About Bowel and Colon Cancer
Bowel and Colonic cancers (colorectal) are cancers 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 develops 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, eventually developing into cancer. As a result, the patient will have symptoms, which may 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 may break off from the tumour and move through veins or lymph vessels to metastasize elsewhere, particularly in lymph glands or in the liver.
The average age when bowel cancer is first discovered is 65, and it becomes increasingly common with advancing age. It may much younger adults from the age of 20, but this is a rare occurrence. 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, 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 appendectomy seems to have no effect on the subsequent risk of bowel cancer.