The AP chest x-ray shows a dense opacification of the left hemithorax with a mediastinal shift to the right. This is due to mass effect (massive pleural effusion) on the left.

Massive pleural effusions are most commonly due to malignancy, especially secondaries from the lung & breast carcinoma.

Causes of opacification of hemithorax are:

• Massive pleural effusion – mediastinal shift to the opposite side.
• Consolidation – no mediastinal shift.
• Collapse – mediastinum pulled to the same side.

Reference: Grainger & Allison’s Diagnostic Radiology.