Bowel cancer
Written by Prof. Jonathan Rhodes, consultant gastroenterologist
What is bowel cancer?
Bowel is the general term for the long muscular tube that starts at the bottom of the stomach and ends at the anus.
The first part of the bowel is involved with the digestion of food and is known as the 'small bowel' because the tube is narrower here.
The 'large bowel' follows the small bowel and in a healthy person, the main part of the large bowel (colon) is responsible mostly for absorbing water from the faeces. The last part of the large bowel is known as the rectum, which leads to the anus.
Bowel (colorectal) cancer is cancer of the colon or rectum, and it arises from the cells that line the bowel. The small bowel is strikingly free from cancer risk, and almost all bowel cancers arise in the large bowel.
About 6 per cent of the population in Western countries develop bowel cancer at some time during their lives, making this the second commonest cause of cancer-related death. However, it is curable in 40 to 50 per cent of cases, usually by surgery.
The cancer develops when one of the cells in the colon develops a series of changes (mutations) in some of the genes that control how the cell divides and survives. As a result, the cell divides uncontrollably to form a clump of malignant (cancerous) cells. Initially, these cell changes commonly produce a polyp (a clump of abnormal cells the size of a pea on the end of a stalk of normal cells) called an adenoma.
At this stage, an adenoma is still pre-cancerous (a stage at which it may or may not become cancer), and probably only about 5 per cent of the polyps progress further to become life-threatening cancers.
The polyp enlarges very slowly, probably over about 10 years, up to between 1cm and about 5cm in diameter. The abnormal cells first invade the stalk of the polyp, then the underlying tissue of the colon to which the stalk is attached. This invasion indicates that cancer has developed. The patient will then usually have symptoms, which can include bleeding from the ulcerated tip of the cancer and diarrhoea caused by disturbance in the muscle activity of the colon or to obstruction. The risk of invasive cancer becomes appreciable once the polyp diameter has exceeded 1cm.
About 30 per cent of bowel cancers arise from flat lesions and do not pass through a polyp stage. This particularly occurs with cancers of the proximal (right-sided) colon and caecum.
If the cancer is not removed quickly, cancerous cells can break off from the tumour and move through veins or lymph vessels to form tumour growths (called metastases or secondaries) elsewhere, particularly in lymph glands or in the liver. The cure rate falls sharply once this has happened.
The average age when bowel cancer is first discovered is 65, and it becomes increasingly common with advancing age. Very occasionally, it may affect much younger adults from the age of 20. The rates do not differ strikingly between the sexes, although men are slightly more prone to developing rectal cancer and women to developing cancer of the caecum. This is the point where the appendix is attached.
The appendix itself is rarely the site of cancer, although it can be the site of a much rarer tumour called a 'carcinoid'. Previous appendicectomy (removal of the appendix) seems to have no effect on the subsequent risk of bowel cancer.
What causes bowel cancer?
No cancers are fully understood, but bowel cancer is better understood than most.
Studies of migrating populations, for example Japanese migrants who move to Hawaii, have shown that people rapidly acquire the risk of developing bowel cancer that is found in the country to which they have moved.
About 90 per cent of the risk for bowel cancer is thought to be due to dietary factors, with the other 10 per cent due to genetic (inherited) factors.
Dietary factors
Dietary factors that increase bowel cancer risk are not yet clearly defined. Populations with a high-fibre intake tend to have a low risk of bowel cancer. However, the results of studies in which people, usually those who have already developed polyps, have been given high-fibre diets are disappointing.
It now seems as though the beneficial effect of fibre is not simply due to its mechanical effect on helping the bowel to regularly pass faeces.
Evidence suggests that vegetable fibre is more protective that cereal fibre. Recent studies have also shown that specific chemicals in vegetables, for example the isothiocyanates, which give brassicas (cabbage, broccoli, brussel sprouts, cauliflower) their characteristic pungent taste, might be especially protective against cancer. A high intake of calories and obesity are both risk factors for bowel cancer, and a high intake of red meat is also linked with increased risk.
