Colon cancer is cancer that develops in the colon or the rectum. The colon and rectum are parts of the digestive system, which is also called the gastrointestinal, or GI, system. The digestive system processes food for energy and rids the body of solid waste (fecal matter or stool).
After food is chewed and swallowed, it travels through the esophagus to the stomach. There it is partially broken down and sent to the small intestine where digestion continues and most of the nutrients are absorbed. The word "small" refers to the diameter of the small intestine, which is smaller than that of the large intestine. The small intestine is actually the longest part of the digestive system – about 20 feet long. Cancer almost never arises in the small intestine.
The small intestine joins the large intestine in the lower right abdomen. The first and longest part of the large intestine is the colon, a muscular tube about 5 feet long. Water and mineral nutrients are absorbed from the food matter in the colon. Waste (feces) left from this process passes into the rectum, the final 6 inches of the large intestine, and is then expelled. (The small and large intestine are sometimes called the small and large bowel).
The colon has 4 sections:
The first section is called the ascending colon. It begins where the small intestine attaches to the colon and extends upward on the right side of a person's abdomen.
The second section is called the transverse colon, since it crosses the body from the right to the left side.
The third section, the descending colon, continues downward on the left side.
The fourth section is known as the sigmoid colon because of its S-shape. The sigmoid colon joins the rectum, which in turn joins the anus.
Colorectal cancer usually develops slowly over a period of many years. Before a true cancer develops, it usually begins as a noncancerous polyp which may eventually change into cancer. A polyp is a growth of tissue that develops on the lining of the colon or rectum. Certain kinds of polyps, called adenomatous polyps or adenomas, are most likely to become cancers.
More than 95% of colorectal cancers are adenocarci-nomas, which evolve from glandular tissue. For approximately 85% of colon and rectum cancers, the tumor arises from an adenomatous polyp that is visible through a scope or on an x-ray.
Once cancer forms in the large intestine, it eventually can begin to grow through the lining and into the wall of the colon or rectum. Cancers that have invaded the wall can grow into blood vessels or lymph vessels, which are thin channels that carry away cellular waste and fluid. Cancer cells first drain into nearby lymph nodes, which are bean-shaped structures that help fight against infections. The process through which cancer cells travel to distant parts of the body through blood or lymphatic vessels is called metastasis.
The extent to which a colorectal cancer has spread is described as its stage. Cancers that have not yet begun to invade the wall of the colon or rectum are called carcinomas in situ, and are not counted in cancer statistics. More than one system is used for the clinical staging of cancer. In this document, we will describe colorectal cancer stages as:
Local: Cancers that have grown into the wall of the colon and rectum, but have not extended through the wall to invade nearby tissues
Regional: Cancers that have spread through the wall of the colon or rectum and have invaded nearby tissue, or that have spread to nearby lymph nodes
Distant: Cancers that have spread to other parts of the body, such as the liver and lung.