Metastasis

Metastasis is the spread of a pathogenic agent, most often a cancerous tumor, from its original or primary site to a different, secondary site within the body. The resulting secondary growths are called metastases, and the process is a central concern of oncology because it marks the point at which a cancer can no longer be treated as a single localized disease. The subject falls under the Neoplasms chapter of medical classification because it is defined and studied as a feature of tumor spread.