The chest x-ray shows
sail shaped density behind the heart with a parallel left heart border silhouetting the medial left hemi-diaphragm (double left heart border or sail sign).
The left main bronchus is depressed & there is volume loss in the left hemithorax.
This constellation of findings should raise suspicion for an endobronchial lesion given history of smoking.
Other causes of left lower lobe collapse are – mucus plugging, foreign body obstruction. Rarely a mediastinal mass & severe cardiomegaly can lead to extrinsic compression of the left lower lobe bronchus causing left lower lobe collapse.
Reference: Brant & Helms’ Fundamentals of Diagnostic Radiology.