The differential diagnosis of the cystic lung disease is extensive, ranging from an isolated incidental lung cyst to multiple diffuse lung cysts with other varied associated lung abnormalities. High resolution computed tomography (HRCT) thorax is the imaging modality of choice in the evaluation of these diseases, by improving the characterisation of the lung cysts (number, size, shape, distribution, wall thickness and regularity), and associated pulmonary and extrapulmonary abnormalities (like ground glass densities, nodules, interstitial fibrosis, mediastinal lymphadenopathy). After differentiating from the common cyst mimickers (cavity, centrilobular emphysema and cystic bronchiectasis), they should be grouped by location into subpleural cysts (paraseptal emphysema, bulla and honeycombing) and intraparenchymal cysts. Intraparenchymal cysts are in turn categorised into solitary / localised cysts (incidental cyst, congenital pulmonary airway malformations, intrapulmonary bronchogenic cyst and hydatid cyst) and multiple diffuse cysts. The next step will be the categorisation of these multiple diffuse cysts into those without other lung abnormalities (lymphangioleiomyomatosis and Birt-Hogg-Dube syndrome) and those with other lung abnormalities such as predominant ground glass densities (Pneumocystis jirovecii pneumonia, desquamative interstitial pneumonia, lymphoid interstitial pneumonia, subacute hypersensitivity pneumonitis and pneumatoceles) or predominant nodules (pulmonary Langerhans cell histiocytosis, amyloidosis, light chain deposition disease, follicular bronchiolitis, recurrent respiratory papillomatosis, cystic tuberculosis, cystic lung metastasis, sarcoidosis, and pulmonary mesenchymal cystic hamartomas). We conclude that this orderly radiologic approach in a given HRCT chest study of a cystic lung disease (in addition to correlation with clinical and laboratory findings), can lead us to a specific diagnosis in majority of instances, thereby optimising their treatment management, without the need of an invasive biopsy. KEY WORDS HRCT Thorax, Lung Cysts, Ground Glass Densities, Lung Nodules
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