The Facts
Colon cancer begins in the colon, which is part of the digestive -- or gastrointestinal (GI) -- tract. This is where food is processed to create energy and rid the body of waste matter.
The colon has four sections: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. Cancer can start in any of the four sections. Each of these sections of the colon has several layers of tissues. Cancer starts in the innermost layer and can grow through some or all of the other layers. The stage (extent of spread) of colon cancer depends to a great degree on which of these layers it affects.
Colon cancers usually develop slowly over a period of several years. Before a true cancer develops, there are often earlier changes in the lining of the colon. One type of change is a growth of tissue called a polyp. Removing the polyp early may prevent it from becoming cancer.
Over 95% of colon cancers are adenocarcinomas. These are cancers of the cells that line the inside of the colon and rectum. There are also some other, more rare, types of tumors of the colon such as carcinoid tumors, gastrointestinal stromal tumors, and lymphomas.
