What are the characteristic chest imaging findings of silicosis on plain radiograph and high‑resolution CT?

Characteristic Chest Imaging Findings of Silicosis

Silicosis on plain radiograph characteristically shows bilateral small rounded opacities (nodules) with upper lobe predominance, hilar lymph node enlargement (often with eggshell calcification), and may progress to progressive massive fibrosis—features that distinguish it from asbestosis, which presents with lower lobe irregular opacities. 1

Plain Radiograph Findings

Simple Silicosis

• Small rounded opacities (nodules) are the hallmark finding, classified as type p, q, or r opacities using the International Labor Organization (ILO) classification system 2, 3
• Upper and middle lung zone predominance is typical, contrasting with the lower lobe distribution seen in asbestosis 1, 4
• Bilateral distribution with profusion scores ranging from 1/0 (presumptively diagnostic) to higher categories indicating more severe disease 4
• Nodules are typically well-defined and range from 1-10mm in diameter 2

Lymph Node Abnormalities

• Hilar and mediastinal lymph node enlargement is a characteristic feature that helps distinguish silicosis from asbestosis 1
• Eggshell calcification of hilar nodes (peripheral rim calcification) is highly suggestive of silicosis, though not always present 2

Progressive Massive Fibrosis (PMF)

• Large conglomerate masses typically in the upper lobes, representing coalescence of smaller nodules 1, 3
• These masses are bilateral, irregular, and non-homogeneous, often with calcification 4
• PMF indicates advanced disease and is associated with significant functional impairment 3

High-Resolution CT (HRCT) Findings

Nodular Patterns

• Small nodules with perilymphatic distribution are the most specific HRCT finding for silicosis 2
• Nodules are located along the bronchovascular bundles, interlobular septa, and subpleural regions 2
• Centrilobular nodules may be present, particularly in early disease 5
• Branching centrilobular structures are seen in 68% of silica-exposed workers and may represent early disease 5

Distribution and Profusion

• Upper and posterior lung zone predominance is characteristic 4
• HRCT demonstrates better inter-reader agreement (κ = 0.84) compared to plain radiographs (κ = 0.54) for diagnosing silicosis 5
• Profusion of opacities on HRCT correlates inversely with pulmonary function, particularly total lung capacity and forced vital capacity 5

Advanced Disease Features

• Progressive massive fibrosis appears as large, irregular masses with non-homogeneous density and calcification 4, 3
• Conglomerate masses may contain areas of necrosis or cavitation, particularly when complicated by tuberculosis 4
• Architectural distortion with traction bronchiectasis may be present in advanced fibrosis 3

Additional HRCT Findings

• Ground-glass opacities may be seen, particularly in acute silicoproteinosis 3
• Diffuse interstitial fibrosis with chronic inflammation can occur in rapidly progressive disease 3
• Focal alveolar proteinosis has been reported in some cases 3
• Polarized light microscopy of tissue reveals birefringent mineral dust particles (silica and silicates) rather than predominantly carbonaceous material 3

Key Distinguishing Features from Other Pneumoconioses

Silicosis vs. Asbestosis

• Upper lobe rounded opacities, hilar node enlargement, and PMF are NOT features of asbestosis; their presence suggests silicosis or another cause 1
• Asbestosis shows lower lobe irregular opacities with subpleural predominance 1
• Asbestosis is associated with pleural plaques and diffuse pleural thickening, which are uncommon in pure silicosis 1

Silicosis vs. Coal Worker's Pneumoconiosis

• Both show small rounded nodules, but silicosis more commonly demonstrates eggshell calcification of lymph nodes 2
• Mixed dust lesions may be present in workers with combined exposures 3
• Classic coal worker's pneumoconiosis lesions were found in only 4 of 13 cases in one series of rapidly progressive disease, with most showing features of accelerated silicosis 3

Clinical Correlation and Diagnostic Accuracy

Radiographic-Pathologic Correlation

• Specimens with histologic features of silicosis are significantly associated with rounded (type p, q, or r) opacities on chest imaging (P = 0.047) 3
• Grade 3 interstitial fibrosis is associated with irregular (type s, t, or u) opacities (P = 0.02) 3

Functional Correlation

• HRCT profusion scores correlate with pulmonary function impairment, particularly reduced total lung capacity and forced vital capacity 5
• The presence of PMF indicates severe functional impairment and increased mortality risk 3

Common Pitfalls and Caveats

• Plain radiographs may miss early disease: 15-20% of pathologically confirmed pneumoconiosis cases show no radiographic evidence of parenchymal fibrosis 1
• HRCT is more sensitive for detecting early parenchymal abnormalities and provides better inter-reader agreement 5, 6
• Mixed exposures (silica plus asbestos, coal dust, or welding fumes) can produce combined disease patterns that may confound diagnosis 1
• Tuberculosis risk is 2.8 to 39-fold higher in silicosis patients; cavitation or rapid progression should prompt evaluation for silicotuberculosis 4
• Rapidly progressive pneumoconiosis in coal miners often shows features of accelerated silicosis rather than classic coal worker's pneumoconiosis, suggesting high silica exposure 3
• Berylliosis can mimic silicosis on CT but typically shows a sarcoidosis-like pattern 2