Rare idiopathic interstitial pneumonias: LIP and PPFE and rare histologic patterns of interstitial pneumonias: AFOP and BPIP

In the 2013 reclassification of the idiopathic interstitial pneumonias (IIPs), two rare IIPs (idiopathic lymphoid interstitial pneumonia (LIP), idiopathic pleuroparenchymal fibroelastosis (IPPFE)) and two rare histologic patterns (acute fibrinous and organizing pneumonia (AFOP), bronchiolocentric pattern of interstitial pneumonia (BPIP)) are described. All these entities are rare with small series published to date, mostly containing primary and secondary forms of disease. LIP is histologically characterized by diffuse polyclonal lymphoid cell infiltrate surrounding the airways and expanding the interstitium. Thin‐walled cysts and diffuse ground glass are considered the typical radiologic features. The clinical course is highly variable with corticosteroid responsiveness evident in approximately half of cases. IPPFE is defined histologically by coexisting upper lobe pleural and intra‐alveolar fibrosis with elastosis. Dense subpleural irregular fibrosis and consolidation are the cardinal radiologic features. A history of recurrent lower respiratory tract infection is frequent. Responses to immunomodulation have not been reported and the rate of progression appears to be highly variable. AFOP is a rare histologic pattern lying within the spectrum of acute/subacute lung injury, characterized by organizing pneumonia and intra‐alveolar fibrin deposition without hyaline membranes. BPIP is characterized histologically by fibrosis and/or inflammation confined to the alveolar interstitium around bronchovascular bundles, overlapping with peribronchial metaplasia and fibrosis in some series. Currently, AFOP and BPIP are both best viewed as histological entities rather than true clinical disorders, in the absence of characteristic associated imaging patterns and clinical features.