A normal variant produced by an indentation in the R. upper lobe of the lung by four layers of pleura (two layers of visceral pleura and two layers of parietal pleura). It is produced by invagination of the azygous vein. This is not a true lobe, since it does not have a separate bronchus.
Approximately 0.5% of routine chest radiographs reveal an azygos lobe. This normal variant consists of the upper right lung developing around the azygos vein. The vein remains in the substance of the lungs but as the upper lobe develops inferiorly to superiorly, a double fold of visceral pleura (two layers of visceral pleura and two layers of parietal pleura) develops. This invagination can be identified on a chest x-ray. The classic finding on a chest film created by the double pleural fold is the “reverse comma sign�? that extends towards the top of the right lung in a curvilinear fashion ending at the apex of the parietal pleura. The base of the comma is formed by the vein itself. It is important to remember that the azygos lobe is not a true lobe with a separate segmental bronchus. However, this lobe can be involved independent of the other lobes of the lung with disease such as cancer, tuberculosis, or pulmonary metastasis.