Desquamative interstitial pneumonia.

Desquamative interstitial pneumonia (DIP) is one of the rarest of the idiopathic interstitial pneumonias. It is characterized by the accumulation of macrophages in large numbers in the alveolar spaces associated with interstitial inflammation and/or fibrosis. The macrophages frequently contain light brown pigment, and because of their association with smoking have been called 'smoker's macrophages'. Lymphoid nodules are common, as is a sparse but distinct eosinophil infiltrate. Most cases of DIP are caused by cigarette smoking, but drugs and other inhaled agents, including marijuana smoke, can also produce the same disease. Although respiratory bronchiolitis-interstitial lung disease (RB-ILD) is a closely related process, there are prognostic reasons for continuing to separate it from DIP when possible. The proposed relationship of DIP to fibrotic non-specific interstitial pneumonia (NSIP) remains uncertain. The prognosis of DIP appears to be significantly better than that of fibrotic NSIP, so while there can be morphological overlap between the two, merging them into one disease may hide important prognostic information. Although the majority of DIP patients improve on treatment, some patients develop progressive irreversible fibrosis.