| Key histologic features |
| Uniform involvement of lung parenchyma. |
| Prominent accumulation of alveolar macrophages (may show fine granular positivity with iron stains). |
| Mild to moderate fibrotic thickening of alveolar septa. |
| Mild interstitial chronic inflammation (lymphoid aggregates). |
| Pertinent negative findings |
| Dense and extensive fibrosis: inconspicuous or absent. |
| Smooth muscle proliferation: inconspicuous or absent. |
| Honeycomb fibrosis absent. |
| Fibroblastic foci and organizing pneumonia: inconspicuous or absent. |
| Eosinophils: inconspicuous, absent, or only focal. |
