- HRCT is the best imaging tool to assess the size and distribution of the cysts. Sometimes the presence of lung cysts is only part of a pathological entity whose spectrum of imaging findings is broader and more varied.

- In other cases, the presence of pulmonary cysts is of paramount importance in the definition of the underlying entity. The latter includes Langerhans cell histiocytosis, lymphangioleiomyomatosis and tuberous sclerosis.

- When it comes to the diagnosis, one should ask if it is a true cyst or a cystic simulating entity. Then the pattern of distribution and the accessory pulmonary imaging findings should be analyzed. These can include ground glass opacities, micronodules, consolidation or fibrosis.

- Extrapulmonary findings can also play a role in the diagnosis. The differential diagnosis can be even more narrowed when the findings are combined with the patient’s profile and history. 

Cyst

- Thin-walled

- Well-defined

- Air-containing lung lesion (occasionaly contains fluid or solid material)

- >=1cm

Cystic Lung Diseases

Etiologies

Fig. 1

Pulmonary Langerhans Cell Histiocytosis (Fig. 2; Fig. 3)

- Unknown etiology

- Most often recognized in chilhood

- Also known as histiocytosis X or eosinophilic granuloma

- 40% of patients have lung involvement

- Over 90% of adults with PLCH are smokers

- Evolution of the pulmonar lesions: centrilobular nodules→Thick-walled cysts→Thin-walled cysts

Imaging features

- Cystic appearance may mimick honeycombing

- Usually bilateral

-  ++ Middle and upper lung zones 

-  Variable cyst wall thickness (ranging from an emphysema appearance to a thick walled cavity)

- Some cysts may have an unusual appearance (cloverleaf)

- Important tip: true honeycombing does not occur.

Lymphangioleiomyomatosis and Tuberous Sclerosis (TS) (Fig. 4 - Fig. 7)

- It occurs in association with the TS complex or in a sporadic form

- Due to a proliferation of perivascular epithelioid cells (PEC)

- According to the new WHO classification of lung tumors, it is considered a low-grade metastising malignancy.

- Very small nodules (1 to 3mm) also may be present.

- Angiomyolipomas: present in 15% of cases (Fig. 6). 

- +++ women of childbearing age, between 17 and 50 years of age.

- Progression is typical and most patients die within 5 to 10 years from the onset of symptoms.

- LAM is considered an indication to lung transplantation. However, the disease may recur in the transplanted lung.

Imaging features: 

- Lung volumes (as with PLCH) may be increased despite the presence of reticulation.

- 50% of patients have pneumothorax at the time of presentation.

- Cysts: round, thin-walled, diffuse, involve the costophrenic angles.

Neurofibromatosis

- Incidence: 1 in 3000 individuals.

- Lung disease is present in 10-20% of adult patients.

- Bullae in the upper lobes; interstitial fibrosis at the lung bases.

- Patients typically present with dyspnea.

Lymphoid Interstitial Pneumonia (Fig. 8 - Fig. 10)

- Benign lymphoproliferative disorder 

- Diffuse interstitial infiltrate of lymphocytes and plasma cells

- Alveolar walls are extensively involved

- Cysts appear to arise from peribronchiolar lymphoid infiltration resulting in bronchiolar stenosis or occlusion, leading to airtrapping and distal airway dilatation.

- Cysts occur in 80% of patients

- An association with collagen vascular diseases can occur.

- ∼5% of patients with LIP develop lymphoma, but whether this is a consequence of LIP or whether LIP is a manifestation of underlying lymphoma is still unclear (Fig. 8) 

Imaging features:

- An array of imaging presentations can occur such as:

- Nodular pattern, patchy or diffuse ground-glass opacities

- Multiple thin walled cysts (++ Sjögren syndrome): fewer than those seen in LAM or LCH

Sjogren Syndrome

Clinical triad:

1. Keratoconjunctivitis sicca

2. Xerostomia

3. Recurrent swelling of the parotid gland

Imaging findings: (Fig. 11; Fig. 12)

- Pulmonary manifestations are relatively common, including LIP, NSIP, UIP, OP and pleuritis.

