Colorectal cancer is a term used to refer to cancer that develops in the colon or the rectum. These cancers are sometimes referred to separately as colon cancer or rectal cancer, depending on where they start. Colon cancer and rectal cancer have many features in common, which is why they are discussed together in this document.
The normal digestive system
The colon and rectum are parts of the digestive system, which is also called the gastrointestinal (GI) system. The first part of the digestive system processes food for energy, while the last part (the colon and rectum) absorbs fluid to form solid waste (fecal matter or stool) that then leaves the body.
To understand colorectal cancer, it helps to have some basic knowledge about the normal structure and function of the digestive system
After food is chewed and swallowed, it travels through the esophagus to the stomach. There, it is partly broken down and then sent to the small intestine, also known as the small bowel. It is called small because it is narrower than the large intestine (colon and rectum), but actually the small intestine is the longest segment of the digestive system – about 20 feet. The small intestine continues breaking down the food and absorbs most of the nutrients.
The small intestine joins the large intestine (or large bowel) in the right lower abdomen. Most of the large intestine is made up of the colon, a muscular tube about 5 feet long. The colon absorbs water and salt from the food matter and serves as a storage place for waste matter.
The colon has 4 sections:
The first section is called the ascending colon. It starts with a small pouch (the cecum) where the small bowel attaches to the colon and extends upward on the right side of the abdomen. The cecum is also where the appendix attaches to the colon.
The second section is called the transverse colon since it goes across the body from the right to the left side in the upper abdomen.
The third section, the descending colon, continues downward on the left side.
The fourth and last section is known as the sigmoid colon because of its “S” or “sigmoid” shape.
The waste matter that is left after going through the colon is known as feces or stool. It goes into the rectum, the final 6 inches of the digestive system, where it is stored until it passes out of the body through the anus.
Abnormal growths in the colon or rectum
Most colorectal cancers develop slowly over several years. Before a cancer develops, a growth of tissue or tumor usually begins as a non-cancerous polyp on the inner lining of the colon or rectum. A tumor is abnormal tissue and can be benign (not cancer) or malignant (cancer). A polyp is a benign, non-cancerous tumor. Some polyps can change into cancer, but not all do. The chance of changing into a cancer depends upon the kind of polyp. The 2 main kinds of polyps are:
Adenomatous polyps (adenomas) are polyps that have the potential to change into cancer. Because of this, adenomas are called a pre-cancerous condition.
Hyperplastic polyps and inflammatory polyps, in general, are not pre-cancerous. But some doctors think that some hyperplastic polyps can become pre-cancerous or might be a sign of having a greater risk of developing adenomas and cancer, particularly when these polyps grow in the ascending colon.
Another kind of pre-cancerous condition is called dysplasia. Dysplasia is an area in the lining of the colon or rectum where the cells look abnormal (but not like true cancer cells) when seen under a microscope. These cells can change into cancer over time. Dysplasia is usually seen in people who have had diseases such as ulcerative colitis or Crohn’s disease for many years. Both ulcerative colitis and Crohn’s disease cause chronic inflammation of the colon.