Cancer starts when cells in the body change (mutate) and grow out of control. To help you understand what happens when you have cancer, it's important to know how your body works normally. Your body is made up of tiny building blocks called cells. Normal cells grow when your body needs them and die when your body doesn't need them any longer.

Cancer is made up of abnormal cells that grow even though your body doesn't need them. Genetic changes in cancer cells keep them multiplying. They don't turn off without treatment. In most cancers, the abnormal cells grow to form a lump or mass called a tumor. If cancer cells are in the body long enough, they can grow into (invade) nearby areas. They can even spread to other parts of the body (metastasis).

Colorectal cancer is cancer that starts in either your colon or your rectum. These make up the lower part of your digestive tract. In most cases, cancer doesn't start in both places. But colon and rectal cancer have a lot in common. So they're often called colorectal cancer.

The colon is a muscular tube about 5 feet long. It forms the last part of the digestive tract. It absorbs water from the remaining food matter. The rectum is the last 6 inches of the digestive tract. It's the storage space for waste (feces or stool) before it leaves the body through the anus.

Together, the colon and rectum make up the large intestine. This is sometimes called the large bowel. The colon and rectum have an inner lining made of millions of cells. Changes in these cells can lead to growths (polyps) that can become cancer.

These are the types of cancer that can start in the colon and rectum:

• Adenocarcinoma. Nearly all colorectal cancers are adenocarcinoma. This cancer starts in the lining of internal organs. The tumors start in gland cells that release, or secrete, fluids.

Other types of cancer that can start in the colon or rectum are much less common:

• Gastrointestinal stromal tumor. These tumors start in special nerve cells in the wall of the digestive tract. They may be found anywhere in the digestive tract. But they rarely start in the colon. They may be benign, or not cancer, at first. But over time, many do turn into cancer.
• Lymphoma. This cancer starts in a type of immune cell called a lymphocyte. Lymphomas often start in bean-sized groups of lymphocytes, called lymph nodes. But they can also start in the colon, rectum, or other organs.
• Carcinoid. This cancer starts in certain cells in the intestine that make hormones.
• Sarcoma. These tumors start in blood vessels, muscle, or connective tissue in the colon and rectum wall.

Most colorectal cancers start as polyps. These are growths caused by changes in the cells that line the inside of the colon or rectum. Over time, some types of polyps can become cancer. Finding and removing polyps while they're small may stop cancer from ever forming.

Polyps are fleshy clumps of tissue that form on the inner lining of the colon or rectum. Small polyps are seldom cancer. But over time, the cells in a type of polyp called an adenomatous polyp, or adenoma, can change and become cancer. The longer a polyp is there and the bigger it gets, the more likely this is to happen.

Colorectal cancers most often start when cells in a polyp start growing abnormally. As a cancer tumor grows, it can invade into the deeper layers of the colon or rectal wall. Over time, the cancer can grow beyond the colon or rectum and into nearby organs. Or it can spread to nearby lymph nodes. The cancer cells can also travel to other parts of the body, where they can form new tumors. This is known as metastasis. If colorectal cancer spreads, it most often goes to the liver first. But it can also spread to other organs, like the lining of the belly (peritoneum) and the lungs.

If you have questions about colorectal cancer, talk with your doctor. Your doctor can help you understand more about this cancer.