Rounded atelectasis is a form of chronic, localized lung collapse that appears as a rounded or oval-shaped mass-like opacity on imaging. It is often associated with pleural disease, particularly pleural thickening or fibrosis, and typically results from mechanical folding and collapse of the lung tissue adjacent to pleural abnormalities. Despite its mass-like appearance, it is benign and non-neoplastic.

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Radiological Features

1. Chest X-Ray (CXR):
   • Appearance:
     • A rounded or oval opacity located near the pleura.
     • Often found in the lower lobes.
   • Associated Features:
     • Pleural thickening or calcifications (indicative of asbestos exposure or other pleural diseases).
   • May mimic a mass, requiring further imaging for differentiation.
2. CT (High-Resolution CT):
   • Rounded Mass-Like Opacity:
     • Sharply defined, subpleural, rounded or oval opacity.
     • Size typically ranges from a few centimeters to larger areas.
   • Comet-Tail Sign:
     • Curved bronchovascular structures leading toward the hilum, characteristic of rounded atelectasis.
   • Associated Pleural Abnormalities:
     • Pleural thickening or plaques, often from prior asbestos exposure.
   • Volume Loss:
     • Subtle reduction in volume of the affected region or adjacent lung parenchyma.
3. Key Features for Differentiation:
   • No invasive growth.
   • Lack of enhancement (rules out malignancy).

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Common Causes and Associations

Rounded atelectasis occurs due to pleural disease, which leads to adjacent lung collapse:

1. Asbestos-Related Disease:
   • Strongly associated with asbestos exposure.
   • Pleural plaques or diffuse pleural thickening are common.
2. Pleural Effusion:
   • Chronic effusion causing adjacent lung compression.
3. Pleural Fibrosis:
   • Scarring leads to folding and collapse of lung parenchyma.
4. Post-Infectious or Inflammatory Pleural Disease:
   • Chronic inflammation can induce pleural thickening and atelectasis.

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Differential Diagnosis

Rounded atelectasis can mimic other pleural or parenchymal conditions:

1. Malignancy (e.g., Primary or Metastatic Lung Cancer):
   • Distinguished by lack of comet-tail sign and invasive behavior.
2. Pulmonary Abscess:
   • Typically associated with fluid levels and infectious symptoms.
3. Benign Tumors:
   • Often isolated without pleural abnormalities.

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Clinical Relevance

• Rounded atelectasis is benign and does not require treatment.
• Correct identification is crucial to avoid unnecessary invasive investigations for suspected malignancy.
• Close association with pleural disease, particularly asbestos exposure, underscores the need for monitoring asbestos-exposed individuals for other complications like mesothelioma.