Colorectal adenocarcinoma is the most important malignant tumor of the large intestine. It is the second most common cause of cancer death in Ontario and is the leading cause of cancer deaths in non-smokers. The peak incidence is in the >60 age group with <20% of cases occurring under the age of 50.
When colon cancer occurs in a young person, predisposing conditions such as inflammatory bowel disease or one of the hereditary syndromes should be suspected, particularly hereditary non-polyposis colon cancer (HNPCC) or familial adenomatous polyposis (FAP). In North America, about 30% of tumors occur in the rectum, about 25% occur in the sigmoid colon, and the remainder are evenly distributed.
Adenocarcinomas of the colorectum most frequently arise within adenomatous polyps. There is a predictable sequence of events, both in terms of genetic abnormalities and morphology, that occurs from normal mucosa through to the development of adenocarcinoma.
Dietary factors and exercise are important in the etiology of colorectal cancer. Decreased intake of fresh fruits and vegetables (and therefore antioxidants) is one of the most important factors, now thought to be even more important than dietary fibre. Diets low in fibre do predispose to colon cancer by slowing transit times with longer exposure of the mucosa to potential carcinogens. Fibre may also play a role in binding carcinogens. Diets high in fat lead to increased bile acid concentrations in the colon, and some may be carcinogenic. Individuals with diets low in calcium are also at increased risk for developing colon cancer.
Physical activity has been shown to be important in preventing colon cancer. Studies have shown that the preventative effects of physical exercise are better for colon cancer than for any other human cancer. Other risk factors include advanced age, hereditary non-polyposis colon cancer (HNPCC), prior colorectal cancer, idiopathic inflammatory bowel disease (IIBD) (especially ulcerative colitis and [[Crohn's disease), hereditary polyposis syndromes (e.g., familial adenomatous polyposis (FAP) and juvenile polyposis).
Colorectal cancers may be polypoid, ulcerating or infiltrative. Polypoid tumors are more common on the right side, while infiltrative tumors are more common on the left side. Histologically, the vast majority of colon cancers are adenocarcinomas. Pathological examination is vital for estimating prognosis and for making treatment decisions, such as the need for adjuvant chemotherapy. Pathological staging (using the Duke's or TNM staging systems) involves determining the extent of spread through the bowel wall (e.g., into or through muscularis propria) and whether lymph nodes are involved.
Colon cancer is often clinically silent for long periods of time, during which occult bleeding may be the only clue to the cancer's presence. Ultimately, tumours can present with noticeable rectal bleeding. Tumors on the left side, where the luminal calibre is smaller tend to present with obstructive symptoms, whereas right sided tumors commonly present with anemia due to blood loss.
Digital rectal examination can detect up to a third of all tumoirs and is an essential component of the physical examination of any patient; barium enemas, sigmoidoscopy and colonoscopy can detect the remainder. Colon cancers are primarily treated with surgery, with or without adjuvant chemotherapy or radiation therapy.
The 5-year survival rate depends directly on the pathological stage (90% without full thickness penetration through muscularis propria, 40-50% with nodal involvement).