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Article Summary
Idiopathic Interstitial Pneumonia: CT Features

Lynch DA, Travis WD, Müller NL, Galvin JR, Hansell DM, Grenier PA, King TE Jr. Radiology. 2005;236:10-21.

Discussion

• The classification of idiopathic interstitial pneumonia (IIP) is based on histologic criteria; each histologic pattern is associated with a characteristic imaging pattern.
• Similar morphologic patterns of lung injury may occur in other conditions, including collagen vascular disease, hypersensitivity pneumonitis, and drug toxicity; these conditions must be excluded clinically.
• In the correct clinical context, the CT features of UIP and organizing pneumonia are often diagnostic.
• Distinction of UIP from the other interstitial pneumonias is important because UIP is associated with a substantially poorer prognosis than the other entities.
• A multidisciplinary approach by clinician, thoracic radiologist, and pathologist is recommended to generate an accurate diagnosis.

Purpose

• The purpose of the review is to illustrate the radiological aspects of IIP, with emphasis on differential diagnosis.

CT Diagnosis

Morphologic Pattern	CT Features Suggesting IIP Subtype	CT Features Suggesting Other Disease
UIP	Reticular opacities (often patchy) Honeycombing Ground-glass opacity less extensive than reticular pattern Architectural distortion & traction bronchiectasis Lobar volume loss Basal & peripheral distribution	Micronodules Mosaic attenuation or air trapping Nonhoneycomb cysts Extensive ground-glass opacification Consolidation Predominantly peribronchovascular distribution
NSIP	Ground-glass opacity (predominant feature) Often seen with findings of fibrosis (lobar volume loss, reticular pattern, and/or traction bronchiectasis) Basal predominance	Honeycombing Consolidation
DIP	Ground-glass opacity Lower zone/peripheral distribution Limited irregular linear opacities and reticular pattern (usually in lung bases) Well defined, thin walled, and less than 2 centimeter cysts can be seen in areas of ground-glass opacification	Honeycombing Centrilobular nodules
RB & RB-ILD	Mild centrilobular nodularity Small patches of ground-glass opacity Radiographic findings more extensive in RB-ILD	--
COP	Unilateral or bilateral consolidation Often subpleural or peribronchial More frequently in lower lungs Air bronchograms with mild cylindric bronchial dilatation Ground-glass opacity in 60% of cases Reticular opacities less common	--
AIP	Ground-glass opacity (bilateral and patchy) Bronchial dilatation Architectural distortion Consolidation, particularly in dependent lung In comparison with ARDS: More often symmetric lower-lobe distribution Greater prevalence of honeycombing	--
LIP	Dominant finding: ground-glass opacity Sometimes perivascular cysts Less commonly, perivascular honeycombing	--

UIP, usual interstitial pneumonia; NSIP, nonspecific interstitial pneumonia; DIP, desquamative interstitial pneumonia; RB, respiratory bronchiolitis; RB-ILD, respiratory bronchiolitis-associated interstitial lung disease; COP, cryptogenic organizing pneumonia; AIP, acute interstitial pneumonia; LIP, lymphoid interstitial pneumonia.

IPF Diagnostics:

• The positive predictive value of a thoracic radiologist's "certain" reading of IPF is 96%¹
• The CT appearances of UIP and COP may be diagnostic in the correct clinical context but there is substantial overlap in the CT appearances of the other IIPs.
• Other conditions to be considered in the differential diagnosis:
• Collagen vascular disease
• Chronic hypersensitivity pneumonitis
• Asbestosis
• Inhalation exposures²

• The thoracic radiologist must distinguish patients with typical features of UIP, who will usually not require biopsy. While those with atypical features may require a biopsy to define the underlying diagnosis.²
• Acute exacerbation of IPF is a distinguished clinical entity occurring in IPF patients:
• Acute onset of progressive dyspnea or cough
• Diffuse or peripheral ground-glass opacification on CT imaging

Summary:

• Classification of IIP is based on a histologic pattern that is associated with a characteristic imaging pattern and a clinical syndrome.
• Diagnosis of UIP and organizing pneumonia can often be made on the basis of the clinical profile and distinctive CT features. Other IIPs require histologic input for definitive diagnosis.
• The diagnosis of IIP requires dynamic integration of the morphologic patterns identified by the thoracic radiologist and pathologist with the clinical features evaluated by the clinician.

References

1. Hunninghake GW, Zimmerman MB, Schwartz DA, et al. Utility of a lung biopsy for the diagnosis of idiopathic pulmonary fibrosis. Am J Respir Crit Care Med. 2001;164:193-196.
2. American Thoracic Society, European Respiratory Society. American Thoracic Society/European Respiratory Society International Multidisciplinary Consensus Classification of the Idiopathic Interstitial Pneumonias. Am J Respir Crit Care Med. 2002;165:277-304.

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