A 58-year-old woman with advanced lung cancer presents with progressive dyspnea over 3 days. On examination, she has reduced breath sounds on the left side with dullness to percussion. Chest X-ray reveals a wedge-shaped opacity in the left lower lobe with the apex pointing toward the hilum. The left hilum is displaced medially, and the left hemidiaphragm is elevated. The mediastinum is shifted toward the left. No air bronchograms are visible within the opacity. What is the most likely radiological diagnosis?

Explanation

## Collapse: Radiological Signs and Mechanisms **Key Point:** Collapse (atelectasis) is characterized by volume loss with mediastinal/hilar shift, elevated hemidiaphragm, and absence of air bronchograms. The wedge-shaped opacity with apex toward the hilum is classic for lobar collapse. ### Radiological Features of Collapse | Feature | Collapse | Consolidation | |---------|---|---| | **Shape** | Wedge-shaped, apex toward hilum | Homogeneous, amorphous | | **Hilar shift** | Toward affected lobe (medial in left lower) | No shift | | **Mediastinal shift** | Toward affected side | No shift | | **Hemidiaphragm** | Elevated on affected side | Normal position | | **Air bronchograms** | Absent | Present | | **Volume** | Decreased (loss of air) | Normal | **High-Yield:** The combination of **mediastinal shift + hilar shift + elevated hemidiaphragm + absent air bronchograms** = collapse, not consolidation. ### Pathophysiology of Collapse in Lung Cancer 1. **Endobronchial obstruction** — tumor occludes left lower lobe bronchus 2. **Air absorption** — distal air is reabsorbed into bloodstream 3. **Volume loss** — alveoli collapse as air is removed 4. **Mediastinal shift** — negative pressure pulls mediastinum toward collapsed lobe 5. **Hilar displacement** — hilum moves medially with collapsing lobe **Clinical Pearl:** In malignancy, collapse is often due to endobronchial tumor obstruction. Post-obstructive pneumonia may develop distal to the obstruction, but the primary finding here is collapse (wedge shape, shift, no air bronchograms). **Mnemonic: SHIFT for Collapse** - **S**hift of mediastinum and hilum - **H**emidiaphragm elevated - **I**ncrease in density (wedge opacity) - **F**ew or no air bronchograms - **T**otal volume loss ### Why This Is Collapse and Not Consolidation 1. **Wedge-shaped opacity** — classic collapse morphology 2. **Mediastinal shift toward left** — indicates volume loss pulling mediastinum 3. **Hilar displacement medially** — hilum moves with collapsing lobe 4. **Elevated left hemidiaphragm** — volume loss effect 5. **No air bronchograms** — airways are collapsed, not patent 6. **Clinical context** — lung cancer with endobronchial obstruction ### Distinction from Post-Obstructive Pneumonia While post-obstructive pneumonia can coexist with collapse, the primary radiological finding here is collapse: - Consolidation would show air bronchograms and normal mediastinal position - Collapse shows mediastinal shift and absent air bronchograms - If both were present, collapse would dominate the radiological picture [cite:Felson's Principles of Chest Roentgenology Ch 3]