Digestive Problems
Diseases: Colon and Rectum: Colorectal Cancer
The inside of a healthy bowel.
The large bowel (colon and rectum) is a hollow flexible structure approximately three feet long. The image to the right shows the inside of a normal large bowel. The image also illustrates a side view of the bowel, indicating that the large bowel is formed mostly from muscle with an inside lining (mucosa). This mucosa is able to absorb water. The main functions of the bowel are to absorb water and to retain waste products until they are evacuated.
Colorectal cancer occurs frequently in developed countries and is the second most common cancer in American men and women. 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.
Colorectal cancer is more common with increasing age. Incidence rates double each decade from 40 to 80 years. The disease affects men and women equally. Many researchers have tried to identify the cause of colorectal cancer. It is more common in those who eat lots of fat and meatbut little fiber, and is very rare in some countries where the population eats a low meat, high-fiber diet. Other factors associated with the development of colorectal cancer include having an inherited disease such as familial adenomatous polyposis or having a close relative with the disease. In addition patients with ulcerative colitis or Crohn's disease have a greater chance of developing colorectal cancer than the normal population.
A colorectal cancer forms initially in the mucosa 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. Polyp means "many feet" in the Greek language, because a polyp looks like a little toe or a mushroom projecting out of the wall into the hollow center of the bowel. The cells within the polyp tend to grow faster than the cells in the surrounding skin, and so they become heaped up.
A polyp inside the colon.
When looked at under a microscope, the polyp cells look abnormal but, as can be seen in the image to the left, the abnormal cells are confined to the polyp and do not spread into the stalk of the polyp or into the bowel wall. Polyps are therefore benign, but if left long enough about one in 20 become cancerous or malignant.
Polyps only become cancers when the abnormal cells begin to spread and invade through normal tissue. In the below-right image, the cancer has spread beyond the skin and muscular structure of the bowel wall to reach the underlying tissues. For this reason, colorectal cancer screening is recommended by many health care agencies. Polyps can be removed at the time of colonoscopy, preventing cancer formation.
Cancerous tissue inside the colon.
Colorectal cancers may cause a number of bowel symptoms, 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 the bowel movements. Bleeding from a cancer in the upper part of the bowel may, however, not be seen because the blood may have broken down before reaching the rectum. In this case, constant blood loss will 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 include stomach pains, back pain and weight loss. The 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. Rarely, the cancer may perforate the bowel wall, leading to an infection of the abdominal cavity.
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 will give the doctor information about the presence of anemia, infection and more specialized blood tests may be able to detect specific tumor markers in the blood. Making a firm diagnosis of colorectal cancer is usually relatively easy. A barium enema or colonoscopy can show the position of a cancer within the bowel. In addition, a small piece of tissue can be taken for microscopic analysis at the time of colonoscopy. A computerized tomogram (CT) or abdominal ultrasound may be performed to look for evidence of tumor in other parts of the abdomen.
The main treatment for colorectal cancer is surgical removal of the tumor. In the past, many operations involved the formation of a colostomy, which meant wearing a bag on the abdominal wall to collect bowel movements. Many patients found this distressing but, fortunately, surgery has progressed significantly over the past 25 years, and only about one in 10 patients will have an operation that involves a colostomy.
The removal of a tumor.
Most operations involve removal of the cancer and a section of bowel on either side. (See set of images to left.) The first image (A) shows a cancer in the sigmoid colon. During the operation, the surgeon cuts above and below (B) to isolate the tumour. This section is then removed (C) and the upper and lower part of the bowel are reconnected (D). Interestingly, most patients retain very good control of bowel movements, even when a large section of the colon is removed.
Following the operation, the removed bowel and cancer will be examined microscopically by a pathologist. The pathologist can then tell how far the tumor has penetrated through the bowel and surrounding tissues, if lymph nodes are involved and if the tumor has reached the edge of the surgical specimen. Using these features, and other results from the operation and from scans, the cancer will have a "stage" assigned to it.
The four stages of colorectal cancer.
As shown in the images to the right, stage I (or stage A) colorectal cancer has not passed through the bowel wall. In stage II, the tumors extend past the bowel wall but do not involve lymph nodes. Stage III tumors have cancer cells present in lymph nodes, and stage IV cancers 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 had a complete cure and to allow decisions to be made about additional medical therapies (chemotherapy and/or radiotherapy). About 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 colorectal cancer will be cured. Medical therapies 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.
Some patients may not be suitable for surgery either because of other serious illness or because surgery would not benefit the patient. In these cases, the doctor my place a hollow tube (a stent) across the cancer to ensure that the bowel does not become obstructed.
Having a previous cancer or polyp increases the risk of developing a colorectal cancer. Therefore, following the initial illness, patients will be offered regular colonoscopy to detect polyps or early cancer in the remaining bowel.