What stage in the development of cancer would a metastasis occur?

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A metastasis (a deposit of tumor cells that has spread some distance away from the cancer) can occur early in the lifespan of a cancer or late in its lifespan or not at all. Some patients with very small tumors are found to already have a metastasis to an organ in a distant area of the body. Conversely, some with very large tumors are found not to have spread or "metastasized" at all. A metastasis can occur in another organ (lung cancer spread to the liver, for example) or to nearby lymph nodes (lung cancer spread to lymph nodes in the chest, for example). It is the presence or absence or such metastases (deposits of tumor cells) in lymph nodes or other organs that forms the basis of what is known as the "staging" of cancer,

The exact staging of tumors varies somewhat depending on the specific type of cancer (there are differences between cancers found in the lung, breast, colon, etc.). However, in general the following principles are true for most cancers. Small tumors that have not spread to lymph nodes or distant organs comprise the earliest form of tumor and are termed Stage I disease. When a tumor has spread to lymph nodes right next to tumor, it is a Stage II. If metastasis is found in lymph nodes far away from the tumor it is called Stage III. Finally, if cancer deposits are found in an organ other than where the cancer started (i.e. colon cancer found in the lung) then it is called Stage IV.