Metastasis (Greek: displacement, μετά=next + στάσις=placement, plural: metastases), or Metastatic disease, sometimes abbreviated mets, is the spread of a disease from one organ or part to another non-adjacent organ or part. Only malignant tumor cells and infections have the established capacity to metastasize; however, this is recently reconsidered by new research.

Cancer cells can break away, leak, or spill from a primary tumor, enter lymphatic and blood vessels, circulate through the bloodstream, and settle down to grow within normal tissues elsewhere in the body. Metastasis is one of three hallmarks of malignancy (contrast benign tumors). Kumar, Abbas, Fausto; Robbins and Cotran: Pathologic Basis of Disease; Elsevier, 7th ed. Most tumors and other neoplasms can metastasize, although in varying degrees (e.g., glioma and basal cell carcinoma rarely metastasize).

When tumor cells metastasize, the new tumor is called a secondary or metastatic tumor, and its cells are like those in the original tumor. This means, for example, that, if breast cancer metastasizes to the lungs, the secondary tumor is made up of abnormal breast cells, not of abnormal lung cells. The tumor in the lung is then called metastatic breast cancer, not lung cancer.

Modes and sites of metastatic dispersal

Metastatic tumors are very common in the late stages of cancer. The spread of metastases may occur via the blood or the lymphatics or through both routes. The most common places for the metastases to occur are the lungs, liver, brain, and the bones. There is also a propensity for certain tumors to seed in particular organs. This was first discussed as the "seed and soil" theory by Stephen Paget over a century ago in 1889. For example, prostate cancer usually metastasizes to the bones. In a similar manner, colon cancer has a tendency to metastasize to the liver. Stomach cancer often metastasizes to the ovary in women, then it is called a Krukenberg tumor. It is difficult for cancer cells to survive outside their region of origin, so in order to metastasize they must find a location with similar characteristics. Fact: date=November 2008

Cancer cells may spread to lymph nodes (regional lymph nodes) near the primary tumor. This is called nodal involvement, positive nodes, or regional disease. ("Positive nodes" is a term that would be used by medical specialists to describe a patient's condition, meaning that the patient's lymph nodes near the primary tumor tested positive for malignancy. It is common medical practice to test by biopsy at least two lymph nodes near a tumor site when doing surgery to examine or remove a tumor.) Localized spread to regional lymph nodes near the primary tumor is not normally counted as metastasis, although this is a sign of worse prognosis. Transport through lymphatics is the most common pathway for the initial dissemination of carcinomas.