Metastasis is the spread of a pathogenic agent from a primary to a different site within a host. The term is typically used to refer to the spread of cancer from a primary neoplasm to distant organs. The new growths are metastases. Metastasis is distinguished from cancer invasion which is the direct penetration by cancer cells into neighbouring tissues.
Metastasis occurs when malignant cells break away from the original tumour and attach to and degrade the surrounding extracellular matrix. This allows a route for cells to enter the bloodstream or lymphatic system from where they can migrate to other areas of the body. Cells then begin to grow at distant sites by the process of metastatic colonization. The spread of metastasis may occur via the blood or the lymphatic system or through both routes, with lymphatic spread the most common. The most frequent sites for metastases to occur are the lungs, liver, brain, and the bones.
Approximately 0.01% of all tumour cells that do enter the bloodstream will eventually form a metastasis and the potential of a tumour cell to metastasise is dependent on both the characteristics of the neoplastic cells and the microenvironment of the host tissue. The metastatic cells must survive and be capable of invasion, embolization and angiogenesis. Metastasis is one of the hallmarks of cancer, distinguishing it from benign tumours.

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