When a tumor is malignant, or cancerous, it can spread from the initial site to other parts of the body. Invading cells move through the lymph system. This is a system of narrow vessels similar to blood vessels. The system transports white blood cells and clear fluid, called lymphatic fluid.
Cancer can also grow directly outward.
This is called local extension. When tumor cells spread to other areas of the body, it is called metastasis. As cancer grows and changes, cells lose their ability to stay in a single tumor mass. When cancer cells get into the lymph system or blood vessel system, they break off. Cells can spread as a single cell or as many tiny cells.