Liver metastases are tumors that have spread to the liver from other areas of the body. Cancer cells often have aggressive tendencies and will invade other areas of the body. They usually do so by floating in the bloodstream and then replicating themselves in a new place. The liver, in particular, provides a fertile soil for metastatic spread because of its rich blood supply and the presence of humoral factors (other bodily fluids) that promote cell growth. In terms of blood flow per minute, the blood supply of the liver is second only to the lung. The risk of cancer spreading to the liver depends on the site of the original cancer. For example, cancers of the GI tract often spread to the liver because their blood drains directly through the liver. Melanoma usually spreads through the body's blood vessels to the liver. Liver metastases are sometimes present when the original (primary) cancer is diagnosed, or it may occur months or years after the primary tumor is removed. After the lymph nodes, the liver is the most common site of metastatic spread. Most liver metastases originate from the colon, rectum, pancreas, stomach, esophagus, breast, lung, melanoma, and some less common sites.