Metastasis is the spread of cancer from where it began to other places in the body. Because metastasis affects treatment options and possible outcomes, understanding the process of metastasis can help patients and their families communicate with their health care providers.
The process of metastasis
Metastasis usually begins when cancer cells break away from a tumor and move into nearby tissue; this step is called invasion. The cancer cells then enter the bloodstream or the lymphatic system (the network of lymph vessels and lymph nodes that are part of the immune system) to circulate throughout the body.
Most cancer cells in the blood or lymph are identified and destroyed by the immune system. Any cells that go undetected may stay suspended in the blood or lymph and never settle down, or they may become stuck to vessel walls. Occasionally, cells that have become stuck will move into nearby tissues and begin to multiply, creating a new tumor. This new tumor is called a metastasis, a metastatic lesion, or a secondary tumor.
Cancer cells in the lymphatic system are likely to settle in a lymph node near the primary tumor, an occurrence called regional lymph node metastasis; other cancer cells may move to distant parts of the body and form distant metastases.
While metastasis can occur in almost any type of cancer, the process of metastasis can be shorter in some cancers—such as leukemia and lymphoma—in which cancer cells are already circulating through the body.
Some types of cancer tend to metastasize to particular places. When breast cancer spreads, it often moves to the lungs, liver, bones, or brain. In contrast, metastatic melanoma occurs in the bones and brain more frequently than it does in other organs. No matter where the metastatic tumors develop, they are still named for the primary cancer, not the locations of metastases. For example, if lung cancer metastasizes to the brain, the brain tumors will be called metastatic lung cancer.
Metastasis in cancer staging
Metastasis is one factor doctors use to determine cancer stage, which guides treatment. A cancer that has spread to nearby organs or lymph nodes is usually called locally advanced cancer and is typically classified as stage III, but such a cancer might be stage II depending on factors such as the cancer type and the tumor size. Cancer that has spread to distant parts of the body is said to be stage IV.
Metastatic cancer can be difficult to identify. Occasionally, the metastatic tumor is diagnosed before the primary tumor. But in most cases, the primary cancer is identified first, prompting doctors to look for metastases.
Metastases can be found by multiple methods. A physical examination may identify unusual lumps that could be tumors. Imaging techniques like x-ray, computed tomography, and magnetic resonance imaging can locate and measure tumors inside the body. Additionally, if surgery is performed to examine or remove a primary tumor, doctors may examine nearby lymph nodes and organs for signs of metastasis.
Prognosis and treatment
For many types of cancer, the most effective way to prevent metastasis is finding the primary cancer early, before cancer cells invade the bloodstream, and removing the primary tumor completely. The risk of metastasis may also be reduced by radiation and chemotherapy. If the primary cancer is successfully treated and there is no sign of local or distant spread, the possibility of such spreading requires regular follow-up examinations so that if metastases do occur, they can be found and treated.
Once cancer has metastasized, treatment can become more difficult. However, new techniques and therapies have made it possible to treat metastases that were not treatable 5 or 10 years ago in some types of cancer.
Metastasis is a serious development, but surveillance and treatment can help patients fight the battle against cancer on all fronts.
–K. Werner
For more information, ask your physician, visit www.mdanderson.org, or call askMDAnderson at 866-929-6223.
OncoLog, May 2016, Volume 61, Issue 5