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Colon Cancer (Colorectal Cancer)

What is colorectal cancer?

The colon, or large intestine, is a hollow flexible tube composed of a soft inner lining, or mucosa, with a tough muscular covering. The colon is approximately five feet long and the last 6 inches of the large intestine are called the rectum. The main functions of the colon and rectum are to absorb water and to retain waste products until they are evacuated.

An endoscopic image of a benign, or non-cancerous, polyp. 
An endoscopic image of a benign, or non-cancerous, polyp.

Cancers may develop in either the colon or the rectum. These cancers start in the mucosal lining of the colon or rectum as polyps. When these polyps initially form, they are benign—that is, they are not cancerous. Over time, these polyps can become cancerous. As these cells grow, they can invade tissue surrounding the colon, and travel through the blood stream to other areas of the body.

Colorectal cancer occurs frequently in developed countries and is the second most common cancer in American men and women. Polyps are found in approximately 20-30% of American adults and about 5% (1 in 20) American adults will develop colorectal cancer. It is estimated that 140,000 new cases are diagnosed each year in the United States, and 60,000 colorectal cancer deaths occur each year. The disease affects both men and women equally.

Colon cancer is more common as a person grows older, with the odds of occurrence doubling each decade from the ages of 40–80. It is for this reason that it is important to have recurring colonoscopy screenings during this period in your life. Pre-cancerous polyps can be removed, and this greatly reduces the chances of developing colon cancer.

What causes colon cancer?

Many researchers have tried to identify the cause(s) of colon cancer. But here is some of what is known:

  • colon cancer is more common in those who eat lots of fat and meat, but little fiber;
  • colon cancer is very rare in some countries where the population eats a low-meat, high-fiber diet;
  • having an inherited disease such as familial adenomatous polyposis (FAP) may be a factor in the development of colon cancer;
  • having a close relative with colon cancer increases the risk of also developing it;
  • patients with ulcerative colitis or Crohn's disease have a greater chance of developing colon cancer compared with the normal population.

How does colon cancer occur?

A colorectal cancer forms initially in the mucosal lining of the bowel. However, the process of cancer formation is quite complex.

In most cases, the first step in the formation of a colon cancer is the appearance of a polyp. A polyp looks like a little toe or a mushroom projecting out of the wall of the colon into its hollow center. The cells within the polyp tend to grow faster than the cells in the surrounding skin, and so they become heaped up.

When looked at under a microscope, benign polyp cells look abnormal but are confined to the polyp itself and do not spread into its stalk or into the bowel wall. However, depending on the size and microscopic appearance of the cells, there is a risk of cancer formation.

Polyps only become cancers when the abnormal cells begin to spread and invade through normal tissue. For this reason, colon cancer screening is recommended by many health care agencies. Polyps can be removed at the time of colonoscopy, thus preventing cancer formation if they are still benign.

Can colorectal cancer be prevented?

Screening, or testing asymptomatic individuals for colorectal cancer, has been shown to decrease the risk of developing colorectal cancer and has been shown to decrease the risk of dying from colorectal cancer. While the recommendations for who should be screened for colorectal cancer change as we get more information about the disease, screening is generally recommended to start at age 45 or 50 for average risk individuals and earlier for those with specific risk factors for colorectal cancer. Ask your doctor about your individual risk and when you should begin screening. There are a number of ways to screen for colorectal cancer including stool tests and procedures such as colonoscopy.

What are the symptoms of colon cancer?

Many patients with colorectal cancer have no symptoms at all and have their cancers identified by screening as noted above. However, colorectal cancer may cause a number of bowel-related symptoms, some of which are more prevalent depending on the position of the cancer within the colon or rectum. The cancer may bleed, which will be seen as bright or dark red blood in bowel movements. Bleeding from a cancer in the upper part of the bowel may not be seen, however, because the blood may have broken down before reaching the rectum. In this case, blood loss may lead to anemia.

Many patients with colorectal cancer complain of diarrhea, constipation, or a feeling of incomplete evacuation after passing a bowel movement. Other symptoms may include abdominal pains, back pain and weight loss.

A tumor can also block the bowel completely, causing obstruction. Patients with obstruction tend to complain of nausea, vomiting, severe or total constipation, abdominal pains and abdominal distention, or bloating. Rarely, the cancer may perforate the bowel wall leading to an infection of the abdominal cavity.

