A tumor starts from a single cell that has become cancer and is dividing to make more cancer cells. At first these cells can use the nearby blood vessels, but as the tumor grows, the cells in the middle of the tumor get farther away from the blood supply. For the tumor to go on growing, it must have new blood vessels. Without new blood vessels, a tumor can't grow larger than about the size of a pin head (about 1 to 2 cubic millimeters).

Last Medical Review: 03/10/2009
Last Revised: 03/10/2009

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