The chest x-ray shows an incidental finding of azygous lobe, which is an anatomical variant. The lobe is outlined by the azygous fissure, at the lower end of which a dense azygous vein can be seen.

The incidence of an azygous lobe, in the right apical area, is about 0.5%. The accessory lobe mostly has no clinical significance. The azygous fissure limits the spread of pathological conditions from the right upper lobe to the azygous lobe. The azygous fissure also excludes the pneumothorax from the apical portion of the pleural space.

Reference: Fundamentals of Diagnostic Radiology by Brant and Helms, 4th Edition.