[Interstitial lung diseases. The pattern is important]

Interstitial lung diseases (ILDs) comprise a number of rare entities with an estimated incidence of 10-25 per 100,000 inhabitants but the incidence greatly increases beyond the age of 65 years. The prognosis depends on the underlying cause. The fibrotic disorders show a set of radiological and histopathological patterns that are distinct but not entirely specific. In the absence of a clear clinical picture and consistent high resolution computed tomography (HRCT) findings, patients are advised to undergo surgical lung biopsies from two or three lung lobes (or transbronchial biopsies) to determine the histopathological pattern. The ILDs are differentiated into disorders of known causes (e.g. collagen vascular disease, drug-related), idiopathic interstitial pneumonia (IIP), granulomatous ILDs (e.g. sarcoidosis) and other forms of ILD (e.g. Langerhans' cell histiocytosis). The IIPs encompass idiopathic pulmonary fibrosis (IPF), non-specific interstitial pneumonia, desquamative interstitial pneumonia, respiratory bronchiolitis-interstitial lung disease, cryptogen organizing pneumonia, lymphocytic interstitial pneumonia and acute interstitial pneumonia. Additionally, a category of unclassified interstitial pneumonia exists. The pathologist has to recognize and address the histopathological pattern. In a multidisciplinary discussion the disorder is allocated to a clinicopathological entity and the histopathological pattern plays a major role in the classification of the entity. Recognition of the underlying pattern and the respective histopathological differential diagnoses is important as the therapy varies depending on the cause and ranges from elimination of the stimulus (if possible) to antifibrotic drug therapy up to preparation for lung transplantation.