Cancer is a disease that affects cells, the body's basic unit of life. To understand any type of cancer, it is helpful to know about normal cells and what happens when they become cancerous.

The body is made up of many types of cells. Normally, cells grow, divide, and produce more cells when they are needed. This process keeps the body healthy and functioning properly. Sometimes, however, cells keep dividing when new cells are not needed. The mass of extra cells forms a growth or tumor. Tumors can be benign or malignant.

• Benign tumors are not cancer. They usually can be removed and, in most cases, they do not come back. Cells in benign tumors do not spread to other parts of the body. Most important, benign tumors are rarely a threat to life.

• Malignant tumors are cancer. Cells in malignant tumors are abnormal and divide without control or order. These cancer cells can invade and destroy the tissue around them. Cancer cells can also break away from a malignant tumor and enter the bloodstream or lymphatic system (the tissues and organs that produce, store, and carry white blood cells that fight infection and other diseases). This process, called metastasis, is how cancer spreads from the original (primary) tumor to form new (secondary) tumors in other parts of the body.

Cancer that begins in the esophagus (also called esophageal cancer) is divided into two major types, squamous cell carcinoma and adenocarcinoma, depending on the type of cells that are malignant. Squamous cell carcinomas arise in squamous cells that line the esophagus. These cancers usually occur in the upper and middle part of the esophagus. Adenocarcinomas usually develop in the glandular tissue in the lower part of the esophagus. The treatment is similar for both types of esophageal cancer.

If the cancer spreads outside the esophagus, it often goes to the lymph nodes first. (Lymph nodes are small, bean-shaped structures that are part of the body's immune system.) Esophageal cancer can also spread to almost any other part of the body, including the liver, lungs, brain, and bones.

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