Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. To learn more about how cancers start and spread, see What Is Cancer?
Colorectal cancer is a term for cancer that starts in either the colon or the rectum. Colon cancer and rectal cancer have many features in common. They are discussed together here except for the section about treatment, where they are discussed separately.
The normal digestive system
To understand colorectal cancer, it helps to know something about the structure of the digestive system and how it works.
After food is chewed and swallowed, it travels to the stomach. There it is partly broken down and sent to the small intestine. The small intestine is only called small because it isn’t very wide compared to the colon. In fact, the small intestine is the longest part of the digestive system -- about 20 feet. The small intestine also breaks down the food and absorbs most of the nutrients.
What remains goes into the colon (large intestine), a muscular tube about 5 feet long. The colon absorbs water and nutrients from the food and also serves as a storage place for waste matter (stool). Stool moves from the colon into the rectum, which is the last 6 inches of the digestive system. From there, stool passes out of the body through the opening called the anus.
The colon begins at the end of the small intestine – on the right side of the body at a place called the cecum. It goes up (the ascending colon in the picture) and bends to go across the top of the belly (the transverse colon in the picture), and turns down again on the left side (the descending colon in the picture). The rectum is in the lower part of the pelvis.
Abnormal growths in the colon or rectum
Most colorectal cancers start as a polyp – a growth that starts in the inner lining of the colon or rectum and grows toward the center. Most polyps are not cancer. Only certain types of polyps (called adenomas) can become cancer. Taking out a polyp early, when it is small, may keep it from becoming cancer.
Over 95% of colon and rectal cancers are adenocarcinomas. These are cancers that start in gland cells, like the cells that line the inside of the colon and rectum. There are some other, more rare, types of tumors of the colon and rectum.
The information here is only for adenocarcinomas in the colon or rectum.
Last Revised: 02/26/2016