The large bowel extends from the end of the small intestine to the anus. It is about 1.5 metres long and, although it is wider at its origin than in its distal part, the diameter varies considerably, depending on the faecal and gas content.It also depends on the tension of the muscles in the wall. The appendix protrudes from the first part, which is called the caecum.
The wall of the bowel has a lot of muscle composed of an outer layer, which runs the length of the bowel, and an inner one which is circular. Muscle contractions and relaxations push the bowel contents towards the anus. Connective tissues contain blood vessels, nerves, lymph channels and glands and there is another thin band of muscle just under the internal lining of the bowel. The lining consists of millions of mucous secreting cells arranged in a regular infolded pattern but, to the naked eye, this inner surface appears smooth. These myriad cells are regularly replaced and it is hardly surprising that an occasional one will have malignant potential.
Normally a cell which has undergone a genetic mutation will be recognised by the body’s defences and attacked by killer cells. Sometimes the change is minimal and the cell escapes undetected. It can then multiply to produce a simple tumour, called an adenoma. Some of the multiplying cells in the adenoma can undergo further change, become frankly malignant and multiply in a disorderly fashion to produce a cancerous tumour. It is thought that 30% of middle-aged to elderly men will have at least one adenomatous polyp in their bowel but only about 1% of them will become malignant. In some families there is a genetic predisposition to the formation of multiple adenomata in the bowel. These patients are very likely to develop cancer by the time they are 40 although their adenomata could have been detected 20 years earlier and removed. Others at risk are patients with ulcerative colitis, in which polyps form, and those who have had their ureters implanted into the bowel after excision of the bladder, resulting in a type of chemical colitis.
Many theories advanced
Many theories have been advanced about the cause of malignant transformation of bowel lining cells. Diet, as usual, is given prominence. Faddists would have us believe they know what causes bowel cancer. They don’t. We don’t know either, otherwise we might have been able to reduce the mortality in the over 50’s, which we haven’t.
Many tumours are not detected until at a late stage. Symptoms may be few or absent until a bowel obstruction occurs, bleeding is noted, or the growth perforates into the peritoneal cavity and causes peritonitis. A change of bowel habit, perhaps with alternating constipation and diarrhoea, or even undue fatigue can be the presenting symptom. However, to detect a growth it is usual to look at the inside of the bowel with a flexible fibre optic scope as well as making an X-ray examination following an enema with a barium salt.
Examining the stools for evidence of hidden bleeding, and digital examination of the distal bowel, have been used as screening tests but most tumours are beyond the reach of the finger. Many tumours secrete a protein, called carcinoembryonic antigen (CEA), which can be detected in the blood
The best treatment is surgical excision of the tumour, which often means a temporary, or less often, a permanent colostomy opening on the body wall. If possible, surgeons will join the two cut ends of the bowel after resecting the tumour. Most bowel cancers are relatively insensitive to both radiotherapy and chemotherapy, but both can be used to make life more bearable for the inoperable patient.
Examination of the resected material will show the extent of spread and whether the lesion has tracked through the bowel wall, has formed secondary tumours elsewhere, such as in the glands or the liver, and should enable the outcome of surgery to be assessed . For instance a tiny cancer found in a polyp, which has not spread from its original site, can be considered a cure. Tumour spread into the wall, but not the glands, means a 70% chance of surviving five years.