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.
The cells circulate until they find a good place to grow. The entire body is supplied by blood and drained by the lymph system. Metastases can appear in any part of the body. A site for metastatic cancer must have a blood supply and an environment where it can grow. Cancer cells may travel throughout the body.