The colon, or large intestine, is where the body extracts water and salt from solid wastes. The waste then moves through the rectum and exits the body through the anus.
It is also the third most common cause of cancer-related death, and in 2017, 95,520 new diagnoses are expected to occur in the United States (U.S.). However, advances in diagnosis, screening, and treatment have led to steady improvements in survival.
Regular screenings are recommended after the age of 50 years.
Colon cancer and rectal cancer may occur together. This is called colorectal cancer. Rectal cancer originates in the rectum, which is the last several inches of the large intestine, closest to the anus.
Here are some key points about colon cancer. More detail is in the main article.
- Colon cancer affects the large intestine and it usually starts with polyps in the wall of the intestine.
- Symptoms may not appear until a later stage, but if they do, gastrointestinal problems are common symptoms.
- Treatment involves a combination of chemotherapy, radiation therapy, and surgery, possibly resulting in a colostomy.
- A healthy lifestyle with a high-fiber, low-fat diet can help prevent colon cancer, and screening can detect it in the early stages.
Symptoms and signs
Colon cancer affects the large intestine. It usually develops from benign polyps.
There are often no symptoms in the earliest stages, but symptoms may develop as the cancer advances.
- diarrhea or constipation
- changes in stool consistency
- loose and narrow stools
- rectal bleeding or blood in the stool
- abdominal pain, cramps, bloating, or gas
- pain during bowel movements
- continual urges to defecate
- weakness and fatigue
- unexplained weight loss
- irritable bowel syndrome (IBS)
- iron deficiency anemia
If the cancer spreads to a new location in the body, additional symptoms can appear in the new area. The liver is most commonly affected.
There are different ways of staging cancer. The stages depend on how far the cancer has spread.
Here is a brief summary of a commonly used four-stage account of where the cancer is at the beginning of each stage.
Stage 0: The cancer is in a very early stage. It is known as carcinoma in situ. It has not grown further than the inner layer of the colon.
Stage 1: The cancer has grown into the next layer of tissue, but it has not reached the lymph nodes or other organs.
Stage 2: The cancer has reached the outer layers of the colon, but it has not spread beyond the colon.
Stage 3: The cancer has grown through outer layers of the colon and it has reached one to three lymph nodes. It has not spread to distant sites.
Stage 4: The cancer has reached other tissues beyond the wall of the colon. As stage 4 progresses, the cancer reaches distant parts of the body.
Cancer develops progressively. Each stage is not fixed but describes a phase during which certain developments take place.
Treatment will depend on the type and stage of the cancer, and the age, health status, and other characteristics of the patient.
There is no single treatment for any cancer, but the most common options for colon cancer are surgery, chemotherapy, and radiation therapy.
Treatments seek to remove the cancer and relieve any painful symptoms.
Surgery to remove part or all of the colon is called a colectomy. The surgeon removes the part of the colon containing the cancer and the surrounding area.
Surgery for colorectal cancer often means a person will need a colostomy. A bag collects waste from a stoma, bypassing the need for the lower part of the large intestine.
Nearby lymph nodes are also usually removed. The healthy portion of the colon will either be reattached to the rectum or attached to a stoma depending on the extent of the colectomy.
A stoma is an opening made in the wall of the abdomen. Waste will pass into a bag, removing the need for the lower part of the colon. This is called a colostomy.
Some small, localized cancers can be removed using endoscopy.
Laparoscopic surgery, using several small incisions in the abdomen, may be an option to remove larger polyps.
Palliative surgery may relieve symptoms in cases of untreatable or advanced cancers. The aim is to relieve any blockage of the colon and manage pain, bleeding, and other symptoms.
Chemotherapy administers chemicals that interfere with the cell division process by damaging proteins or DNA in order to damage and kill cancer cells.
These treatments target any rapidly dividing cells, including healthy ones. The healthy cells can usually recover from any chemically-induced damage, but cancer cells cannot.
Chemotherapy is generally used to treat cancer that has spread because the medicines travel through the whole body. Treatment occurs in cycles, so the body has time to heal between doses.
Common side effects include:
- hair loss
Combination therapies often mix multiple types of chemotherapy or combine chemotherapy with other treatments.
Radiation treatment damages and kills cancer cells by focusing high-energy gamma-rays on them.
Radioactive gamma-rays are emitted from metals such as radium, or from high-energy x-rays. Radiotherapy can be used as a standalone treatment to shrink a tumor or destroy cancer cells, or alongside other cancer treatments.
