The colon and rectum make up the large intestine (or large bowel), which is part of the digestive system, also called the gastrointestinal (GI) system.
The colon is the longest part of the large intestine (the first 1.5-1.8 metres). It receives almost completely digested food from the caecum (a pouch within the abdominal cavity that is considered to be the beginning of the large intestine), absorbs water and nutrients, and passes waste (faeces) to the rectum.2
Most of the large intestine is made up of the colon, a muscular tube about 1.5-1.8 metres long.2
The rectum is the lowest 12-15cm of the large bowel, is relatively fixed in position behind the peritoneum (the tissue that lines the abdominal wall).
What is colorectal cancer
Most colorectal cancers start as a growth, called a polyp, on the inner lining of the colon or rectum.4
Over time (usually many years), some polyps can change into cancer. The chance of a polyp turning into cancer depends on the type of polyp.4 The different types of polyps are:4
- Adenomatous polyps (adenomas): these polyps sometimes change into cancer, and because of this, adenomas are called a pre-cancerous condition. There are three types of adenomas: tubular, villous, and tubulovillous.
- Hyperplastic polyps and inflammatory polyps: these polyps are more common, but in general they are not pre-cancerous. Some people with large (more than 1cm) hyperplastic polyps might need colorectal cancer screening with colonoscopy more often.
- Sessile serrated polyps (SSP) and traditional serrated adenomas (TSA): these polyps are often treated like adenomas because they have a higher risk of becoming cancerous.
When cancer forms in a polyp, it can grow into the wall of the colon or rectum. The walls of the colon and rectum are made up of multiple layers – the cancer starts in the innermost layer (the mucosa) and can grow outward through some or all the other layers.4
When cancer cells are in the wall, they can then grow into blood or lymph vessels. From there, they can travel to nearby lymph nodes or to other parts of the body.4
The stage (extent of spread) of a colorectal cancer depends on how deeply it grows into the wall and if it has spread outside the colon or rectum.
Causes of colorectal cancer
Some factors that increase the risk of colorectal cancer include:1,3,4
- Inherited genetic risk and family history
- Inflammatory bowel disease such as Crohn’s
- High red meat consumption, especially processed meats
- Diet low in fibre
- Polyps
- Being overweight or obese
- High alcohol consumption (≥4 drinks/day)5
- Smoking
- Having a previous diagnosis of colorectal cancer
- Having other disease such as Type II diabetes
Symptoms of colorectal cancer
Symptoms of colorectal cancer include:1,3
- Change in bowel habit including diarrhoea, constipation or the feeling of incomplete emptying
- Change in appearance of consistency of bowel movements such as watery stools
- Blood in stools
- Abdominal pain, bloating or cramping
- Anal or rectal pain
- A lump in the anus or rectum
- Weight loss
- Unexplained fatigue
- Tiredness and/or anaemia (pale complexion, weakness, and breathlessness)
