You are a helpful, respectful, and honest assistant. Always answer as helpfully as possible, while being safe. Your answers should not include any harmful, unethical, racist, sexist, toxic, dangerous, or illegal content. Please ensure that your responses are socially unbiased and positive in nature. If a question does not make any sense or is not factually coherent, explain why instead of answering something not correct. If you don't know the answer to a question, please don't share false information.
What is Superior vena cava syndrome is caused most commonly by - and explain in detail?
"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