What is bowel cancer?
Bowel cancer is cancer in any part of the large bowel (colon or rectum). It is sometimes known as colorectal cancer and might also be called colon cancer or rectal cancer, depending on where it starts.
“The first sign of the bowel cancer was that I had a little bit of bleeding when I went to the toilet. I thought it might be polyps, but when it became more frequent and heavy, I decided to go to the GP.”
The cancer grows from the inner lining of the bowel (mucosa). It usually develops from growths on the bowel wall called polyps. Most polyps are harmless (benign), but some become cancerous (malignant) over time.
If untreated, bowel cancer can grow into the deeper layers of the bowel wall. It can spread from there to the lymph nodes (glands). These small, bean-shaped masses are part of the body’s lymphatic system. If the cancer advances further, it can spread to other organs, such as the liver or lungs (metastasis).
In most cases, the cancer develops slowly and stays in the bowel for months or years before spreading.
Topics on this page:
Who gets bowel cancer?
Bowel cancer is the second most common cancer affecting people in Australia. It is estimated that about 17,000 people are diagnosed with bowel cancer every year.
About one in 19 men and one in 28 women will develop bowel cancer before the age of 75. It is most common in people over 50, but it can occur at any age.
What causes bowel cancer?
The exact cause of bowel cancer is not known. However, some factors increase the chance of developing it.
Risk factors include:
- age – bowel cancer most commonly affects people over the age of 50
- polyps – having a large number of polyps in the bowel
- bowel diseases – people who have an inflammatory bowel disease, such as Crohn’s disease or ulcerative colitis, have a significantly increased risk, particularly if they have had it for more than eight years
- lifestyle factors – being overweight, having a diet high in red meat (particularly processed meats such as salami or ham), drinking alcohol and smoking increase the risk
- other diseases – people who have had bowel cancer once are more likely to develop a second bowel cancer; some people who have had ovarian or endometrial cancer may have an increased risk of bowel cancer
- strong family history – bowel cancer can run in the family (see below)
- rare genetic disorders – such as familial adenomatous polyposis and Lynch syndrome (see below)
Being physically active, maintaining a healthy weight and eating a high-fibre diet may help protect against bowel cancer. Learn more about reducing your risk of developing bowel cancer in Prevention.
Family history of cancer
If one or more of your close family members (such as a parent or sibling) have had bowel cancer, it may increase your risk. This is especially the case if they were diagnosed before the age of 55, or if there are two close relatives on the same side of your family with bowel cancer.
A family history of other cancers, such as endometrial cancer, may also increase your risk of developing bowel cancer.
Inherited genetic conditions
There are two rare genetic conditions that occur in some families. These cause a small number (5–6%) of bowel cancers.
- Familial adenomatous polyposis (FAP) – This condition causes hundreds of polyps to form in the bowel. If polyps caused by FAP are not removed, they will become cancerous.
- Lynch syndrome – Previously known as hereditary non‑polyposis colorectal cancer (HNPCC), this syndrome is characterised by a fault in the gene that helps DNA repair itself. Having Lynch syndrome increases the risk of developing bowel cancer and other cancers.
The parts of the bowel
The bowel is part of the digestive system, which is also called the gastrointestinal (GI) or digestive tract. The digestive system starts at the mouth and ends at the anus. It helps the body break down food and turn it into energy. It also gets rid of the parts of food the body does not use. This solid waste matter is called faeces (also known as stools when it leaves the body through the anus).
The bowel is made up of the small bowel and the large bowel. ‘Bowel cancer’ means cancer of the large bowel. Cancer of the small bowel is very rare and is usually called ‘small bowel cancer’ or ‘small intestine cancer’.
Anal cancer is a different form of cancer that affects the anus, the opening at the end of the digestive tract.
The small bowel is a long tube (4–6 m) that absorbs nutrients from food. Also called the small intestine, it is longer but narrower than the large bowel. The small bowel has three main sections:
- duodenum – the first section of the small bowel; receives food from the stomach
- jejunum – the middle section of the small bowel
- ileum – the final and longest section of the small bowel; transfers waste to the large bowel
The large bowel is a tube that absorbs water and salts and turns what is left over into waste (faeces). Also called the large intestine, the large bowel is about 1.5 m long. It has four main sections:
- caecum – a pouch that receives waste from the small bowel; the appendix is a small tube hanging off the end of the caecum
- colon – the main working area of the large bowel; takes up most of the large bowel’s length and has four parts: ascending colon, transverse colon, descending colon and sigmoid colon
- rectum – the last 15–20 cm of the large bowel
- anus – the opening at the end of the digestive tract; during a bowel motion, the muscles of the anus relax to release faeces from the rectum
What is anal cancer?
Anal cancer is a rare cancer that affects the tissues of the anus. About 350 people in Australia are diagnosed with anal cancer every year, and most are 50 to 60 years old. Risk factors include smoking and sexually transmitted infections.
Symptoms of anal cancer can be similar to the symptoms of bowel cancer, and your doctors may carry out some of the same tests, including blood tests, a proctoscopy or rigid sigmoidoscopy, and scans. Anal cancer is staged using the TNM system.
The main treatment for anal cancer is chemotherapy and radiotherapy. Surgery may also be used. Early stage cancer may need only a local resection, which just removes the tumour. More advanced cancer may require an abdominoperineal resection and a permanent stoma.
Cancer Council NSW is funding some important research into anal cancer and can provide further information and support.