Colon Cancer
(Colorectal Cancer)

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Colon cancer facts

  • Colorectal cancer is a malignant tumor arising from the inner wall of the large intestine.
  • Colorectal cancer is the third leading cause of cancer in males and fourth in females in the U.S.
  • Risk factors for colorectal cancer include heredity, colon polyps, and long-standing ulcerative colitis.
  • Most colorectal cancers develop from polyps. Removal of colon polyps can prevent colorectal cancer.
  • Colon polyps and early cancer can have no symptoms. Therefore regular screening is important.
  • Diagnosis of colorectal cancer can be made by barium enema or by colonoscopy with biopsy confirmation of cancer tissue.
  • Treatment of colorectal cancer depends on the location, size, and extent of cancer spread, as well as the health of the patient.
  • Surgery is the most common treatment for colorectal cancer.
  • Chemotherapy can extend life and improve quality of life for those living with colorectal cancer.

What is cancer?

Every day within our bodies, a massive process of destruction and repair occurs. The human body is comprised of about fifteen trillion cells, and every day billions of cells wear out or are destroyed. In most cases, each time a cell is destroyed the body makes a new cell to replace it, trying to make a cell that is a perfect copy of the cell that was destroyed because the replacement cell must be capable of performing the same function as the destroyed cell. During the complex process of replacing cells, many errors occur. Despite remarkably elegant systems in place to prevent errors , the body still makes tens of thousands of mistakes daily while replacing cells either because of random errors or because there are outside pressures placed on the replacement process that promote errors. Most of these mistakes are corrected by additional elegant systems or the mistake leads to the death of the newly made cell, and another normal new cell is produced. Sometimes a mistake is made, however, and is not corrected. Many of the uncorrected mistakes have little effect on health, but if the mistake allows the newly made cell to divide independent of the checks and balances that control normal cell growth, that cell can begin to multiply in an uncontrolled manner. When this happens a tumor (essentially a mass of abnormal cells) can develop.

Tumors fall into two categories; there are benign tumors and malignant (cancerous) tumors. So what is the difference? The answer is that a benign tumor grows only in the tissue from which it arises. Benign tumors sometimes can grow quite large or rapidly and cause severe symptoms, even death, although most do not. For example, a fibroid tumor in a woman's uterus can cause bleeding or pain, but it will never travel outside the uterus and grow as a new tumor elsewhere. Fibroids, like all benign tumors, lack the capacity to shed cells into the blood and lymphatic system, so they are unable to travel to other places in the body and grow. A cancer, on the other hand, can shed cells that can float like dandelion seeds in the wind through the blood or lymphatic system, landing in tissues distant from the primary tumor and growing into new tumors in these distant tissues. This process of spreading to distant tissues, called metastasis, is the defining characteristic of a cancerous tumor.

Cancer often is referred to as a single entity, but in fact, it is a group of more than 100 different diseases, much like infectious diseases. Cancers are named by the tissues from which the first tumor arises. Hence, a lung cancer that travels to the liver is not a liver cancer but is described as lung cancer metastatic to the liver, and a breast cancer that spreads to the brain is not described as a brain tumor but rather as breast cancer metastatic to the brain. Each cancer is a different disease with different treatment options and varying prognoses (likely outcomes). In fact, each individual with cancer has a unique disease, and the relative success or lack thereof of treatment among patients with the same diagnosis may be very different. As a result, it is important to treat each person with a diagnosis of cancer as an individual regardless of the type of cancer.

What is cancer of the colon and rectum?

Picture of colon (colorectal) cancer.
Picture of colon (colorectal) cancer.