Diagnosing colon cancer

After asking about these symptoms and examining a patient, the doctor will need to perform a number of tests to determine the exact diagnosis.

Blood tests give the doctor information about the presence of anemia and infection. There are even more specialized blood tests that can possibly detect specific tumor markers in the blood.

Making a firm diagnosis of colon cancer is usually relatively easy:

  • a barium enema or colonoscopy can usually show the position of a cancer within the bowel;
  • a small piece of tissue can be taken (called a biopsy) for microscopic analysis at the time of colonoscopy;
  • a computerized tomogram (CT) or abdominal ultrasound may be performed to look for evidence of a tumor in other parts of the abdomen.
Illustration depicting treatment of colon cancer that reads: (A) a cancer in the sigmoid colon; (B) during the operation, the surgeon cuts above and below the tumor to isolate it; (C) isolated section is then removed; (D) upper and lower parts of the bowel are reconnected. 
(A) a cancer in the sigmoid colon; (B) during the operation, the surgeon cuts above and below the tumor to isolate it; (C) isolated section is then removed; (D) upper and lower parts of the bowel are reconnected.

Treating Colon Cancer

The main treatment for colon cancer is surgical removal of the tumor by removing a section of the colon or rectum that includes the tumor and healthy bowel on either side. Treatment of rectal cancer can be more complex and may involve the use of chemotherapy and radiation in combination with surgery to give the best chance of cure. In the past, many operations involved the formation of a colostomy, which meant wearing a bag on the abdominal wall to collect bowel movements. Fortunately, surgery has progressed significantly over the past 40 years or so, and now only rarely involves the creation of a colostomy. Most operations are now performed minimally invasively using laparoscopy or a surgical robot. These approaches use small incisions in the abdomen to introduce a camera and surgical instruments to conduct the operation without the need for a large incision. Minimally invasive techniques have been shown to help patients recover more quickly and with less pain.

After colon cancer surgery

Following the operation, the removed section of bowel containing the cancerous tumor will be examined microscopically by a pathologist. The pathologist can then tell:

  1. how far the tumor has penetrated through the bowel and surrounding tissues;
  2. if lymph nodes are involved; and;
  3. if the tumor has reached the edge of the surgical specimen

The four stages of colorectal cancer

Illustration depicting the four stages of colorectal cancerUsing these features, and other results from the operation and from scans, the cancer will have a "stage" assigned to it. These stages are:

  • stage I — the colon cancer has not yet passed through the bowel wall;
  • stage II — the tumors extend past the bowel wall but do not yet involve lymph nodes;
  • stage III — cancer cells are also present in lymph nodes;
  • stage IV — cancers will have spread to distant sites such as the liver.

The reason for staging tumors is to give some indication about each patient's chance of having a complete cure and to allow decisions to be made about additional medical therapies.

Additional medical therapies for colon cancer

Approximately 90 percent of patients with stage I, 75 percent of patients with stage II, 55 percent of patients with stage III, and less than 5 percent of patients with stage IV colon cancer will be cured. Medical therapies —for example, chemotherapy and/or radiotherapy— are given to patients either because the cancer cannot be removed completely, or because a patient who has had a tumor removed may be at risk of recurrent disease.

As well, certain patients may not be suitable for surgery either because of another serious illness or because surgery would not benefit the patient. In these cases, the doctor may place a hollow tube (called a stent) across the cancer to ensure that the bowel does not become obstructed.

Having a previous cancer does increase the risk of developing another colorectal cancer. Therefore, following treatment of colorectal cancer, patients will be offered regular colonoscopy to detect polyps or early cancer in the remaining bowel. Surveillance imaging will also be used to identify any signs of the cancer coming back.

Cancer-Related Terms

One of the bewildering aspects of a cancer diagnosis is the many words that are used to describe the condition. Here are a few that we thought you should know:

  • Carcinoma — an early form of cancer that has not spread, or a pre-cancer
  • Dysplasia — the earliest form of a pre-cancerous lesion
  • Histology — microscopic study of the anatomy of cells
  • Malignant — a medical condition that becomes progressively worse
  • Metastasis — term used to indicate that a disease has spread to another organ
  • Neoplasm — term used for an abnormal mass. (A tumor is a neoplasm.)
  • Staging — the extent to which a cancer has developed or spread