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Surgical Pathology Criteria

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Cryptogenic Organizing Pneumonia - Bronchiolitis Obliterans Organizing Pneumonia


  • Subacute onset lung disease characterized by intra-air space plugs of granulation tissue

Alternate/Historical Names

  • BOOP

Diagnostic Criteria

  • Organizing pneumonia (BOOP) is a pattern that has many causes including
    • Infection, toxins, drugs, radiation, inflammatory bowel disease
    • May present associated with connective tissue disease
    • When idiopathic, it is termed cryptogenic organizing pneumonia (COP)
  • Clinical
    • Subacute onset of dyspnea, cough
      • May follow an upper respiratory tract infection
      • Systemic symptoms may also be present
    • Average age mid 50's
    • Majority of patients respond to steroids
  • High resolution computed tomography (HRCT)
    • Patchy air space consolidation and nodules
    • No honeycombing
  • Histopathologic features
    • Patchy involvement of lung
    • Distal airways and adjacent alveoli filled by fibromyxoid plugs of granulation tissue
      • All plugs appear to be at the same stage (temporal uniformity)
      • Alveolar ducts and alveoli primarily involved
      • Respiratory bronchioles usually involved
      • Adjacent lung parenchyma is normal
    • Mild to moderate interstitial infiltrate of plasma cells, lymphocytes
      • Neutrophils and eosinophils infrequent
    • Scant fibrin may be seen in air spaces
    • Obstructive pneumonia may be seen distal to plugs
      • Alveoli filled with lipid-laden foamy macrophages
    • Minimal fibrosis
      • No remodeling or honeycomb change
  • Features not seen
    • Hyaline membranes or abundant fibrin
    • Granulomas or necrosis
    • Prominent neutrophils or eosinophils
  • COP/BOOP pattern may occur in the setting of acute exacerbation of a chronic interstitial lung disease such as UIP
    • Underlying architectural changes such as peripheral honeycombing may help identify the diagnosis
    • Clinical or radiographic history of pre-existing pulmonary disease is very helpful

Gerald J Berry MD
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342

Original posting/updates: 11/20/10

Differential Diagnosis

Usual Interstitial Pneumonia Cryptogenic Organizing Pneumonia
Temporal heterogeneity Temporally uniform
Honeycomb change prominent Honeycomb change rare
Interstitial fibrosing process Intralumenal fibrosing process
Fibroblastic foci adjacent to mature collagen, covered by epithelium Granulation tissue foci without significant collagen, not subepithelial
Random distribution Bronchiolocentric distribution
Predominantly collagenous fibrosis Predominantly fibroblastic fibrosis
Organizing pneumonia may be seen focally in UIP or in the setting of acute exacerbation of UIP


Cryptogenic Organizing Pneumonia Acute Interstitial Pneumonia
Patchy distribution Diffuse
No hyaline membranes Hyaline membranes prominent in early stage and fragments in organizing phase
No collagen deposition, architecture preserved Diffuse honeycomb change may be seen in late stage


Cryptogenic Organizing Pneumonia (BOOP) Acute Eosinophilic Pneumonia
Subacute onset Acute, fulminant onset
Eosinophils infrequent Prominent eosinophils in lavage and biopsy specimens
BOOP pattern may be seen in acute eosinophilic pneumonia


  • Specific causes of organizing pneumonia pattern must always be ruled out
    • These include infection, toxins, drugs, radiation, inflammatory bowel disease
  • Organizing pneumonia pattern may be superimposed on a chronic process such as UIP in the setting of an acute exacerbation
    • Presence of more than rare intra-airway granulation tissue plugs should suggest COP/BOOP
    • Clinical and radiographic history is important

Classification / Lists

Smoking Related Lung Diseases

Idiopathic Interstitial Lung Diseases

Other Diffuse Parenchymal Lung Diseases



  • Travis WD, Colby TV, Koss MN, Rosado-de-Christenson ML, Müller NL, King TE Jr.  Non-neoplastic Disorders of the Lower Respiratory Tract, AFIP Atlas of Nontumor Pathology, First Series, Fascicle 2, 2002.
  • American Thoracic Society; European Respiratory Society. ATS/ERS International Multidisciplinary Consensus Classification of the Idiopathic Interstitial Pneumonias. This joint statement of the American Thoracic Society (ATS), and the European Respiratory Society (ERS) was adopted by the ATS board of directors, June 2001 and by the ERS Executive Committee, June 2001. Am J Respir Crit Care Med. 2002 Jan 15;165(2):277-304.
  • Epler GR, Colby TV, McLoud TC, Carrington CB, Gaensler EA. Bronchiolitis obliterans organizing pneumonia. N Engl J Med. 1985 Jan 17;312(3):152-8.
  • Churg A, Wright JL, Tazelaar HD. Acute exacerbations of fibrotic interstitial  lung disease. Histopathology. 2010  [Epub ahead of print].
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