What is colorectal cancer?

Colon, or colorectal, cancer is cancer that starts in the large intestine (colon) or the rectum (end of the colon). In 2010 there were 102,900 new cases of colon cancer; 39,670 new cases of rectal cancer; and 51,370 deaths combined in the United States.

This makes it one of the most common cancers. But unlike some malignant tumors , colorectal  cancer can often be cured by surgery, and new treatments are being introduced to make survival even more likely. The earlier the colorectal  cancer is diagnosed, the greater the likelihood of cure.

Throughout our lives, the lining of the bowel constantly renews itself. This lining contains many millions of tiny cells that grow, serve their purpose, and are then replaced by new cells. Each one of these cells contains genes, which give instructions to the cell on how to behave.

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When genes behave in a faulty manner, this can cause the cells to grow too quickly, which eventually leads to the formation of a growth that is known as a polyp. This is the first step on the road to cancer.

What is a polyp?

A polyp, or a more specific type of polyp called an adenoma, starts as a tiny bump on the inside of the bowel. At first, the genes give instructions for the polyp to grow in an orderly manner. Some polyps remain very small throughout their lives while others slowly grow larger. At this stage, the lump is still benign.

Most polyps remain benign throughout life but about one in 10 will turn into a cancer. Broadly speaking, the larger the polyp, the more likely it is to become cancerous. It is unusual for a polyp to be malignant if it is less than 1cm in diameter. It is believed that all malignancies of the bowel probably start off as benign polyps. Removing benign polyps can prevent cancer from developing later.

How does a polyp turn to cancer?

In some polyps, the instructions that the genes give the cell on how to grow become increasingly disordered. When this happens, the cells grow so quickly and in such a strange way that they grow not just on the lining of the bowel but into the wall of the intestines.

At that stage, the polyp is no longer benign but has become malignant , or cancerous. As the tumor advances, it grows through the wall of the bowel to invade nearby tissues and can spread more widely throughout the body, particularly to the liver and the lungs. When cancer spreads far away from its primary site (in this case the bowel) to distant parts of the body, it is said to have metastasized.