Colorectal cancer is the uncontrolled growth of cells that starts in the colon or the rectum. Depending on the site where they start, these cancers can be called colon cancer or rectal cancer.

Several types of colorectal cancers have been identified and classified according to the type of cell affected. The types include adenocarcinomas, carcinoid tumors, gastrointestinal stromal tumors, lymphomas, and sarcomas.

Researchers have found several factors, such as obesity and consuming processed meats, that can raise the possibility of colorectal cancer, but it is not yet known how all of these factors are able to cause this type of cancer.

Most colorectal cancers begin as an enlargement, called a polyp, on the inner lining of the colon or rectum. Although not all polyps develop into cancer, some can turn cancerous over the course of several years.

Colorectal cancer may not cause symptoms at the early stages, but in case it does, the patient may feel fatigue, full bowel that does not empty completely, and may notice blood (bright red or very dark) in the stool, and diarrhea or constipation.

Several diagnostic procedures may be carried out to confirm that a patient has colorectal cancer. Some of these include blood tests, colonoscopy, biopsy, and imaging tests such as CT scan and magnetic resonance imaging.

Treatment options for this type of cancer may include surgery, chemotherapy, radiation therapy, and targeted drug therapy. In many cases, colon cancer is treatable when caught early.

When treated at an early stage, many people survive at least 5 years after diagnosis. Countries with the highest incidence rates include Australia, New Zealand, Canada, the United States, and parts of Europe. The countries with the lowest risk include China, India, and parts of Africa and South America.

Due to the similarity of the features associated with colon cancer and rectal cancer, they are often grouped together. Colorectal cancer is the cancerous growth that starts in the colon or the rectum. Mostly, colorectal cancer starts as small, noncancerous masses of cells named adenomatous polyps. Some of these polyps can develop into colorectal cancers.

The following are types of cancer that start in the colon and rectum classified according to the type of cell from which they develop:

• Adenocarcinomas: These cancers originate in cells responsible for producing mucus to lubricate the internal layers of the colon and rectum. This type accounts for the vast majority of colorectal cancer cases.
• Carcinoid tumors: These tumors start in specialized hormone-making cells found in the intestine.
• Gastrointestinal stromal tumors: These tumors begin in specialized cells in the wall of the colon called the interstitial cells of Cajal. Some of these tumors are benign.
• Lymphomas: These are cancers of the immune system that start in lymph nodes. These cancers can develop anywhere in the body including the colon and rectum.
• Sarcomas: These cancers begin in blood vessels, muscle layers, or other connective tissues found in the wall of the rectum and colon.

The direct cause of colorectal cancer is still unknown, but several factors have been implicated in its development. How these factors come together to cause it, however, is still not yet fully understood.

Multiple risk factors have been linked with an increased chance of developing cancer in the colon or rectum.The following are some of these risk factors:

• Obesity: Being overweight increases the possibility of having many kinds of cancers, including colorectal cancer.
• Lack of physical exercise: Decreased physical activity can play a role in developing colorectal cancer.
• Medical history: A record of one's history of previous colorectal polyps or colorectal cancer increases the chance of developing a new cancer in the region. Other diseases that affect the bowel, such as inflammatory bowel disease, may also increase the chance of colorectal cancer.
• Family history: People who have family members who had colorectal cancer or adenomatous polyps in the past are at an increased risk of developing the disease themselves.
• Inherited syndromes: Having an inherited syndrome, such as familial adenomatous polyposis (FAP) and Lynch syndrome (hereditary non-polyposis colorectal cancer), is a risk factor for colorectal cancer.
• Smoking: Smoking has been strongly linked to several types of cancer including colorectal cancer.
• Alcohol: Excessive alcohol consumption increases the risk of developing colorectal cancer.
• Age: The possibility of having colorectal cancer increases with age.
• Type 2 diabetes: People with type 2 diabetes are at an increased chance of colorectal cancer.
• Food: Eating particular types of food such as red meats (beef and lamb) and processed meats (hot dogs and some luncheon meats).
• Ethnic background: Race and ethnic background affect the risk of colorectal cancer. For instance, the highest incidence of colorectal cancer and mortality rates of all racial groups in the United States is among African Americans.
• Night shift work: It is not known exactly how night shift work increases the possibility of colorectal cancer, but it may be due to disruption of the sleep\wake cycle and the production of melatonin, the hormone that prepares the body for sleep.