Radiation treatments are not often used until a later stage. They may be employed if early stage rectal cancer has penetrated the wall of the rectum or traveled to nearby lymph nodes.
Side effects may include:
- mild skin changes resembling sunburn or suntan
- appetite and weight loss
Most side effects resolve a few weeks after completing treatment.
Standard preventive measures include:
- maintaining a healthy weight
- consuming plenty of fruits, vegetables, and whole grains
- decreasing saturated fat and red meat intake
People are also recommended to limit alcohol consumption and quit smoking.
Normally, body cells follow an orderly process of growth, division, and death. Cancer happens when cells grow and divide uncontrollably, without dying.
Most colon cancer originates from noncancerous, or benign, tumors called adenomatous polyps that form on the inner walls of the large intestine.
Cancerous cells may spread from malignant tumors to other parts of the body through the blood and lymph systems.
These cancer cells can grow and invade healthy tissue nearby and throughout the body, in a process called metastasis. The result is a more serious, less treatable condition.
The exact causes are unknown, but colon cancer has several potential risk factors.
Colon cancer usually derives from precancerous polyps that exist in the large intestine.
The most common types of polyps are:
- Adenomas: These can resemble the normal lining of the colon but look different under a microscope. They can become cancerous.
- Hyperplastic polyps: Colon cancer rarely develops from hyperplastic polyps, as they are typically benign.
Some of these polyps may grow into malignant colon cancer over time, if they are not removed during the early stages of treatment.
Uncontrolled cell growth can happen if there is genetic damage, or if changes to the DNA affect cell division. A genetic predisposition can be inherited from family members, but most colon cancers occur in people without a family history.
Some people are born with certain genetic mutations that make them statistically more likely to develop cancer later in life. Sometimes, a person will have this genetic feature, but cancer will not develop unless an environmental factor triggers it.
Traits, habits, and diet
Age is an important risk factor for colon cancer. Around 91 percent of those who receive a diagnosis are over 50 years old.
As the colon is part of the digestive system, diet is an important factor. Diets that are low in fiber and high in fat, calories, red meat, alcohol, and processed meats have been linked to a higher risk of colon cancer.
Some conditions and treatments have been associated with an increased risk of colon cancer.
- acromegaly, a growth hormone disorder
- radiation treatment for other cancers
- inflammatory bowel disease, such as ulcerative colitis or Crohn's disease
If colorectal cancer is diagnosed in Stage 1, there is a 92 percent chance of surviving at least another 5 years. At stage 4, the percentage is 11 percent.
For this reason, annual screening is recommended for men and women aged 50 to 75 years, especially since symptoms may not appear until the cancer has progressed.
The American Cancer Society suggest colonoscopy, for early detection of colon cancer, as it is the most accurate method. It visualizes the entire colon and allows the surgeon to remove polyps during the procedure.
Other screening tests include:
- annual fecal occult blood tests
- stool DNA testing
- flexible sigmoidoscopy every 5 years
- CT colonography every 5 years
These frequency recommendations depend on the individual's risk of colon cancer.
A physician will carry out a complete physical exam and ask about personal and family medical histories.
Diagnosis is usually made after colonoscopy or a barium enema x-ray.
A long, flexible tube with a camera on one end is inserted into the rectum to inspect the inside of the colon.
The patient may have to follow a special diet for 24 to 48 hours before the procedure. The colon will also require cleansing with strong laxatives, a process known as bowel prep.
If polyps are found in the colon, they are removed and sent to a pathologist for biopsy, an examination under a microscope that detects cancerous or precancerous cells.
A similar procedure, called a flexible sigmoidoscopy, examines a smaller portion of the colorectal area. This involves less preparation, and a full colonoscopy may not be needed if polyps are not found, or if they are located within a small area.
Double-contrast barium enema
This x-ray procedure uses a liquid called barium to provide clearer imaging results than a standard x-ray can give. Before a barium enema, the patient must fast for several hours.
A liquid solution containing the element barium is then injected into the colon through the rectum, followed by a brief pumping of air to smooth over barium layer for optimal results.
The x-ray of the colon and rectum is then taken. The barium will appear white on the x-ray, and tumors and polyps will appear as dark outlines.
There may also be a blood test for a substance produced by some cancer cells called carcinoembryonic antigen (CEA).
Following a diagnosis, the doctor will determine the stage of the cancer based on the size and extent of the tumor, and on the spread to nearby lymph nodes and distant organs.
A cancer's stage will determine the treatment options and inform the prognosis, or the likely course the cancer will take.