Colorectal cancer is cancer that starts in either the colon or the rectum. Colon and rectal cancers begin in the digestive system, also called the GI (gastrointestinal) system.
In order to understand colorectal cancer, it helps to understand how the GI system works.
The wall of the colon and rectum has many layers of tissues. Colorectal cancer starts in the inner layer and can grow through some or all of the layers. Most colon and rectal cancers begin as a polyp (a growth that starts in the lining and grows in the centre of the colon and rectum). A polyp may or may not be cancerous. A type of polyp known as an adenoma can become cancerous. Removing a polyp early may stop it from becoming cancerous.