Malignant tumors exhibit not only uncontrolled proliferation and local invasion, but the ability to set up distant colonies. Growth and survival of metastatic tumor cells depend upon angiogenesis and the ability of tumor cells to evade detection by the immune system.
Metastasizing cells escape normal cellular adhesion mechanisms and shed from the primary tumor. Local invasion can enable the 'escapee' cells to penetrate lymphatics and/or blood vessels. Local infiltration of lymph nodes is associated with an increased likelihood of metastasis, so determination of whether or not lymph nodes are involved is important in cancer staging.
Having been transported via circulation of lymph or blood, the malignant cells invade distant tissues where they establish focal colonies of proliferating cells (secondaries). Tumors originating in certain tissues often display a propensity for metastasis to specific tissues and organs.
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