Colorectal cancer develops in the colon or rectum. Depending on where they begin, these cancers are also known as colon cancer or rectal cancer. Because they share many characteristics, colon cancer and rectal cancer are frequently lumped together.
Understanding colorectal cancer requires knowledge of the normal structure and function of the colon and rectum.
The large intestine (or large bowel) is made up of the colon and rectum and is part of the digestive system, also known as the gastrointestinal (GI) system.
The colon, a muscular tube about 5 feet (1.5 metres) long, makes up the majority of the large intestine. The parts of the colon are named after the direction in which food passes through them.
After passing through the small intestine, the colon absorbs water and salt from the remaining food matter (small bowel). The waste matter that remains after passing through the colon is deposited in the rectum, the last 6 inches (15cm) of the digestive system.
It is kept there until it is processed by the anus. Ring-shaped muscles (also known as sphincters) surround the anus and prevent stool from exiting until they relax during a bowel movement.
The majority of colorectal cancers begin as a growth on the lining of the colon or rectum. Polyps are the medical term for these growths.
Some polyps can develop into cancer over time (usually many years), but not all polyps do. The likelihood of a polyp developing into cancer is determined by the type of polyp. Polyps come in a variety of shapes and sizes.
If cancer develops in a polyp, it has the potential to spread to the colon or rectum wall over time. The colon and rectum’s walls are made up of many layers.
Colorectal cancer begins in the innermost layer (the mucosa) and can spread to any or all of the other layers.
When cancer cells become embedded in the wall, they can develop into blood vessels or lymph vessels (tiny channels that carry away waste and fluid). They can then spread to nearby lymph nodes or to distant parts of the body.
A colorectal cancer’s stage (the extent to which it has spread) is determined by how deeply it has grown into the wall and whether it has spread outside the colon or rectum.
The majority of colorectal cancers are adenocarcinomas. These cancers begin in cells that produce mucus to lubricate the colon and rectum. When doctors discuss colorectal cancer, they almost always refer to this type. Some subtypes of adenocarcinoma, such as signet ring and mucinous, may have a poorer prognosis (outlook) than others.
Other, much rarer types of tumours can also begin in the colon and rectum. These are some examples: