In order to understand this, you should consider two facts; What is the mediastinum, and how does cancer operate. The mediastinum is nothing more then a central thoracic compartment... Literally in the middle. Now normally the mediastinum itself is more or less connective tissue. However this compartment does contain and touch some very important structures. The Heart, The great vessels (Aorta, Vena Cava), the esophagus, and numerous lymph ducts and nodes.
Cancer as you may know, is what in medicine is called a malignant neoplasm, tumors that grow and spread rapidly, by abnormal cell mutation and rapid cell replication. Cancer has a very high likelihood of metastasizing or spreading throughout the body once it has progressed farther along. The route for such spread is usually via blood or Lymph.
Now considering the fact that the position of the mediastinum and what it covers, and the nature of cancer and how it spreads, we can come to the conclusion that the mediastinum can act like a central port spreading the cancer via it's massive vasculature and lymph system.
