More than 95 percent of colorectal cancers are adenocarcinoma, which is cancer of the gland cells that line the inside wall of the colon and rectum. These usually develop from adenomatous polyps (also called adenomas), abnormal precancerous growths that can form in the inner lining of the colon and rectum. Some adenomatous polyps and other kinds of polyps, such as inflammatory polyps, remain benign.

Uncommon types of colorectal cancer include:

• Carcinoid tumor. Develops from the colon’s hormone-producing cells.
  
  
• Gastrointestinal stromal tumor (GIST). Develops in the colon wall from specialized cells called interstitial cells of Cajal.
  
  
• Lymphoma. Cancers of the immune system that usually start in lymph nodes.