- Not specific

- However, one may think of Sjogren when isolated lung cysts occur or when these are seen with ground glass opacities.

Birt Hogg Dubé Syndrome

- Rare autossomal dominant disease

- Characterized by the presence of lung cysts, fibrofolliculomas and renal tumors (ranging from benign oncoytoma to renal cell carcinoma)

Imaging features:

- CT demonstration of lung cysts in a patient with a renal tumor suggests the diagnosis.

-Regarding the cysts:

      - often subpleural

      - larger than the ones found in LCH and LAM

Thacheobronchial Papillomatosis (Fig. 13; Fig. 14)

- 2 to 5% of patients with squamous papillomas have further spreading to the trancheobronchial tree – tracheobronchial papillomatosis. 

Imaging features:

- Peripheral papillomas arising from small airways may be seen radiographically as  nodular mass lesions within the lung and can cavitate.

- Eventually, cavitary lesions may porgress to large thick/thin walled cysts.

Cyst simulating lesions – Avoiding the pitfalls

Cavities

- Can be caused by primary bronchogenic carcinoma, lung metastasis, vascular inflammatory diseases and infection.

Imaging features: (Fig. 15; Fig. 16)

- Can be differentiated from a lung cyst by the presence of a wall measuring 4 or more mm.

Honeycombing (Fig. 17; Fig. 18)

- Found in patients with end-stage pumonary fibrosis

- Multiple rows of air-filled spaces clustered in the subpleural region, predominantly in the lower lobes

- Typically < 1 cm in diameter.

Bronchiectasis 

- Localized, irreversible dilatation of the bronchial tree

- The term is used to refer only cartilage-containing airways

- There is some degree of correlation between the type of bronchiectasis and anatomical and functional abnormalities (cilindrical: less severe; cystic: most severe).

Classification

-       Cylindrical

-       Varicose

-       Cystic

Cystic bronchiectasis – Imaging features: (Fig. 19 - Fig. 21)

- In the case of cystic bronchiectasis, the airway appears ballooned, sometimes exceeding 2cm in diameter.

- Differentating a cyst from a cystic bronchiectasis is not as easy as one may think:

       - The typical branching appearance of bronchi in areas of bronchiectasis is somestimes almost impossible to follow due to the severe structural changes.

     - Cysts are sometimes isolated and are not associated with other air abnormalities.

     - Cystic formations can be visualized in subpleural topography.

Emphysema

“a condition of the lung characterized by permanent, abnormal enlargement of airspaces distal to the terminal bronchiole, accompanied by the destruction of their walls." – Definition by the American Thoracic Society 

In contrast to true cysts, cyst-like lucencies caused by the destruction of lung parenchyma in emphysema do not have walls.

Classification

As described by the Fleischner Society, emphysema can be classified into three types:

- Centrilobular emphysema:

      - Associated with cigarette smoking

      - Upper lung lobes

      - Central portion of secondary lobules

- Panlobular emphysema:

      - All components of the acinus

      - Involves the entire secondary lobules

- Paraseptal emphysema:

     - Involves the alveolar ducts and sacs in the lung periphery

     - Often associated with spontaneous pneumothorax

- Bullous emphysema

    - reflect a descriprion of emphysema where there is a predominant bullous component. 

Bullae: (Fig. 22)

-       Located in the subpleural region 

-       Diameter >= 1cm, wall thickness <= 1mm.

-       Causes compression of normal lung

-       Often in the upper lobes

Other

-      Several thoracic changes such as those occurring after a thoracic intrevention or post radiotherapy might simulate cystic diseases. Therefore, one should be informed about important medical/surgical interventions that alter the normal lung architecture (Fig. 23; Fig. 24)