It synthesizes and stores amino acids, proteins, vitamins and fats; regulates blood sugar; and makes bile, which breaks down fat in the small intestine. The liver also collects and filters gastrointestinal blood.

Although cancer of the liver is rare in the United States, it ranks as one of the most common malignancies in parts of Africa and Asia, causing about 1 million deaths annually worldwide.

The American Cancer Society estimates in the United States this year, 15,300 new cases of liver cancer will be diagnosed, with 13,800 estimated deaths. Even with removal of a tumor, recurrence of liver cancer is common, and five-year survival is rare.

While the average age of onset for liver cancer is between 60 and 70 years old, liver cancer can strike children as well. For specific information, see the summary on childhood liver cancer.

For adults, cancer that develops in the liver is referred to as primary liver cancer. Primary liver cancer is relatively uncommon in the United States. Cancer that spreads to the liver from other locations, called metastases, occurs more frequently.

The most common primary liver cancer is called hepatocellular carcinoma, which starts in the liver cells and represents about 90 percent of primary cancers. About 7 percent of primary cancers start in the bile ducts of the liver, and the remainder, called angiosarcomas, originate in the blood vessels.

Due to the high volume of blood that flows through the liver, it is a common site for metastases grown in several other locations, such as the lung, breast, colon and rectum. Malignant cells detach from the primary site, enter the blood stream or lymphatic channels, and migrate to the liver. Once a malignant cell takes root in the liver, it thrives on the organ's rich blood supply.