A cancer is an abnormal growth of cells (usually derived from a single abnormal cell).
The cells have lost normal control mechanisms and thus are able to multiply continuously, invade nearby tissues, migrate to distant parts of the body, and promote the growth of new blood vessels from which the cells derive nutrients. Cancer cells can migrate to other organs of body and form a new tumor, called metastasis. Metastatic tumors are very common in the late stages of cancer. The spread of metastasis may occur via the blood or the lymphatics or through both routes. The most common sites of metastases are the lungs, liver, brain, and the bones.