The best available approaches for a low risk of developing bowel cancer are:
- a diet high in green vegetables, particularly cabbage, broccoli, brussel sprouts or cauliflower.
- a diet low in red meat. In particular, avoid burnt meat, which contains cancer-promoting chemicals called cyclic amines.
- keeping to a normal body weight and taking regular exercise.
- Although still controversial, it seems that taking aspirin (eg Disprin) regularly (300mg per day or more ie one standard tablet) reduces the risk by about 50 per cent. However, prolonged use of aspirin carries a risk of intestinal ulceration and bleeding, so whether the benefits would outweigh the risks is unclear at present.
Genetic factors
Approximately 10 per cent of bowel cancers have a strong genetic factor. The commonest is hereditary non-polyposis colon cancer (HNPCC or Lynch syndrome). This condition is caused by mutation in any one of at least five different genes. These genes carry the instructions for manufacturing proteins that can repair damaged DNA.
Inheritance of this type of cancer is autosomal dominant, which means that half the children of someone with HNPCC are at risk of inheriting the condition. When these genes are defective, DNA repair does not take place, and damaged (mutant) DNA accumulates within cells, greatly increasing the cancer risk.
The colon is not the only organ affected. The syndrome also carries an increased risk of cancers of the stomach, ovaries, breasts and uterus.
The bowel cancers in affected individuals tend to develop as flat lesions rather than as polyps. The cancers more commonly affect the proximal (right sided) colon, whereas other cancers are more common in the distal colon (nearer the rectum) or rectum. They occur at a younger age and this condition should be suspected in anyone who develops bowel cancer before the age of 45.
About 1 per cent of bowel cancers occur in people who inherit a defect in the gene for familial polyposis coli. These people develop hundreds of adenomatous (pre-cancerous) polyps in the colon by the time they are in their teens and almost invariably develop bowel cancer by middle age unless the colon is removed.
Patients with ulcerative colitis or Crohn's disease of the colon (conditions that together affect about 1 in 800 of the population in Western countries) have about a five-fold increased risk of bowel cancer.
The risk is greater if the colitis (colon inflammation) seen in both conditions affects the whole colon, and if it has been present for more than ten years. The risk is probably reduced by regularly taking mesalazine (5-aminosalicylic acid), a medication that is widely used to reduce the risk of relapse in these diseases.
What are the symptoms of bowel cancer?
Because early cancers often cause no symptoms, screening of symptom-free individuals is being considered. About 85 per cent of people with bowel cancer are currently not diagnosed until the cancer has penetrated through the bowel wall or spread to lymph nodes or elsewhere. Cure is nevertheless still possible. The earliest symptom is often bleeding from the back passage. Later changes include loss of the normal form of bowel motions sometimes followed by diarrhoea.
Constipation can also occur. If the growth starts to block the bowel then colicky lower abdominal pain (ie coming in waves each of which lasts for a few minutes) can develop. If the bowel becomes completely obstructed, severe abdominal pain and vomiting occurs, followed by complete constipation. This obstruction is a surgical emergency requiring immediate admission to hospital, since the bowel is at risk of becoming gangrenous if the obstruction is not relieved.
In the proximal colon, the lumen (the space inside) is larger (about 4 or 5cm in diameter) and less likely to become obstructed. Cancers of this part of the colon, including the caecum, tend to show themselves very subtly as iron-deficiency anaemia, due to loss of small amounts of blood over a long period of time. The anaemia can lead to symptoms of pallor, shortness of breath or simply tiredness.
Cancers of the rectum typically cause rectal bleeding, which can easily be mistaken for bleeding haemorrhoids (piles). Other symptoms include the feeling that you haven't fully emptied your bowel accompanied by a need to frequently empty the bowel. Loss of appetite and weight loss tend to be late features in bowel cancer.
