Metastatic cancer is a form of the disease that spreads from one area of the body to another. Distant metastasis occurs when cancer cells break loose from a tumor and enter the bloodstream or the lymphatic system. These cells take root and grow in areas more or less distant from the primary site. Local metastsis occurs when the primary tumor grows into areas adjacent to the primary site.

Eventually, the cancer cells can take root in a new area of the body and begin to form another tumor. The cells in the new, metastatic tumor come from the original tumor site and are named for the area where that tumor originated. For instance, if a tumor spreads from the breast to the lungs, it is still considered breast cancer to the lungs, because the cancer cells are those that originated in the breast. If cancer has spread widely throughout the body, physicians may be unable to determine the primary site. This condition is referred to as cancer of unknown primary site or CUPS.

Risk factors associated with metastatic cancer are the same as those for cancer in general. Lifestyle risk factors include tobacco use, alcohol abuse, too little exercise and unhealthy diet. Together, these factors account for about two-thirds of fatal cancers, according the American Cancer Society (ACS). Some cancers have genetic risk factors, meaning individuals inherit genes that place them at higher risk for cancer. Genetic factors are associated with a lower number of cases, approximately 5 to 10 percent of all cancers.

Metastasis should not be confused with local spread. A cancer that spreads locally continues to grow in the same location that it originally developed. Local spread of a cancer may extend into nearby tissues and organs, but it does not spread to other parts of the body. Cancer that begins in the mouth and spreads to the throat is an example of local spread.

Cancer that has metastasized does not necessarily mean it is advanced cancer. Some tumors may spread only minimally but they will still be considered metastatic cancer. However, metastatic cancer that has invaded tissues and organs causing substantial harm is likely to be advanced cancer. Minimally spread metastatic cancer may be cured while advanced cancer will have a poorer prognosis. Each year, more than half a million people will die from advanced cancer in the United States, according to the ACS.

In metastasis, cancer cells spread to other areas, including the lymph nodes via the blood or lymph vessels (tubes that contain lymph cells and immune system cells). The lymph nodes are the bean-shaped collections of immune-system cells located throughout the body that help to fight infections and cancer. They are located throughout the body in areas sucj as the armpits, throat and groin. When cancer cells settle into the lymph nodes, they may continue to grow and the prognosis for survival becomes poorer.

When cancer spreads to the lymph nodes near the primary cancer site, doctors sometimes call this regional spread. For example, breast cancer may regionally spread into the lymph nodes under the arm. This is to differentiate it from distant metastasis, which generally occurs when the cells travel to other organs or tissues and develop new tumors.

In tubular body structures like the bronchi or colon, there may be so called drop metastases, where the malignant cells migrate down the tube and implant and grow, rather than travel through the blood and lymph vessels.

Metastasis develops in several phases:

• Phase one. As a tumor grows, cancer cells divide and are more likely to spread.
  
  
• Phase two. In a process known as angiogenesis, the tumor creates new blood vessels that feed the cancer and cause it to grow.
  
  
• Phase three. Additional cells develop in the tumor that are both faster growing and more likely to spread. Before spreading, these cells produce an enzyme that allows them to break loose from the tumor’s extracellular matrix, which acts like a mortar that holds together the cells in the tumor. These cancer cells must undergo several other changes that enable them to break through the walls of blood vessels or lymphatic vessels.
  
  
• Phase four. Tumor cells enter the blood or lymph circulation. The immune system destroys most of these cells, but some have undergone changes that make them resistant to the immune system. This allows them to survive and travel to other parts of the body.
  
  
• Phase five. The cells have the ability to travel to and grow in distant organs or lymph nodes.
  
  
• Phase six. Tumors actually grow into another organ or lymph nodes by penetrating blood vessel and lymph vessel walls.
  
  
• Phase seven. Angiogenesis must occur in the new environment.
  
  
• Phase eight. The cancer cells must be able to grow in their new environment and avoid the body’s attempts to reject them.

Metastatic cancers are discovered in several ways. Sometimes they are found during the diagnosis of a primary cancer. In other cases, signs of the metastatic cancer – such as enlarged lymph nodes – may be found before a physician is even aware of the presence of a primary cancer. The ACS estimates that with the exception of non-melanoma skin cancers, about one-third of cancer patients have metastasis at the time of diagnosis.

Metastases are responsible for the majority of cancer-related deaths, according to the ACS. Though primary cancers in many organs (including the prostate, breast, colon and lungs) can be removed during surgery, it is metastasis of these cancers that causes the most serious consequences for the patient.