Metastatic cancer is cancer that has spread from its original location to another part of the body, most commonly the lungs, liver and bones. It happens when cancer cells break away from a tumor, travel through either the bloodstream or the lymph system (a collection of vessels that carries fluid and immune system cells) and create a new tumor elsewhere.
Even when cancer has spread to a new area, it’s still named after the part of the body where it started (the primary cancer). Treatment is also based on the primary cancer. For example, if prostate cancer spreads to the bones, it’s still called prostate cancer (not bone cancer), and the doctor will recommend treatments that have been shown to help against metastatic prostate cancer. Likewise, breast cancer that has spread to the lungs is still called breast cancer, not lung cancer, and is treated as metastatic breast cancer.