Cancer can begin in either the colon or the rectum, and it may be called colon cancer or rectal cancer, based on where it started. Sometimes these cancers are referred to together as colorectal cancer. There are many similarities between the two, but there are some differences in the ways they are usually treated.
Your Colon and Rectum
As you eat, your stomach secretes gastric juices that break down your food. The food and gastric juices mix into a thick fluid that empties into your small intestine. Your small intestine breaks down the food further and absorbs most of the nutrients.
Your small intestine joins your colon, which absorbs water and nutrients from your food and stores waste before it exits through your rectum and anus. Together your colon and rectum form your large intestine. The upper five to six feet of the large intestine are the colon, and the lower six inches are the rectum.
Polyps and Cancer
In most cases, colon and rectal cancers develop slowly over many years. Most of these cancers begin as a growth of tissue called a polyp in the inner lining of the colon or rectum. Usually polyps bulge into the colon or rectum; some are flat.
Polyps are common in people older than age 50. Most are benign (noncancerous). However, some polyps may turn into cancer. Certain kinds of polyps are more likely to do this, especially adenomatous polyps (also called adenomas). Adenomas can turn into cancers called adenocarcinomas. More than 95 percent of colorectal cancers are adenocarcinomas. Removing a polyp early may prevent it from turning into cancer.
Other less common types of colorectal cancers include gastrointestinal carcinoid tumors, gastrointestinal stromal tumors, and lymphomas.
The information in this rectal cancer section is about adenocarcinomas.
Early Detection Matters
When colorectal cancers are found early, the cure rate is high. Because colorectal cancer becomes more common as people age, doctors recommend getting screening tests—such as a fecal occult blood test, sigmoidoscopy, or colonoscopy—regularly starting at age 50. You might need screenings earlier or more often if you have a family history of the disease or other risk factors.