This is the most common malignancy of the alimentary tract and as a cause of cancer related death is second only to lung cancer. The tumours are adenocarcinomas with about half arising in the rectum,one-third in the sigmoid colon and the remainder elsewhere in the colon. The tumour may be:
- A soft polypoid mass,projecting into the lumen of the colon or rectum with tendency to ulceration,infection and bleeding.
- A hard fibrous mass encircling the colon,causing narrowing of the lumen and,eventually,obstruction.
The most common predisposing factor for colorectal cancer is diet. People eating a high-fibre,low fat diet,the disease is virtually unknown, whereas ,where large quantities of red meat and saturated animal fat and insufficient fibre are eaten,the disease is much more common. Slow movement of bowel contents may result in conversion of unknown substances present into carcinogenic agents.Genetic factors are also implicated. Predisposing diseases include ulcerative and some benign tumours .
LOCAL SPREAD of intestinal tumours occurs early but may not be evident until there is severe ulceration and haemorrage or obstruction. Spread can be outwards through the wall into peritoneal cavity and adjacent structures.
LYMPH-SPREAD metastases occur in mesenteric lymph nodes,the peritoneum and other abdominal and pelvic organs. Pressure caused by enlarged lymph nodes may cause obstruction or damage other structures.
BLOOD-SPREAD metastases are most common in the liver,brain and bones.
