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What is a bowel cancer

Cancer and oncological diseases Gastroenterology Proctology

Bowel cancer

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What is a 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 feces.

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 40-50 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.

How the bowel cancer growing?

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 diarrhea 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 tumor and move through veins or lymph vessels to form tumor growths (called metastases or secondaries) elsewhere, particularly in lymph glands or in the liver. The cure rate falls sharply once this has happened.

At what age it occurs?

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 tumor 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-fiber 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-fiber diets are disappointing.

It now seems as though the beneficial effect of fiber is not simply due to its mechanical effect on helping the bowel to regularly pass farces.

Evidence suggests that vegetable fiber is more protective that cereal fiber. 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 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 diarrhea.

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 anemia, due to loss of small amounts of blood over a long period of time. The anemia 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.

How is bowel cancer diagnosed?

The doctor might feel a cancer of the rectum by inserting a gloved finger into the rectum. The diagnosis should usually be confirmed by biopsy in which a small (2mm diameter) sample of tissue is taken painlessly with forceps inserted through a small tube (a proctoscope or sigmoidoscope).

To spot cancers further along the colon, the doctor will use either a flexible sigmoidoscope (to see the part of the colon nearest the rectum, including the sigmoid [bendy] colon on the left side of the abdomen) or a colonoscope (to see the whole colon, including the part on the right side of the abdomen).

Colonoscopy is usually performed using intravenous sedation and takes about 30 minutes. Flexible sigmoidoscopy takes about ten minutes and can usually be done without sedation.

Alternatively, a barium enema examination may be used to look at the whole colon. In this test, a liquid suspension of barium sulfate, which shows up on X-rays, is poured into the rectum through a narrow tube inserted through the anus. Usually a small balloon is then inflated in the rectum to pump in a small amount of air.

This gives ‘double contrast’ to show the lining of the bowel in good detail. The procedure takes about 30 minutes.

For both procedures, the colon needs to be cleared by quite vigorous purgation (medicines are given to stimulate the intestines and clear out the bowel), which many patients find the most unpleasant aspect of the procedure.

There are advantages and disadvantages of each approach:

  • colonoscopy allows biopsy and therefore the presence of cancer can be confirmed under the microscope.

But, even in skilled hands, the whole colon is seen in only about 90 per cent of procedures. Colonoscopy can be uncomfortable, but it is usually performed with sedation, which has the added advantage that the individual usually cannot remember the procedure. Complications include a slight risk of perforation (puncturing) of the bowel and a very slight risk of death (about 1 per 5000).

  • the barium enema is usually more comfortable, but it is performed without sedation.

It nearly always shows up the whole colon, but in less detail than colonoscopy so very small polyps can be missed.

  • Scanning techniques.

(CT scanning or ultrasound) are increasingly being studied as possible ways to diagnose bowel cancer, but they are not yet reliable enough for routine use.

  • Screening is now recommended in the USA for all individuals over the age of 50.

It is done by a combination of an annual test for non-visible (occult) blood in the feces (fecal occult blood test), plus some form of endoscopy, either flexible sigmoidoscopy or colonoscopy, every three to five years. Experts have not yet firmly established whether this approach can prolong life, and screening has not yet been introduced routinely in the UK.

A positive fecal occult blood test is linked with approximately a 10 per cent chance of cancer or a 34 per cent chance of a polyp. However, the test is negative with up to 50 per cent of cancers (this figure falls to about 30 per cent when the test is done on three consecutive days). Therefore, this test is not sufficiently reliable for routine diagnosis of symptom free patients.

The need for screening is different if you have a strong family history of bowel cancer. In individuals who have a first-degree relative (eg a parent, brother, sister or child) who developed colorectal cancer before the age of 45, the life-time risk for colorectal cancer is 1 in 10.

The consensus of opinion is that such individuals should be offered screening, probably by full colonoscopy, every five years, starting 5-10 years younger than the age at which the relative was diagnosed. Individuals who have two first-degree relatives with colorectal cancer have a one in six life-time risk and should be similarly offered screening.

What other conditions might cause similar symptoms?

This depends on the symptoms.

Rectal bleeding

It is extremely common to notice a few spots of blood on the toilet paper after passing stools. If there is no blood in the toilet bowl, this is most likely to be due to minor damage to a blood vessel in the skin of the anus. If it persists, the problem requires investigation to exclude significant disease of the anus.

Blood in the toilet bowl that is separate from the feces and is bright red is commonly due to bleeding from hemorrhoids (piles). However, this sort of bleeding cannot be distinguished from bleeding due to a rectal cancer without further investigation.

