What is metastatic cancer?

Metastatic cancer is a cancer that has spread from the part of the body where it started (the primary site) to other parts of the body. When cancer cells break away from a tumor, they can travel to other areas of the body through either the bloodstream or the lymph system (a collection of vessels that carry fluid and immune system cells).

The Lymph System

If the cells travel through the lymph system, they may end up in the lymph nodes (small, bean-sized collections of immune cells) or spread to other organs. If the cells travel through the bloodstream they can go to any part of the body. Most often, the cancer cells break off and travel in the bloodstream. Many of these cells die, but some settle in a new area, begin to grow, and form new tumors. This spread of cancer to a new part of the body is called metastasis.

In order for cancer cells to spread to new parts of the body, they have to go through several changes. They have to be able to break away from the original tumor and enter the bloodstream or lymph system, which can carry them to another part of the body. At some point they need to attach to the wall of a blood or lymph vessel and move through it into a new organ. They then need to be able to grow and thrive in their new location. All the while, they need to be able to avoid attacks from the body's immune system. Going through all these steps means the cells that start new tumors may no longer be exactly the same as the ones in the tumor they started in. This may make treatment more difficult.

Even when cancer has spread to a new area, 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 (not bone cancer). Likewise, breast cancer that has spread to the lungs it is still called breast cancer, not lung cancer.

Sometimes the metastatic tumors have already begun to grow when the cancer is first found and diagnosed. And in some cases, a metastasis may be found before the original (primary) tumor is found. If a cancer has already spread to many places before it is found, it may be hard to figure out where it started. If this happens the cancer is called cancer of unknown primary. This is discussed in a separate document, Cancer - Unknown Primary.