Metastasis refers to the spread of cancer to distant organs in the body. Cancerous tumors can spread just by conquering nearby areas, which is called local-regional spread, or they can hop a ride through the blood vessels or the lymphatic system. This metastatic spread can damage otherwise healthy organs and body tissue, and it makes treatment more difficult.

When a person gets cancer in organ or area of the body, it's usually named for that organ, such as breast cancer or bladder cancer. The original cancer is called the primary cancer or primary tumor. Often, it's not until the cancer has metastasized that doctors find the cancerous cells. That's because some cancers can metastasize quickly and the tumors may cause symptoms that affect the areas to which the cancer has spread. For example, cancer that has spread to the bones may cause weakness and pain in the bones, cancer that spreads to the liver may cause lack of energy and appetite and cancer that spreads to the lungs could cause a persistent cough and chest infections. When cancer metastasizes to other organs such as the bones, liver or lungs, however, it's not a new cancer. It's the primary cancer showing up in a new place and it still takes the name of the primary cancer (for example, breast cancer metastasized to the bone).

Doctors can see features of the original cancer under special microscopes that let them know the new tumor really is formed from the primary cancer cells, and that it even shares some of the same features at the molecular level [source: National Cancer Institute]. The rapid spread of cancer cells is one reason why it is so dangerous and why early detection of cancers through available screening programs is so important. Examples are screening mammography for breast cancer, colonoscopies for colorectal cancer and digital rectal exams and prostate-specific antigen tests to detect prostate cancer.

The other reason that metastases are so dangerous is that they may be the reason some people with cancer die, not the primary tumor. For example, women with breast cancer face many physically and psychologically painful issues as part of their treatment for the primary breast tumor, but the breast is not a critical organ. Breast cancer is prone to metastatic spread to the lungs, liver, bones and brain. Because of the potential for the cancer to reach these vital organs at the time of diagnosis or years later, metastasis is a serious issue for all people who have cancer, a cancer recurrence or who live as cancer survivors. Detection of metastases continues to improve with advanced imaging, knowledge of genetic markers and increased attention on care plans and follow-up for cancer survivors.