Blood that is mixed in with the stool is more likely to have a worrying cause but benign (non-cancerous) possibilities include bleeding from diverticular disease or from colitis.


Or other change in bowel habit.

Diarrhea that has only been present for a few days, or even up to three weeks, is most commonly due to infection ie gastroenteritis. Diarrhea that carries on for longer than three weeks is rarely due to infection and requires urgent investigation.

Benign causes include:

  • colitis, an inflammation of the colon (eg ulcerative colitis or Crohn’s colitis).
  • diverticulitis, an inflammation of the little pea-sized pouches that can develop in the wall of the bowel, in which case the diarrhea may be bloody.
  • lactose intolerance.
  • irritable bowel syndrome (IBS), in which case the diarrhea is watery and not bloody. Irritable bowel syndrome mimics some of the symptoms of bowel cancer including colicky abdominal pain and diarrhea. However, the diarrhea is always non-bloody, the symptoms are intermittent and IBS typically starts in the adolescent or young adult at an age when bowel cancer is very rare.


Iron deficiency anemia indicates that someone has been losing small amounts of blood over a long time. Causes include heavy menstruation (periods), coeliac disease (gluten intolerance), oesophagitis (inflammation of the gullet), Crohn’s disease and cancer of the stomach. Iron deficiency because of lack of iron in the diet is an uncommon cause, except in adolescents with a poor diet.

Abdominal pain

Bowel cancer may cause lower abdominal pain that is typically colicky. Similar pain occurs in irritable bowel syndrome, but is often associated with diarrhea that alternates with formed or even constipated stools, whereas bowel disturbance in bowel cancer is usually more persistent.

Irritable bowel syndrome may be triggered by stress or by an episode of infectious gastroenteritis. It most commonly presents in adolescents or young adults at an age when bowel cancer is rare.

Crohn’s disease, a form of inflammatory bowel disease, can also present with colicky pains with or without diarrhea. Diagnosis is usually based on barium radiology or colonoscopy when it can be readily distinguished from cancer. Crohn’s disease commonly affects the small intestine, a part of the bowel that is exceptionally rarely the site of cancer.

Pain at the very lowest part of the abdomen (supra-pubic pain) can indicate a bladder problem such as cystitis, and pain low down to the right or left in a woman can indicate disease of the ovaries. Urine testing and pelvic ultrasound examination are usually done if these are possible alternative diagnoses.

What can your doctor do?

You should see your doctor promptly if you have:

  • persistent rectal bleeding.
  • a change in bowel habit (persistent diarrhea or constipation that is unusual for you).
  • recurring abdominal pains or unexplained tiredness.

Your doctor will probably feel your abdomen and perform an internal rectal examination using a gloved finger. He or she might also send off blood tests, especially a full blood count to check for anemia. Occasionally, the doctor’s practice might be equipped for sigmoidoscopy.

Unless your symptoms are considered low risk for cancer (perhaps because of a combination of your youth and the lack of recurrence or persistence of symptoms), you are likely to be referred to your district hospital.

You will usually be seen either by a physician who specializes in bowel diseases (a gastroenterologist) or by a surgeon with a gastroenterological practice. In either case, the procedures they use to make a diagnosis are likely to be the same. They will consist of some form of endoscopic examination (either sigmoidoscopy or colonoscopy), often followed by a barium enema radiological examination.

In the United States, the July 2017 two week cancer guidelines suggest that anyone over 55 with rectal bleeding, or anyone with a combination of rectal bleeding and altered bowel habit, should be seen at a hospital within two weeks of referral by their GP.

Fortunately, bowel cancers are fairly slow growing; estimates are that it takes about 10 years on average for a small polyp to develop into an invasive cancer. Nevertheless, even if your symptoms and age do not put you into the category of people needing to be seen within two weeks, a delay of more than about two months should be regarded as unacceptable.

What can you do yourself?

Prevention and early diagnosis

Ensure a regular daily intake of green vegetables, particularly brassicas (cabbage, broccoli, sprouts or cauliflower). Do not eat red meat (beef and lamb) more than about once per week. Keep your weight normal and take regular exercise.

See your doctor to discuss screening if you have a first-degree relative who has developed bowel cancer before the age of 45, or if you have two or more first-degree relatives who have developed bowel cancer.

See your doctor promptly if you notice rectal bleeding (other than very occasional spotting on the paper only), diarrhea that persists for more than a week, recurring lower abdominal pain or persistent tiredness or shortness of breath.


