Colorectal Cancer: Introduction

What is cancer?

Cancer is when cells in the body change and grow out of control. Your body is made up of tiny building blocks called cells. Normal cells grow when your body needs them, and die when your body does not need them any longer.

Cancer is made up of abnormal cells that grow even though your body doesn't need them. 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).

What is colorectal cancer?

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 does not start in both the colon and rectum. But both types of cancer have a lot in common. So they are often called colorectal cancer.

Understanding the colon and rectum

The colon is a muscular tube about 5 feet long that 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 acts as a storage space before waste (feces or stool) 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 in the colon that can become cancer.  

What are the types of cancer in the colon and rectum?

Here is an overview of the types of cancer that can start in the colon and rectum:

  • Adenocarcinoma. The most common type of colorectal cancer. More than 95% of 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 (GIST). These tumors start in special cells in the wall of the digestive tract. They may be found anywhere in the digestive tract. But they rarely appear in the colon. They may be benign, or not cancer, at first. But 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 special hormone-making cells in the intestine.

  • Sarcoma. These tumors start in blood vessels, muscle, or connective tissue in the colon and rectum wall. 

How colorectal cancer starts and grows

Changes that occur in the cells that line the inside of the colon or rectum can lead to growths called polyps. Over time, some types of polyps can become cancer. Removing polyps early 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 usually not cancer. But over time, the cells in a type of polyp known as an adenomatous polyp, or adenoma, can change and become cancer. The longer a polyp is there and the larger it grows, the more likely this is to happen.

  • Colorectal cancers most often start when cells in a polyp begin 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.

Talk with your healthcare provider

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