Key histologic features |
Cellular pattern* |
Mild to moderate interstitial chronic inflammation |
Type II pneumocyte hyperplasia in areas of inflammation |
Fibrosing pattern* |
Dense or loose interstitial fibrosis lacking the temporal heterogeneity pattern and/or patchy features of UIP |
Lung architecture may appear lost on examination of H&E-stained section, but is relatively preserved with elastin stains |
Interstitial chronic inflammation - mild or moderate |
Pertinent negative findings |
Cellular pattern |
Dense interstitial fibrosis: absent |
Organizing pneumonia: absent or not prominent |
Diffuse severe alveolar septal inflammation: absent |
Fibrosing pattern |
Temporal heterogeneity pattern: fibroblastic foci with dense fibrosis are inconspicuous or absent - this is especially important in cases with patchy involvement and subpleural or paraseptal distribution |
Both patterns |
Acute lung injury pattern, especially hyaline membranes: absent |
Eosinophils: inconspicuous or absent |
Granulomas: inconspicuous or absent |
Viral inclusions and organisms on special stains: not present |
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