The colon is part of a section of the digestive tract called the large intestine. The large intestine is a tube that is 5 to 6 feet in length. The first 5 feet make up the colon, which connects to about 6 inches of rectum, and ends with the anus. By the time food reaches the colon (about 3 to 8 hours after eating), the nutrients have been absorbed and it has become a liquid waste product. The colon's function is to change this liquid waste into stool. The stool can spend anywhere from 10 hours to several days in the colon. It has been suggested that the longer stool remains in the colon, the higher the risk for colon cancer, but this has not been proven.

What is colon cancer?

Colon cancer is malignant tissue that grows in the wall of the colon. The majority of tumours begin when normal tissue in the colon wall forms an adenomatous polyp, or pre-cancerous growth projecting from the colon wall. As this polyp grows larger, the tumour is formed. This process can take many years, which allows time for early detection with screening tests.

Some tumours and polyps may bleed intermittently, and this blood can be detected in stool samples by a test called faecal occult blood testing (FOBT). By itself, FOBT only finds about 24% of cancers. The sigmoidoscope is a slender, flexible tube that has the ability to view about ﾽ of the colon. If a polyp or tumour is detected with this test, the patient must be referred for a full colonoscopy.

The colonoscope is similar to the sigmoidoscope, but is longer, and can view the entire colon. If a polyp is found, the physician can remove it, and send it to a pathology lab to determine if it is adenomatous (cancerous). As a screening method, the American Cancer Society recommends that a colonoscopy be done every 10 years after age 50. Patients with a family or personal history should have more frequent screenings, beginning at an earlier age than their relative was diagnosed. Patients with a history of ulcerative colitis are also at increased risk and should have more frequent screening than the general public. Patients should talk with their doctor about which screening method is best for them, and how often it should be performed.

What are the Signs of Colon Cancer?

Unfortunately, the early stages of colon cancer may not have any symptoms. This is why it is important to have screening tests done even though you feel well. As the polyp grows into a tumour, it may bleed or obstruct the colon, causing symptoms. These symptoms include:

• Bleeding from the rectum
• Blood in the stool or toilet after a bowel movement
• A change in the shape of the stool (i.e. thinning)
• Cramping pain in the abdomen
• Feeling the need to have a bowel movement when you don't have to have one

As you can see, these symptoms can also be caused by other conditions. If you experience these symptoms, you should be checked by a doctor.

How is Colon Cancer Diagnosed and Staged?

After a cancer has been found, the stage must be determined to decide on appropriate treatment. The stage tells how far the tumour has invaded the colon wall, and if it has spread to other parts of the body.

• Stage 0 (also called carcinoma in situ) - the cancer is confined to the outermost portion of the colon wall.
• Stage I - the cancer has spread to the second and third layer of the colon wall, but not to the outer colon wall or beyond. This is also called Dukes' A colon cancer.
• Stage II - the cancer has spread through the colon wall, but has not invaded any lymph nodes (these are small structures that help in fighting infection and disease). This is also called Dukes' B colon cancer.
• Stage III - the cancer has spread through the colon wall and into lymph nodes, but has not spread to other areas of the body. This is also called Dukes' C colon cancer.
• Stage IV - the cancer has spread to other areas of the body (i.e. liver and lungs). This is also called Dukes' D colon cancer.

After the tumour and lymph nodes are removed by a surgeon, they are examined by a pathologist, who determines how much of the colon wall and lymph nodes have been invaded by tumour. Patients with invasive cancer (stages II, III, and IV) require a staging workup, including full colonoscopy, carcinoembryonic antigen (CEA) level (a marker for colon cancer found in the blood), chest x-ray, and CT scan of the abdomen and pelvis, to determine if the cancer has spread.

What are the Treatments for Colon Cancer?

Surgery

Surgery is the most common treatment for colon cancer. If the cancer is limited to a polyp, the patient can undergo a polypectomy (removal of the polyp), or a local excision, where a small amount of surrounding tissue is also removed. If the tumour invades the bowel wall or surrounding tissues, the patient will require a partial resection (removal of the cancer and a portion of the bowel) and removal of local lymph nodes to determine if the cancer has spread into them. After the tumour is removed, the two ends of the remaining colon are reconnected, allowing normal bowel function. In some situations, it may not be possible to reconnect the colon, and a colostomy (an opening in the abdominal wall to allow passage of stool) is needed.

Chemotherapy

Despite the fact that a majority of patients have the entire tumour removed by surgery, as many as 40% will develop a recurrence. Chemotherapy is given to reduce this chance of recurrence. There is some controversy over patients with stage II disease receiving chemotherapy. Studies have not consistently shown a benefit in treating these patients. Generally, patients with stage II disease who present with a bowel perforation or obstruction, or have poorly differentiated tumours (determined by a pathologist), are considered at higher risk for recurrence, and are treated with 6 to 8 months of Fluorouracil (5-FU) and Leucovorin (LV) (both chemotherapy agents). Other patients with stage II disease are followed closely, but generally receive no chemotherapy. Patients who present with stage III colon cancer are typically treated with a regimen of Fluorouracil and Leucovorin for 12 months.

Forty to fifty percent of patients have metastatic (disease that has spread to other organs) at the time of diagnosis, or have a recurrence of the disease after therapy. Unfortunately, the prognosis for these patients is poor. The standard therapy for patients with advanced disease is Fluorouracil, Leucovorin, and irinotecan (CPT-11). This regimen was found to be more effective than Fluorouracil and Leucovorin alone in these patients. With this therapy, an average of 39% of patients have a response, but the average survival is still only 15 months. Patients and their physicians must weigh the benefits of therapy versus the side effects of the treatment. Younger patients and those in better physical shape are better able to tolerate therapy.

Two new medications, capecitabine (Xeloda) and oxaliplatin, are also being used in the treatment of advanced colon cancer. Capecitabine is currently approved by the FDA for the treatment of advanced colon cancer that has failed treatment, but is still being investigated in untreated patients. Oxaliplatin is widely used in Europe, but has not yet been approved by the FDA for use in the United States. Currently, patients can only receive this medication in a clinical trial.

Radiotherapy

Colon cancer is not typically treated with radiation therapy. If the cancer has invaded another organ, or adhered to the abdominal wall, radiation therapy may be one option. One way to understand this is that radiation needs a "target". If the tumour has been surgically resected, there is no target to radiate. If the tumour has spread to other organs, chemotherapy is needed to reach all the tumour cells, whereas radiation can only treat a small area.

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