The colon and the rectum are the final portions of the tube that extends from the mouth to the anus. Food enters the mouth where it is chewed and then swallowed. It then travels through the esophagus and into the stomach. In the stomach, the food is ground into smaller particles and then enters the small intestine in a carefully controlled manner. In the small intestine, final digestion of food and absorption of the nutrients contained in the food occurs. The food that is not digested and absorbed enters the large intestine or colon and finally the rectum. The large intestine is about six feet long and acts primarily as a storage facility for waste; however, additional water, salts, and some vitamins are further removed. In addition, some of the undigested food, for example, fiber, is digested by colonic bacteria and some of the products of digestion are absorbed from the colon and into the body. (It is estimated that 10% of the energy derived from food comes from these products of bacterial digestion in the colon.) The remaining undigested food, dying cells from the lining of the intestines, and large numbers of bacteria are stored in the colon and then periodically passed into the rectum. Their arrival into the rectum initiates a bowel movement that empties the colonic contents from the body as stool.

Most of the large intestine rests inside a cavity in the abdomen called the peritoneal cavity. Parts of the colon are able to move quite freely within the peritoneal cavity as the undigested food is passing through it. As the colon heads towards the rectum, it becomes fixed to the tissues behind the peritoneal cavity, an area called the retroperitoneum. The end portion of the large intestine, the part that resides in the retroperitoneum, is the rectum. Unlike much of the rest of the colon, the rectum is fixed in place by the tissues that surround it. Because of its location, treatment for rectal cancer often is different than treatment for cancer of the rest of the colon, as we'll explain later.

Picture of colon cross section.
Picture of colon cross section.

Although the large intestine is a tube, it is structurally a complicated tube, more like a steel belted radial tire than a garden hose. The tube is comprised of four layers. The first is an inner layer of cells that line the cavity through which the undigested food travels, called the mucosa. The mucosa is attached to a thin second layer, the submucosa, that is attached itself to a layer of muscle, the muscularis. The entire tube is surrounded by fibrous (scar-like) tissue called the serosa. The most common cancers of the large intestine (the type called adenocarcinoma) arise from the mucosa, the inner layer of cells. These cells are exposed to toxins from food and bacteria as well as mechanical wear and tear and are constantly dying off and being replaced. Mistakes (usually a series of mistakes involving genes within the replacement cells) lead to abnormal cells and uncontrolled proliferation of the abnormal cells that give rise to cancer.

Picture of colon cancer formation.
Picture of colon cancer formation.

Cancers of the colon and rectum (colorectal cancer) start when the process of the normal replacement of lining cells goes awry. Mistakes in mucosal cell division occur frequently. For reasons that are poorly understood, sometimes mistakes occur that escape our editing systems. When this occurs, these cells begin to divide independently of the normal checks and balances that control growth. As these abnormal cells grow and divide, they can lead to growths within the colon called polyps. Polyps vary in type, but many are precancerous tumors that grow slowly over the course of years and do not spread. As polyps grow, additional genetic mutations further destabilize the cells and can make the cells more bizarre. When these precancerous tumors change direction (growing through the tube rather than into the middle of it) and invade other layers of the large intestine (such as the submucosa or muscular layer), the precancerous polyp has become cancerous. In most cases this process is slow, taking at least 8 to 10 years to develop from those early aberrant cells to a frank cancer.

Once a colorectal cancer forms, it begins to grow in two ways. First, the cancer can grow locally and extend through the wall of the intestine and invade adjacent structures, making the mass (called the primary tumor) more of a problem and harder to remove. Local extension can cause additional symptoms such as pain or fullness, or cause blockages of the colon or nearby structures. Second, as the cancer grows it begins the process of metastasis, shedding thousands of cells a day into the blood and lymphatic system that can cause cancers to form in distant locations. Colorectal cancers most commonly spread first to local lymph nodes before traveling to distant organs. Once local lymph nodes are involved, spread to the liver, the abdominal cavity, and the lung are the next most common destinations of metastatic spread.

Medically Reviewed by a Doctor on 10/4/2012

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Colon Cancer Screening

Colorectal cancer is both preventable and curable. Colorectal cancer is prevented by removing precancerous colon polyps.