Cancer is a class of diseases characterized by out-of-control cell growth. Colon cancer forms when this uncontrolled cell growth happens in the cells of 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 - a procedure looking at the inner lining of the intestine.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 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.
Causes of colon cancer
Normal cells in the body follow an orderly path of growth, division, and death. Cancer is ultimately the result of cells that uncontrollably grow and do not die.
Programmed cell death is called apoptosis, and when this process breaks down cancer results. Colon cancer cells do not die in the normal way, 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:
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
Cells can experience uncontrolled growth if there is damage or mutations to DNA, and therefore, damage to the genes involved in cell division.
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 certain gene functions, leading to uncontrollable cell growth.
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.
Age is an important risk factor for colon cancer; around 90 percent 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 and processed meats increase the risk of developing colon cancer.
In fact, Western diets increase the risk of colon cancer compared with diets found in developing countries. Heavy alcohol consumption may also increase the risk of colon cancer. Being overweight and physically inactive are also risk factors for developing colon cancer.
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.
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
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.
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.
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 not eating or drinking several hours before the procedure. A liquid solution containing the element barium is then injected into the colon through the rectum. After the barium lines the large intestine, an X-ray of the colon and rectum is taken. The barium will appear white on the X-ray and tumors and polyps will appear as dark outlines.
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 abdomen to see how far the cancer has spread. It is also not uncommon for a doctor to test blood for CEA (carcinoembryonic 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
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.
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 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 marginal area close to the cancer.
Also, 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 can 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 might 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 to relieve pain, bleeding, and other symptoms.
Chemotherapy utilizes chemicals that interfere with the cell division process - damaging proteins or DNA - so that cancer cells will become damaged and die. These treatments target any rapidly dividing cells (not necessarily just cancer cells), but normal cells can usually 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.
One large-scale study has shown that daily low-dose aspirin improves the survival of patients with certain gastrointestinal cancers, such as colon cancer. While the mechanism is not well understood and aspirin's role in prevention has not been well studied, this information provides another possible adjunctive treatment option.
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 die.
Radiotherapy uses high-energy gamma-rays that are emitted from metals such as radium or high-energy x-rays. Radiotherapy can be used as a standalone treatment to shrink a tumor or destroy cancer cells; it is also used in combination with other cancer treatments.
Radiation treatments are not often used 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.
Colon cancer is the third most common cause of cancer death in the U.S., and it is the second most prevalent type of cancer. Regular annual screening for both men and women aged 50 to 75 years is recommended.
The American Cancer Society suggests screening tests, particularly colonoscopy, for early detection of colon cancer. Colonoscopy is the best method, because it will visualize the entire colon and can remove polyps during the procedure. Other screening tests include fecal occult blood tests (annually), stool DNA testing, flexible sigmoidoscopy (every 5 years), and CT colonography (every 5 years).
These frequency recommendations depend, however, on a person's particular risk of colon cancer due to other risk factors.
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.
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