What Is Colon Cancer? What Causes Colon Cancer?

The colon is the longest part of the large intestine and the lowest part of the digestive system. Inside the colon, water and salt from solid wastes are extracted before the waste moves through the rectum and exits the body through the anus.

What is colon cancer?

Cancer is a class of diseases characterized by out-of-control cell growth, and colon cancer forms when this uncontrolled cell growth initiates with cells in the large intestine. Most colon cancers originate from small, noncancerous (benign) tumors called adenomatous polyps that form on the inner walls of the large intestine. Some of these polyps may grow into malignant colon cancers over time if they are not removed during colonoscopy. Colon cancer cells will invade and damage healthy tissue that is near the tumor causing many complications.

After malignant tumors form, the cancerous cells may travel through the blood and lymph systems, spreading to other parts of the body. These cancer cells can grow in several places, invading and destroying other healthy tissues throughout the body. This process itself is called metastasis, and the result is a more serious condition that is very difficult to treat.

Colon cancer is not necessarily the same as rectal cancer, but they often occur together in what is called colorectal cancer. Rectal cancer originates in the rectum, which is the last several inches of the large intestine, closest to the anus.

In the United States, about 112,000 people are diagnosed with colon cancer each year.

What causes colon cancer?

Cancer is ultimately the result of cells that uncontrollably grow and do not die. Normal cells in the body follow an orderly path of growth, division, and death. Programmed cell death is called apoptosis, and when this process breaks down, cancer results. Colon cancer cells do not experience programmatic death, but instead continue to grow and divide. Although scientists do not know exactly what causes these cells to behave this way, they have identified several potential risk factors.

Polyps

Colon cancer usually derives from precancerous polyps that exist in the large intestine. The most common types of polyps are:

• Adenomas: can become cancerous but are usually removed during colonoscopy
• Hyperplastic polyps: rarely become colon cancer
• Inflammatory polyps: usually occur after inflammation of the colon (colitis) and may become cancerous

Genes - the DNA type

Cells can experience uncontrolled growth if there are damages or mutations to DNA, and therefore, damage to the genes involved in cell division. Four key types of genes are responsible for the cell division process: oncogenes tell cells when to divide, tumor suppressor genes tell cells when not to divide, suicide genes control apoptosis and tell cells to kill themselves if something goes wrong, and DNA-repair genes instruct cells to repair damaged DNA.

Cancer occurs when a cell's gene mutations make the cell unable to correct DNA damage and unable to commit suicide. Similarly, cancer is a result of mutations that inhibit oncogene and tumor suppressor gene functions, leading to uncontrollable cell growth. If you have DNA mutations of oncogenes or tumor suppressor genes that lead to colon cancer, it is likely that the mutation was a result of factors that affect DNA after you were born rather than a result of inheritance from parents.

Genes - the family type

Cancer can be the result of a genetic predisposition that is inherited from family members. It is possible to be born with certain genetic mutations or a fault in a gene that makes one statistically more likely to develop cancer later in life. About 20% of colon cancers are thought to be caused by inherited gene mutations. Genetic syndromes that are associated with colon cancer include familial adenomatous polyposis, attenuated adenomatous polyposis, and hereditary nonpolyposis colon cancer.

Traits, Habits, and Diet

Age is an important risk factor for colon cancer, as some 90% of those diagnosed are over 50. Colon cancers are more likely to occur in people with sedentary lifestyles, obese people, and those who smoke tobacco.

Diet is an important factor associated with colon cancer. Diets that are low in fiber and high in fat, calories, and red meat increase the risk of developing colon cancer. In fact, Western diets that are commonly followed in the United States and Europe increase the risk of colon cancer compared to diets found in developing countries. Heavy alcohol consumption may also increase the risk of colon cancer.

Other medical factors

There are several diseases and conditions that have been associated with an increased risk of colon cancer. Diabetes, acromegaly (a growth hormone disorder), radiation treatment for other cancers, ulcerative colitis, and Crohn's disease all increase the risk of colon cancer.

What are the symptoms of colon cancer?

Cancer symptoms are quite varied and depend on where the cancer is located, where it has spread, and how big the tumor is. It is common for people with colon cancer to experience no symptoms in the earliest stages of the disease. However, when the cancer grows, symptoms include:

• Diarrhea or constipation
• Changes in stool consistency
• Narrow stools
• Rectal bleeding or blood in the stool
• Pain, cramps, or gas in the abdomen
• Pain during bowel movements
• Continual urges to defecate
• Weakness or fatigue
• Unexplained weight loss
• Irritable bowel syndrome (IBS)
• Iron deficiency (anemia)

If the cancer spreads, or metastasizes, additional symptoms can present themselves in the newly affected area. Symptoms of metastasis ultimately depend on the location to which the cancer has spread, and the liver is the most common place of metastasis.

How is colon cancer diagnosed and staged?

In order to diagnose colon cancer, physicians will request a complete physical exam as well as personal and family medical histories. Diagnoses are usually made after the physician conducts a colonoscopy or a barium enema x-ray (lower G series).

