[A case of unification anomalies of the superior vena cava and absence of the left brachiocephalic vein with dilated azygos and hemiazygos veins].

We report a case of dilated azygos vein simulating a tumor in the posterior mediastinum. The patient was a 47-year-old male who was found to have anomalies of unification of the superior vena cava and absence of the left brachiocephalic vein. He was admitted to our hospital because chest radiographs and computed tomography (CT) scans showed a well-circumscribed oval mass 2.5 cm in diameter, behind the bifurcation of the trachea. This lesion, which was markedly enhanced in chest CT, magnetic resonance imaging (MRI) and MR angiography (MRA) was diagnosed as a dilated azygos vein. An enlarged hemiazygos vein and collaterals were also recognized. Upon digital subtraction venography of both upper extremities, the right atrium and the left brachiocephalic vein could not been distinguished from the superior vena cava and the infraclavicular vein, respectively. Collaterals entering the azygos and hemiazygos veins were, however, recognized. It was considered that MRA and disital subtraction venography were very useful for confirmation of the diagnosis in this case.