Metastatic cancer cells spread (metastasize) throughout the body by traveling through the bloodstream or the lymph system (a collection of vessels that carry fluid and immune system cells).
When metastatic cells form a tumor, it's called a metastasis (or metastatic tumor), even though it has the same name and type of cancer cells as the primary cancer. For example, a metastatic kidney tumor is a cancer that has spread from the kidneys to another part of the body.
The lungs, bones and liver are the most common sites of cancer metastasis. Treatment for metastatic cancer usually depends on a number of factors, including the type of cancer as well as the size, number and location of the metastatic tumors.