A colonoscopy is a procedure where a long, flexible tube with a camera on one end is inserted into the rectum to inspect the inside of the colon. If polyps are found in the colon, they are removed and sent to a pathologist for biopsy - an examination under a microscope that is used to detect cancerous or precancerous cells. A barium enema begins with the patient swallowing a special liquid that contains the element barium. After the barium lines the large intestine, an x-ray of the colon and rectum is taken. Tumors and polyps will appear as dark shadows on the x-ray.

If a colon cancer diagnosis is made after a biopsy, doctors will often order chest x-rays, ultrasounds, or CT scans of the lungs, liver, and abdomens to see how far the cancer has spread. It is also not uncommon for a doctor to test blood for CEA (carcinoembyonic antigen) - a substance produced by some cancer cells.

After a diagnosis is made, doctors determine the stage of the cancer. The stage determines which choices will be available for treatment and informs prognoses. The standard cancer staging method is called the TNM system. T indicates the size and direct extent of the primary tumor or degree of invasion into the intestinal wall, N indicates the degree to which the cancer has spread to nearby lymph nodes, and M indicates whether the cancer has metastasized to other organs in the body. A small tumor that has not spread to lymph nodes or distant organs may be staged as (T1, N0, M0), for example.

Colon cancer is also staged from 0 to IV, derived from the TNM classification. Stage 0 is written as (Tis, N0, M0) where Tis stands for carcinoma in situ. This is when the tumor has not grown beyond the inner layer of the colon or rectum and has not invaded deeper tissues nor spread outside of the colon. Stage IV is written as (Any T, Any N, M1) and describes cancer that has spread to distant sites and other organs throughout the body.

How is colon cancer treated?

Cancer treatment depends on the type of cancer, the stage of the cancer (how much it has spread), age, health status, and additional personal characteristics. There is no single treatment for cancer, but the most common options for colon cancer are surgery, chemotherapy, and radiation therapy. Treatments seek to remove the cancer and/or relieve painful symptoms that the cancer is causing.

Surgery

Surgery to remove part of or the entire colon is called colectomy. During colectomy, a surgeon will remove the part of the colon containing the cancer as well as the maringal area close to the cancer. In addition, nearby lymph nodes are also usually removed. Depending on the extent of the colectomy, the healthy portion of the colon will either be reattached to the rectum or attached to an opening in the wall of the abdomen called a stoma. This latter option is called a colostomy, and waste will exit the body through the stoma into a colostomy bag instead of exiting through the rectum.

In addition to invasive surgeries like colectomy, doctors are able to remove some small, localized cancers using endoscopy.

Laparoscopic surgery (using several small incisions in the abdomen) may also be an option to remove larger polyps.

Palliative surgery may also be employed to relieve symptoms for cancers that are advanced or untreatable. This type of surgery will try to relieve blockage of the colon or to inhibit other conditions in order to relieve pain, bleeding, and other symptoms.

Chemotherapy

Chemotherapy utilizes chemicals that interfere with the cell division process - damaging proteins or DNA - so that cancer cells will commit suicide. These treatments target any rapidly dividing cells (not necessarily just cancer cells), but normal cells usually can recover from any chemical-induced damage while cancer cells cannot. Chemotherapy is generally used to treat cancer that has spread or metastasized because the medicines travel throughout the entire body. Treatment occurs in cycles so the body has time to heal between doses. However, there are still common side effects such as hair loss, nausea, fatigue, and vomiting. Combination therapies often include multiple types of chemotherapy or chemotherapy combined with other treatment options.

Radiation

Radiation treatment, also known as radiotherapy, destroys cancer by focusing high-energy rays on the cancer cells. This causes damage to the molecules that make up the cancer cells and leads them to commit suicide. Radiotherapy utilizes high-energy gamma-rays that are emitted from metals such as radium or high-energy x-rays that are created in a special machine. Radiotherapy can be used as a standalone treatment to shrink a tumor or destroy cancer cells, and it is also used in combination with other cancer treatments.

Radiation treatments are not used often for early stage colon cancers, but may be employed if early stage rectal cancer has penetrated the wall of the rectum or traveled to nearby lymph nodes. Side effects of radiation therapy may include mild skin changes resembling sunburn or suntan, nausea, vomiting, diarrhea, and fatigue. Patients also tend to lose their appetites and have trouble maintaining weight, but most side effects subside a few weeks after completing treatment.

How can colon cancer be prevented?

Regular screening - especially after age 50 - is one of the best ways to prevent or detect colon cancer. The American Cancer Society suggests screening tests such as fecal occult blood tests (annually), stool DNA testing, flexible sigmoidoscopy (every 5 years), colonscopy (every 10 years), and CT colongraphy (every 5 years). These frequency recommendations depend, however, on a person's particular risk of colon cancer due to other risk factors. Some drugs may also reduce a person's risk of colon cancer. These include aspirin, NSAIDs like ibuprofen and naproxen, and COX-2 inhibitors such as Celebrex.

In general, physicians recommend standard preventive measures such as keeping a healthy weight, exercising, and increasing consumption of fruits, vegetables, and whole grains while decreasing saturated fat and red meat intake. In addition, people are recommended to limit alcohol consumption and quit smoking.