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 parts of the body through the bloodstream or the lymph system. (Lymph vessels are much like blood vessels, except they carry a clear fluid and immune system cells.)
This image shows some parts of the lymph system, like lymph nodes and lymph vessels, as well as organs and tissues that contain many lymphocytes (immune cells). If the cells travel through the lymph system, they could end up in nearby lymph nodes (small, bean-sized collections of immune cells) or they could spread to other organs. More often, cancer cells that break off from the main tumor travel through the bloodstream. Once in the blood, they can go to any part of the body. Many of these cells die, but some may settle in a new area, start to grow, and form new tumors.

This spread of cancer to a new part of the body is called metastasis.
Cancer cells have to go through several steps to spread to new parts of the body:
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.
They need to attach to the wall of a blood or lymph vessel and move through it into a new organ. They need to be able to grow and thrive in their new location.
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 might make them harder to treat.

Even when cancer has spread to a new area, it’s still named after the part of the body where it started.
For instance, breast cancer that has spread to the lungs is called “metastatic breast cancer to the lungs” – it’s not lung cancer. Treatment is also based on where the cancer started. If prostate cancer spreads to the bones, it’s still prostate cancer (not bone cancer), and the doctor will recommend treatments that have been shown to help against metastatic prostate cancer. Likewise, colon cancer that has spread to the liver is treated as metastatic colon cancer, not liver 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 when it’s found, it may be very hard to figure out where it started. If this happens the cancer is called cancer of unknown primary.