Lipoid Pneumonia

Helen T. Winer-Muram, MD

Key Facts

Terminology

Exogenous lipid pneumonia: Aspiration or inhalation of animal, vegetable, or mineral oils

Imaging Findings

Low CT attenuation areas (-10 and -150 HU) in consolidated lung
Initial: Predominant ground-glass opacities in centrilobular (less aspiration) or panlobular distribution (more aspiration)
1 week: Aspirated segment increases in density to frank consolidation
2-4 weeks: Aspirated segment decreases in density, returns to primarily ground-glass opacities
2-4 months: As ground-glass opacities evolve, fibrosis develops, volume loss (up to 50% of affected segment) common
Fat attenuation 80% (fat not seen in all cases)

Top Differential Diagnoses

Hamartoma
Diaphragmatic Hernias

Pathology

Mineral oil most common agent, common laxative in elderly or children with chronic constipation

Clinical Issues

History important but possibility of lipoid pneumonia usually not considered until after radiographic diagnosis

Diagnostic Checklist

Consider lipoid pneumonia in masses that at 1st glance look like bronchogenic carcinoma

Axial CECT shows the typical features of chronic lipoid pneumonia with an irregular mass-like opacity in the left lower lobe

Axial CECT in the same patient at the same level shows that the consolidation contains focal fat

on mediastinal windows.

TERMINOLOGY

Abbreviations and Synonyms

Endogenous lipid pneumonia (“golden pneumonia”) and exogenous lipid pneumonia

Definitions

Endogenous lipid pneumonia: Accumulation of lipid containing macrophages secondary to airway obstruction
Exogenous lipid pneumonia: Aspiration or inhalation of animal, vegetable, or mineral oils

IMAGING FINDINGS

General Features

Best diagnostic clue: Low CT attenuation areas (between -10 and -150 Hounsfield units) in consolidated lung
Patient position/location: Exogenous: Gravity-dependent areas and right middle lobe (RML)

CT Findings

Endogenous: Lipid-laden macrophages show no fatty attenuation at CT
Controlled exogenous lipoid aspiration in animal models, sequence of events
- Initial: Predominant ground-glass opacities in centrilobular (less aspiration) or panlobular distribution (more aspiration)
  - HU of attenuated material low (close to attenuation of original material, -150 HU)
- 1 week: Aspirated segment increases in density to frank consolidation
  - HU of affected segment increases (may be all water density)
- 2-4 weeks: Aspirated segment decreases in density, returns to primarily ground-glass opacities
  - “Crazy-paving” pattern evolves during this time (due to macrophage and inflammatory cell migration to interstitial lymphatics)
  - HU of attenuated segment again decreases
  - Ground-glass opacities may completely resolve, leaving smooth interlobular septal thickening
- 2-4 months: As ground-glass opacities evolve, fibrosis develops, volume loss (up to 50% of affected segment) common
  - Volume loss signs include vascular crowding, traction bronchiectasis, fissural displacement
- Clinical cases will mirror this experimental sequence of events
  - Consolidation most common finding (90%)
  - Fat attenuation (80%) (fat not seen in all cases)
  - Ground-glass opacities (50%)
  - “Crazy-paving” pattern (33%)
  - Irregular-shaped mass (66%)
Acute vs. chronic aspiration, findings often overlap
- Small effusions common in acute (50%), not seen with chronic
- Masses more common in chronic (66%), not seen with acute aspiration
- Signs of volume loss more common in chronic (80%) vs. acute (25%)
Fat attenuation
- Seen with both acute and chronic lipoid aspiration
- Acute may have larger volume of low attenuation material
  - Fat may shift to dependent lung with postural change
- Chronic mass-like areas of aspiration often have smaller (< 10% total volume) foci of fat attenuation
- 15-20% will not have fat attenuation
  - Percentage may actually be smaller with routine use of MDCT and thin-collimation
- Partial volume artifact of abnormal lung admixed with air may give false-positive fat attenuation
Distribution
- Gravity dependent: Posterior segments of upper lobes and superior segments of lower lobes in supine position, basilar segments in upright position
- Right middle lobe also common for unknown reason
Resolution
- Acute aspiration usually improves (80%) but rarely clears completely
- Chronic aspiration usually stable and indolent over time
Complications
- Superinfection with atypical mycobacteria (usually M. fortuitum)
  - Suspect if have cavitated nodules
- Occasional reports of bronchogenic carcinoma developing in chronic aspiration
  - Suspect if mass enlarging or cavitation
Fire-eater’s pneumonia
- Unique form of lipoid pneumonia due to aspiration of paraffin hydrocarbons used to fuel fire
- Aspiration results in acute lipoid pneumonia
- Besides fat attenuation in lung, often develop pneumatoceles

Radiographic Findings

Radiography
- Radiographic appearance nonspecific, either consolidation or interstitial opacities
- Prominent mass leads to workup for presumptive diagnosis of bronchogenic carcinoma

MR Findings

May show fat: High T1 and T2 signal or demonstrate chemical shift on in-out phase T1-weighted imaging
- Hemorrhage (if acute animal fat ingestion) will also be high at T1- and T2-weighted imaging

Imaging Recommendations