What Is Metastatic Cancer?
The main reason that cancer is so serious is its ability to spread in the body. Cancer cells can spread locally by moving into nearby normal tissue. Cancer can also spread regionally, to nearby lymph nodes, tissues, or organs. And it can spread to distant parts of the body. When this happens, it is called metastatic cancer. For many types of cancer, it is also called stage IV (four) cancer. The process by which cancer cells spread to other parts of the body is called metastasis.
When observed under a microscope and tested in other ways, metastatic cancer cells have features like that of the primary cancer and not like the cells in the place where the cancer is found. This is how doctors can tell that it is cancer that has spread from another part of the body.
Metastatic cancer has the same name as the primary cancer. For example, breast cancer that spreads to the lung is called metastatic breast cancer, not lung cancer. It is treated as stage IV breast cancer, not as lung cancer.
Sometimes when people are diagnosed with metastatic cancer, doctors cannot tell where it started. This type of cancer is called cancer of unknown primary origin, or CUP. See the Carcinoma of Unknown Primary page for more information.
When a new primary cancer occurs in a person with a history of cancer, it is known as a second primary cancer. Second primary cancers are rare. Most of the time, when someone who has had cancer has cancer again, it means the first primary cancer has returned.
How Cancer Spreads
Cancer cells spread through the body in a series of steps. These steps include:
Growing into, or invading, nearby normal tissue
Moving through the walls of nearby lymph nodes or blood vessels
Traveling through the lymphatic system and bloodstream to other parts of the body
Stopping in small blood vessels at a distant location, invading the blood vessel walls, and moving into the surrounding tissue
Growing in this tissue until a tiny tumor forms
Causing new blood vessels to grow, which creates a blood supply that allows the tumor to continue growing
Most of the time, spreading cancer cells die at some point in this process. But, as long as conditions are favorable for the cancer cells at every step, some of them are able to form new tumors in other parts of the body. Metastatic cancer cells can also remain inactive at a distant site for many years before they begin to grow again, if at all.
Where Cancer Spreads
Cancer can spread to most any part of the body, although different types of cancer are more likely to spread to certain areas than others. The most common sites where cancer spreads are the bone, liver, and lung. The following list shows the most common sites of metastasis, not including the lymph nodes.