Superior vena cava syndrome is caused most commonly by -

"Superior vena cava syndrome results from obstruction of the superior vena cava by central upper lobe tumors, usually the small cell tumors" - Cecil, 5/e, p 1207 (Cecil is a standard text book of Medicine by Saunders publication) "Superior vena cava (SVC) syndrome most frequently occurs with small-cell carcinoma, with bulkly enlargement of involved mediastinal lymph nodes and compression of the SVC." - Schwartz 9/e