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.

https://www.cancer.org/cancer/colon-rectal-cancer/about/what-is-colorectal-cancer.html