Once a cancer has developed, treatment is aimed at removing the original (primary) growth and at preventing secondary spread. This will be with some combination of surgery, chemotherapy or radiotherapy.

You should:

  • ensure that you seek advice as early as possible after symptoms develop.
  • get good nutrition.
  • stay positive, remembering that more than half of patients with bowel cancer are cured.

Do not hesitate to nag, or have someone nag on your behalf, if you feel you are not being investigated or treated appropriately or speedily.

What can your doctor do?

Once the diagnosis of bowel cancer has been made, the first treatment is usually surgical removal of the cancerous tumor under general anesthetic.

If the cancer is in the rectum, the operation will usually be accompanied by radiotherapy (by external beam irradiation) to reduce the risk of tumors reappearing in the same area. The radiotherapy may sometimes be given first, followed a few months later by the surgery.

For cancer of the colon, radiotherapy is not routinely used, but if examination of cells from the removed cancer shows that the cancer has spread to lymph glands, then some form of chemotherapy will normally be given, usually oral 5-fluorouracil combined with either folinic acid or levamisole.

Chemotherapy is very likely to cause side effects, including nausea and hair loss, but the nausea can usually be well controlled by drugs.

In any form of bowel surgery, the patient is normally warned that the surgeon might have to create a colostomy stoma (opening of the bowel onto the abdomen that is covered by a bag). This might be a temporary measure to divert feces from the site of the bowel that has been repaired after removal of the tumor.

If the tumor is very low down in the rectum, then the primary operation will include cutting out and closing the anus (abdomino-perineal resection) so the stoma will be permanent. Fortunately, modern stoma accessories are excellent, and colostomies are generally well managed and odor free.

In most cases, a bowel cancer higher up the colon can be surgically removed and the bowel repaired without the need for a colostomy.

The average length of stay in hospital for bowel cancer surgery is about 7 to 10 days. The abdominal wound is usually in the middle of the abdomen. Stitches will be removed by about 7 to 10 days, but the scar will usually cause some discomfort for four to six weeks. Pain relief immediately after the operation should nowadays be very effective and is often under the patient’s own control.

The best way to monitor patients after surgery is not yet clearly established, but some surgeons review patients at regular intervals to have a blood test done (carcino-embryonic antigen) to look for any evidence that the cancer has returned.

This test is partly done because tumors that have re-appeared in the same area can be removed and partly because surgeons are now more optimistic about the chances of curing bowel cancer that has spread to the liver, provided it is caught early.

Prognosis (outlook)

Colorectal cancer has a relatively good prognosis compared with most other solid cancers. Between 50 and 60 per cent of people with colorectal cancer survive for five years, after which a return of the cancer is uncommon. If the disease is caught at a time when the tumour has not spread through the bowel wall (so-called Dukes grade A), then the cure rate is over 90 per cent.


Colonoscopy carries a perforation rate of about 1 per 300 procedures, and a death rate of 1 per 5000. Perforation may be the result of polypectomy (polyp removal), particularly in the right colon where the bowel wall is thinner and where polyps more commonly have a flat base.

Significant bleeding requiring blood transfusion occurs in about 1 per 100 cases after polypectomy and is usually due to bleeding from an incompletely clotted artery in the remaining polyp stalk.

The rectal balloon catheter used for barium enema can very rarely cause perforation.

The laxatives used as bowel preparation for colonoscopy or barium enema can occasionally cause a significant fall in blood pressure and fainting. Considerable changes in body fluids and in salts such as sodium and potassium can also occur.

Significant problems rarely happen in individuals who are otherwise in good health, but particular care is needed in people with kidney disease or with heart problems. If these other conditions are significant, the bowel preparation might have to be performed in hospital.


About 5 in every 100 patients will die by 30 days after an operation to remove bowel cancer. Possible complications after surgery include:

  • leakage from the repaired bowel that can sometimes require a second operation at the same site.
  • paralysis of the intestines (ileus), which is usually temporary and recovers spontaneously after a few days.
  • the complications of any operation under general anaesthesia, including deep vein thrombosis, pulmonary embolism and pneumonia.

Patients are routinely given antibiotics to prevent infection from any minor leakage of the repaired bowel, and preventive anti-coagulation (blood thinning treatment) with heparin to protect against possible deep vein thrombosis.


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Julia Anderson
Doctor oncologist with 17 years of experience. Specializes on modern methods of preventive maintenance, treatment and rehabilitation, various diagnostics including mammography decoding, breast ultrasound.