What Is Colorectal Cancer?

Colorectal cancer is a cancer that starts in the colon or the rectum. These cancers can also be named colon cancer or rectal cancer, depending on where they start. Colon cancer and rectal cancer are often grouped together because they have many features in common.

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?

How does colorectal cancer start?

Most colorectal cancers begin as a growth called a polyp on the inner lining of the colon or rectum Some types of polyps can change into cancer over the course of several years, but not all polyps become cancer. The chance of changing into a cancer depends on the kind of polyp. The 2 main types of polyps are:

  • Adenomatous polyps (adenomas): These polyps sometimes change into cancer. Because of this, adenomas are called a pre-cancerous condition.
  • Hyperplastic polyps and inflammatory polyps: These polyps are more common, but in general they are not pre-cancerous.

Other polyp characteristics that can increase the chances a polyp may contain cancer or increase someone’s risk of developing colorectal cancer besides the type include the size (larger than 1cm), the number found (more than two), and if dysplasia is seen in the polyp after it is removed. 

Dysplasia, another pre-cancerous condition, is an area in a polyp or in the lining of the colon or rectum where the cells look abnormal (but not like true cancer cells).

For more detailed information on the types of polyps and conditions that can lead to colorectal cancer, see Understanding Your Pathology Report: Colon Polyps.

If cancer forms in a polyp, it can eventually begin to grow into the wall of the colon or rectum.

The wall of the colon and rectum is made up of several layers. Colorectal cancer starts in the innermost layer (the mucosa) and can grow outward through some or all of the other layers. When cancer cells are in the wall, they can then grow into blood vessels or lymph vessels (tiny channels that carry away waste and fluid). From there, they can travel to nearby lymph nodes or to distant parts of the body.

The stage (extent of spread) of a colorectal cancer depends on how deeply it grows into the wall and if it has spread outside the colon or rectum. For more information on staging, see Colorectal Cancer Stages.

The normal colon and rectum

The colon and rectum are parts of the digestive system, which is also called the gastrointestinal (GI) system (see illustration). The colon and rectum make up the large intestine (or large bowel).

Most of the large intestine is made up of the colon, a muscular tube about 5 feet long. The parts of the colon are named by which way the food matter is traveling.  

  • The first section is called the ascending colon; it begins with a pouch called the cecum, where undigested food is received from the small intestine, and extends upward on the right side of the abdomen.
  • The second section is called the transverse colon because it travels across the body from the right to the left side.
  • The third section is called the descending colon because it descends (travels down) on the left side.
  • The fourth section is called the sigmoid colon because of its “S” shape; the sigmoid colon joins the rectum, which connects to the anus. 

The ascending and transverse sections are collectively referred to as the proximal colon, and the descending and sigmoid colon are referred to as the distal colon.

The colon absorbs water and salt from the remaining food matter after it goes through the small intestine (small bowel). The waste matter that is left after going through the colon 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.

illustration of the digestive system showing the liver, gallbladder, ascending colon, small intestine, cecum, appendix, rectum, esophagus, stomach, transverse colon, descending colon, sigmoid colon and anus

Types of cancer in the colon and rectum

Adenocarcinomas make up more than 95% of colorectal cancers. These cancers start in cells that make mucus to lubricate the inside of the colon and rectum. When doctors talk about colorectal cancer, they are almost always talking about this type. Some subtypes of adenocarcinoma, such as signet ring and mucinous, may have a worse prognosis (outlook). 

Other, less common types of tumors can also start in the colon and rectum. These include:

Carcinoid tumors start from specialized hormone-making cells in the intestine. They are discussed in Gastrointestinal Carcinoid Tumors.

Gastrointestinal stromal tumors (GISTs) start from specialized cells in the wall of the colon called the interstitial cells of Cajal. Some are non-cancerous (benign). These tumors can be found anywhere in the digestive tract, but are not common in the colon. They are discussed in Gastrointestinal Stromal Tumor (GIST).

Lymphomas are cancers of immune system cells that typically start in lymph nodes, but they can also start in the colon, rectum, or other organs. Information on lymphomas of the digestive system is included in Non-Hodgkin Lymphoma.

Sarcomas can start in blood vessels, muscle layers, or other connective tissues in the wall of the colon and rectum. Sarcomas of the colon or rectum are rare. They are discussed in Soft Tissue Sarcoma.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master’s-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: October 15, 2016 Last Revised: March 1, 2017

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