How Cancer Spreads
Metastatic cancer is a cancer that has spread from its primary site (the part of the body in which it developed) to other parts of the body. If cells break away from a cancerous tumor, they can travel to other areas of the body. There, they may settle and form "colony" tumors. In their new location, the cancer cells continue growing. The spread of a tumor to a new part of the body is called metastasis.

Metastasis involves spread of cancer cells through the bloodstream or the lymph system. The lymph system consists of lymph vessels (similar to veins except that they contain tissue waste products and immune system cells instead of blood). These lymph vessels lead to lymph nodes, bean-shaped collections of immune system cells that are important in fighting infections.

Cancer cells that break off from tumors and enter the lymph vessels can be carried to lymph nodes where they may continue to grow and form metastases. Doctors sometimes call metastasis to lymph nodes near the place a cancer developed regional spread. This is to distinguish it from distant spread or distant metastasis. Distant spread generally occurs when cancer cells break off from tumors and enter the bloodstream, travel to other organs, and continue to grow into new tumors.

When cancer spreads, it is still named after the part of the body where it started. For example, if prostate cancer spreads to the bones, it is still called prostate cancer, and if breast cancer spreads to the lungs it is still breast cancer. Breast cancer is a completely different disease than lung cancer, even though both diseases are cancer.