Differential:
1.   Persistent left SVC
2.   Anomalous pulmonary venous return
3.   Mediastinal adenopathy

Discussion:
   Persistence of the embryologic left cardinal vein results in a "double" superior vena cava and is seen in 0.4% of the population. The left sided SVC may be distinguished from mediastinal adenopathy by observation of its crainocaudal extent, and from an anomalous pulmonary vein by observation of its origin and termination. Anomalous pulmonary veins will arise in the lung parenchyma and often course over the aortic arch into the left brachiocephalic vein. The left sided SVC may drain into the coronary sinus and thence the right atrium, the left atrium, the left brachiocephalic vein, or the right SVC.


REFEERNCES:
1.   Lee, Computed Tomography of the Whole Body, pp. 844, 847
2.   Marino, Thomas. The Embryologic Basis for Cardiovascular Congenital Abnormalities, at - LINK -
3.   Bergman, Afifi, and Miyauchi Superior Vena Cava and Brachiocephalic Trunk, in the Illustrated Encyclopedia of Human Anatomic Variation. - LINK -