diameter. The lucencies may have a dot in the center, representing a branch of pulmonary artery.

Figure 9

Cystic bronchiectasis. Nontapering tubular structures that course peripherally to within 1 cm of pleural surface. The contiguity of these dilated bronchi can be appreciated on adjacent transverse sections of airways and on coronal planes, distinguishing them from true individual cysts.

Figure 10

Honeycombing. Lower lobe predominance, with cysts of similar diameter, demonstrating thicker walls. Cysts are present in one or more layers. There is architectural distortion of the lung parenchyma with traction bronchiectasis, representing end-stage lung disease.

Figure 7

The mimics of cystic lung disease along with their radiographic and clinical features. PFT = pulmonary function test.

Figure 11

The major etiologies of cysts that are present predominantly in the subpleural areas. Of note, Birt-Hogg-Dubé (BHD) syndrome may also give rise to cysts in the subpleural area. See Figure 7 legend for expansion of abbreviation.

Figure 12

Bulla. Large (> 1 cm), air-containing structures with very thin walls (≤ 1 mm). Walls of bulla are formed by lung tissues, septa, or pleura. Bullae are usually associated with emphysema.

Figure 13

Paraseptal emphysema. Subpleural lucencies marginated by interlobular septa and pleura. They exhibit upper lobe predominance and are usually associated with centrilobular emphysema.

Figure 14

Intraparenchymal cysts. These are either solitary or multiple. See Figure 6 legend for expansion of abbreviations.

Figure 15

Algorithm for solitary intraparenchymal cysts.

Figure 16

Incidental cyst. These cysts can occur in any lobe of the lung. The average size is 10 mm, and they may represent the normal aging process or persist as a remnant of previous infection or trauma.

Figure 17

Bronchogenic cyst. Usually asymptomatic, these cysts arise from abnormal budding of the tracheobronchial tree. Rarely, they may communicate with the tracheobronchial tree and become infected. An air-fluid level is seen in this large bronchogenic cyst.

Figure 18

Algorithmic approach to intraparenchymal cysts–multiple. See Figure 6 and 7 legends for expansion of abbreviations.

Figure 19

Fibrofolliculomas (dome shaped, whitish papules) of the face, neck and upper trunk are seen in Birt-Hogg-Dubé (BHD) syndrome.

Figure 20

A-D, Axial CT chest images of a patient with Birt-Hogg-Dubé (BHD) syndrome. Multiple, thin-walled cysts are seen in approximately 80% of patients. These cysts are distributed predominantly in the peripheral lung zones, at lung bases, and along the mediastinum. These are less in number generally, compared with those seen in lymphangioleiomyomatosis (LAM).

Figure 21

Lymphoid interstitial pneumonia. Cysts are thin-walled, random in distribution, and typically involve < 10% of lung parenchyma. Note the scattered foci of increased lung density in close proximity to the medial aspect of a cyst in the right lower lobe (solid black arrow). There are multiple small nodules. The thin black arrow on the right side shows these nodules in the fissure.

Figure 22

Metastatic colon cancer with thin-walled cysts. The images are from a 62-year-old male patient who was an ex-smoker with metastatic colon cancer. This CT scan was obtained 10 months after resection of the colon cancer.

Figure 23

The cyst in a patient with coccidioidomycosis is well-delineated with little reaction in the surrounding lung parenchyma.

Figure 24

A, Lymphangioleiomyomatosis (LAM). Thin-walled round (2-5 mm) cysts usually involve juxtaphrenic recesses and spare the extreme apices. The intervening lung between cysts is normal. B, LAM pathologic findings. (a) Low power histology shows multiple cysts surrounded by bundles of smooth muscle. (b) Surrounding cystic spaces are nodules of haphazardly arranged smooth muscle bundles. (c) HMB-45 immunohistochemistry shows positive cytoplasmic staining of the LAM cells.

Figure 25

A, Pulmonary Langerhans cell histiocytosis. Generally seen in young smokers (most commonly between 20 and 40 years of age). Cysts are seen predominantly in upper and middle lobes, are variable in size, are thick- or thin-walled, and have bizarre shapes. The costophrenic angles are spared. The intervening lung parenchyma exhibits architectural distortion. B, Langerhans cell histiocytosis pathologic findings. (a) Low-power histology shows a stellate nodule composed of fibrosis with some cellular infiltrates. At the edges of the fibrotic scar is paracicatricial emphysema, giving some cystic appearance. (b) High-power shows a cellular proliferation of Langerhans cells.

Figure 26

Multiple intraparenchymal cysts with associated findings on high-resolution CT scan (HRCT) imaging. See Figure 6 legend for expansion of abbreviation.

Figure 27

Algorithm for multiple intraparenchymal cysts with associated nodules on high-resolution CT scan (HRCT) imaging and HRCT scan. CVD = collagen vascular disease; CVID = common variable immunodeficiency; LCDD = light-chain deposition disease. See Figure 6 legend for expansion of other abbreviation.

Figure 28

A, Light-chain deposition disease (LCDD). Cysts are thin-walled, round, and may reach up to 2 cm in diameter (white arrow). Associated nodules are diffuse, irregular, and small. Consolidation of lung parenchyma may be present, as well as associated mediastinal lymphadenopathy. B, LCDD pathologic findings. (a) Low power shows a subpleural cystic area. (b) Focally in the wall of one of the cysts is a nodular focus of amorphous eosinophilic material characteristic of LCDD (black arrow).

Figure 29

Amyloidosis. Cysts are thin-walled with or without nodules. Interlobular septal thickening may be prominent. Ground-glass opacities can be seen, along with associated mediastinal lymphadenopathy.

Figure 30

Algorithm for multiple intraparenchymal cysts with ground-glass opacities. D(A-a)O2 = alveolar arterial oxygen gradient; LDH = lactate dehydrogenase; PaO2 = partial pressure of oxygen in arterial blood. See Figure 6 legend for expansion of other abbreviations.

Figure 31

Desquamative interstitial pneumonia (DIP). Ninety percent of cases of DIP are seen in smokers. Small cysts are seen in the milieu of universal ground-glass opacities and scanty ground-glass nodules.

Figure 32

A, Pneumocystis jirovecii: Diffuse ground glass opacities with septal thickening and occasional cysts (long-standing). B, P jirovecii pathologic findings. (a) Low- power histology from a case of long-standing P jirovecii pneumonia shows cystic spaces surrounded by thickened fibrotic walls containing some calcification. (b) High power of GMS (Gomori methenamine silver) stain shows characteristic cysts of P jirovecii with darkly stained foci.