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Background
A lung cyst is circumscribed airspace that is surrounded by an epithelial or distinct fibrous wall < 2 mm in thickness and shows a well-defined interface with normal lung tissue. Cysts are frequently seen on chest CT by the widespread use of CT scans in daily clinical practice, and differential diagnosis can be challenging due to including a heterogeneous group of pathologies and diseases, which could be such as lymphangioleiomyomatosis, Birt-Hogg-Dubé Syndrome, pulmonary Langerhans cell histiocytosis, desquamative interstitial pneumonia, and lymphocytic interstitial pneumonia. Patients with cysts usually have no symptoms and are discovered on chest imaging for another reason, or sometimes present cough or shortness of breath. A radiologic assessment is especially crucial in narrowing the differential diagnosis and coming to the correct diagnosis, although a definite diagnosis requires a multidisciplinary approach incorporating the patient's clinical history, physical examination, laboratory findings, and (rarely) biopsy. The radiological diagnosis is achieved according to the characterization of cysts that includes the shape, size, number, morphology, and distribution of cysts within the lung parenchyma, and moreover probable findings of associated lung parenchymal abnormalities; for example, nodules or ground glass opacity. The aim of this presentation is to provide a useful and systematic guide to classify cystic lung diseases and differentiate them from other mimics, based on the radiologic diagnostic algorithm.

Educational Goals / Teaching Points
Cyst definition and differentiation from its mimics. Review pathology, epidemiology, and clinical manifestations of cystic lung diseases. The stepwise approach to the diagnosis of cystic lung diseases on CT is as follows. Step 1: Are these true cysts or similar air-filled structures? Step 2: Are intraparenchymal cysts solitary or multifocal/diffuse? Step 3: Are multifocal/diffuse cysts associated with other radiological findings? Step 4: Multifocal/diffuse cysts associated with nodules. Step 5: Multifocal/diffuse cysts associated with ground glass opacity. Show relevant cases.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Chest CT, cystic lung disease, air-filled lesions, lung cysts.

Conclusion
A variety of pathological processes and diseases can present as diffuse cystic lung disease. Chest CT remains the most useful noninvasive diagnostic tool for the evaluation of cystic lung diseases. This algorithmic radiologic approach provides an easy method for accurate diagnosis of various cystic lung diseases.