A colon polyp is a protuberant, fleshy growth found on the inner lining of the colon or rectum. It may cause no symptoms, or may become complicated in various ways, including becoming malignant. Larger polyps have more chance of becoming malignant. Polyps may be single or multiple, and more than one malignant polyp may be found at the same time. They may take the form of a stalk, or be flat.
Polyps do not generally cause symptoms unless they are very large or develop complications. Symptoms that do arise can include:
- Rectal bleeding or blood in the stool;
- Change in bowel habit – either diarrhoea or constipation;
- Abdominal pain;
- Fatigue; and
- Bowel obstruction with associated constipation, nausea and vomiting.
There are different ways of classifying the types of polyps, but a convenient working classification is:
- Adenomatous – two-thirds of all polyps are adenomas, and have a 1 in 20 chance of becoming malignant, the average time taken being around seven years. They are more common in men, and in middle age.
- Hyperplastic – The most common non-malignant polyp, these are small and found mainly in the recto-sigmoid part of the colon. They very rarely become malignant.
- Inflammatory – these are the result of inflammation, ulceration and regeneration of the bowel lining, often linked to bouts of ulcerative colitis or Crohn's disease. The polyps themselves are not a danger, but the diseases leading to them bring an increased risk of colon cancer.
Risk factors and associated conditions
Not all polyps become cancerous, but nearly all colon cancers (especially recto-sigmoid ones) start out as polyps. There are a number of established risk factors for the development of polyps and the subsequent risk of colon cancer:
Age and gender: males and persons over 50 have an increased risk of polyps and cancer.
Family history of colon cancers.
Chronic inflammatory bowel disease such as Crohn's or ulcerative colitis.
Smoking and alcohol use: smokers' risk of dying from colon cancer is almost 40% more than non-smokers'. Alcohol excess also increases the risk, and the combination of alcohol and smoking further increases it.
Dietary fibre is protective, probably by reducing the contact of cancer-inducing substances with the colon lining.
Obesity and a sedentary lifestyle are linked, each increasing the risk of colon cancer.
Genetic mutations – these are mainly autosomal dominant, so that having even one parent affected confers a 50% risk of their offspring being affected. There are several recognised types:
- Hereditary non-polyposis colon cancer (HNPCC) is the commonest inherited form, in which there are only a few colon polyps, but with tumours often found in other organs;
- Familial polyposis (FP) is a rare disorder causing the growth of multiple (hundreds) of polyps, starting in teen years. More polyps means more risk of cancerous change.
- Gardner's syndrome, a variation of FP, having polyps throughout the large and small bowel, with non-malignant tumours elsewhere, for example skin and abdomen.
As most polyps have few or no symptoms, those at risk will benefit from screening: early detection and treatment gives a high survival rate. Combining screening techniques can be very effective, for diagnosis as well.
Rectal examination is a simple examination, but limited to the lower rectum, and may not detect small soft polyps.
Occult blood test can detect even traces of blood in the stool. If present, investigations must be taken further to exclude other causes (like haemorrhoids). A negative result, however, does not exclude cancer, because not all polyps or cancers bleed.
Endoscopy involves inspecting the inside of the colon, either the lower part (sigmoidoscopy) or the entire length (colonoscopy). Colonoscopy is considered the best single investigation for detecting colon cancer. This may require prior bowel preparation (dietary instructions for the day before) and is often done under sedation, as it may be a prolonged and uncomfortable examination. Whole polyps can be removed by special instruments, or tissue samples can be taken for analysis. There is a small risk of colon perforation, and active inflammatory disease may preclude this test.
Barium enema involves administering a barium-containing enema, which coats the lining of the bowel, making it visible on X-ray. Introducing air into the colon as well gives a double contrast picture. This is also a lengthy and often uncomfortable test, with a risk of bowel perforation. The disadvantages are that some polyps may be missed, and biopsies are not possible.
3D scans of the colon yield clear pictures of the colon in selected cases.
Pill camera : A tiny camera in a capsule is swallowed, and this can then identify polyps as it travels through the small intestine. This test is seldom used, though, because small bowel polyps are rare.
Malignant polyps cannot be identified by appearance alone, hence all polyps found during colonoscopy are usually removed and sent for analysis. Special loops are used to snare and cut off the polyp and cauterise their base to prevent bleeding.
For large or inaccessible polyps, surgery is done, using laparoscopic or conventional techniques, often removing entire sections of affected colon. In FP, the entire colon and rectum may be resected.
Regular screening and follow-up studies are done to detect recurrences in proven cases of malignancy.
Preventive steps are recommended for everybody, and more so for those with a high risk profile.
Diet: Fruit, vegetables and whole grains provide fibre and antioxidants which are considered protective. Limiting fat (especially saturated fat from animal sources) and alcohol intake is also advised. Foods high in calcium may be protective.
Stay active and maintain an appropriate body weight.
Aspirin use decreases the risk of colon cancer: please consult your doctor first, as there may be a good reason for you to avoid aspirin.
Hormone replacement therapy in menopausal women may be beneficial, but the possible disadvantages and risks must be considered.
Regular screening and/or genetic tests may be appropriate for those with a family history of colon polyps and cancer.
(Dr AG Hall, Health24, January 2008)