Synopsis

 intraluminal organizing fibrosis in distal airspaces

• bronchiles
• alveolar ducts
• alveoli

 patchy distribution (pulmonary nodules)
 preservation of lung architecture
 uniform temporal appearance
 mild interstitial chronic inflammation
 negative elements

• lack of interstitial fibrosis (except incidental scars or apical fibrosis)
• lack of granulomas
• lack of neutrophilic infiltration or abcesses
• lack of necrosis
• lack of hyaline membranes or prominent airspace fibrin
• lack of prominent eosinophilic infiltration

Variants

 diffuse organizing pneumonia
 localized organizing pneumonia

Etiology

 organizing pulmonary infections

• Pneumocystis carinii pneumonia

 organizing diffuse alveolar damage
 drug reaction (drug-associated organizing pneumonia)
 toxic-associated organizing pneumonia

• cocaine abuse

 dysimmune diseases/auto-immune diseases (collagen vascular diseases, connectivitis)
 HIV infection (HIV-associated organizing pneumonia)
 viral infections (virus-associated organizing pneumonia)

 hypersensitivity pneumonitis (extrinsic allergic alveolitis)
 chronic eosinophilic pneumonia
 myelodysplastic syndrome
 ionizing radiations
 pulmonary hemorrhages
 pulmonary vasculitis

• Wegener granulomatosis

 airway diseases complicated by infections

• broncitis and emphysema
• bronchiectasis
• pulmonary cystic fibrosis
• aspiration pneumonia
• chronic bronchiolitis

 pulmonary airways obstruction
 peripheral reaction around

• pulmonary abcess
• pulmonary infarcts
• pulmonary tumors
• Wegener granulomatosis

 bronchiolitis obliterans organizing pneumonia
 acute fibrinous organizing pneumonia
 cryptogenic organizing pneumonia (COP) (or cryptogenic BOOP)

Differential diagnosis

 UIP pattern

• interstitial pulmonary fibrosis (UIP)

 acute interstitial fibrosis (AIP)
 DAD pattern

• organizing phase of DAD (diffuse alveolar damage)

 NSIP pattern (cellualr pattern of NSIP)
 acute fibrinous and organizing pneumonia (AFOP)
 eosinophilic pneumonia (if eosinophilic infiltration)