The silhouette sign and the inferior vena cava.

Pulmonary lesions causing obliteration of the normal inferior vena cava (IVC) shadow on the lateral radiograph are described. Retrocardiac mediastinal mass lesions and subpulmonic pleural effusions may also obliterate the IVC contour. Loss of the IVC shadow is an additional radiographic sign of right lower lobe collapse and frequently of disease involving the medial basal segment of the right lower lobe. This finding should be an indication that further investigation is needed to explain the loss of normal aeration.