Most colorectal cancers are recognized as an enlargement called a polyp on the inner lining of the colon or rectum. Although not all polyps become cancer, some types of polyps can develop into cancer over the course of several years. The possibility of changing into a cancer depends on the kind of polyp. The two major types of polyps are:

• Adenomatous polyps (adenomas): These are called pre-cancerous conditions because sometimes it can turn into cancer.
• Hyperplastic polyps and inflammatory polyps: Generally, these are not pre-cancerous and are more common than adenomatous polyps.

 The stages of colon cancer can be summarized as the following:

• Stage I: Cancer has not spread beyond the colon or rectum wall, but it has grown through the superficial lining (mucosa) of the colon or rectum.
• Stage II: Cancer has not spread to nearby lymph nodes, but it has grown into or through the wall of the colon or rectum.
• Stage III: Cancer is not affecting other parts of the body yet, but it has invaded nearby lymph nodes.
• Stage IV: Cancer has spread to other locations that may include major organs, such as the liver or lungs.

The symptoms of colorectal cancer may not be noticeable at first. When symptoms start to appear, they may include one or more of the following:

• Fatigue
• Nausea that may be accompanied by vomiting
• Feeling full or bloated
• A feeling that the bowel does not empty completely
• Bright red or very dark blood in the stool
• Narrower than usual stools
• Frequent gas pains or cramps
• Unexplained weight loss
• Diarrhea or constipation

Usually, colorectal cancer is detected after symptoms begin to appear. The diagnostic procedures could include:

• Analyzing medical history: The doctor may analyze the person’s medical and family history to try to determine the risk of developing colorectal cancer. The doctor may also ask if there are any symptoms and for how long the person had them.
• Physical examination: Physical examinations in the attempt to diagnose colorectal cancer may include feeling the abdomen for masses or lumps, or performing a digital rectal exam in which the doctor inserts a finger into the patient’s rectum to feel for any abnormalities.

• Colonoscopy: A colonoscopy is conducted to examine the entire length of the colon and rectum to detect cancer. It is performed with a long, thin tube fitted with a camera at the end called a colonoscope.
• Biopsy: It is done by performing tests on a small piece of tissue removed from the colon or rectum with a special instrument inserted through a scope.
• Imaging: Several imaging tests can be performed to look for colorectal cancer. These include:

• Computerized tomography (CT): This produces X-ray images that can reveal tumors.
• Magnetic resonance imaging: This creates a detailed image of body structures using radio waves and strong magnets instead of X-rays. It can detect cancer growing in the colon or rectum.
• Positron emission tomography: This test is performed by injecting radioactive sugar into the blood and create images that indicate which structures use energy most. this helps detect cancer because cancerous cells take-up energy actively.
• Double-contrast barium enema: A type of X-ray test which uses contrast dye to outline and give better detail of the colon and rectum to help detect any abnormalities.

• Blood tests: Blood tests are another method which can help doctors diagnose colorectal cancer, but they are unreliable alone and require further testing. Blood tests may include:

• Tumor markers: Colorectal cancer may produce tumor markers known as CA 19-9 and carcinoembryonic antigen (CEA) that can be detected in the blood; however, these markers are not reliable alone to diagnose cancer, because their levels can be affected by other conditions. This test is more commonly used to check how the patient responds to treatment.
• Complete blood count (CBC): This test can reveal if the person has anemia, which may be caused by a tumor bleeding for an extended period of time in some patients with colorectal cancer.

The recommended treatment for this cancer depends largely on the stage of cancer. The primary treatment options are:

• Surgery for early-stage colorectal cancer: If the cancer is very small in size, the doctor may suggest a minimally invasive approach to surgery, such as removing polyps during colonoscopy, endoscopic mucosal resection, and removing the polyps using laparoscopic surgery.
• Surgery for invasive colorectal cancer: This usually involves the removal of part of the colon (partial colectomy) or all or part of the rectum (proctectomy).
• Surgery for advanced cancer: Some operations can be performed to relieve certain conditions associated with colorectal cancer, such as blockage of the colon, bleeding, and pain. The purpose of these surgical procedures is to relieve symptoms, and they do not cure cancer.
• Chemotherapy: It is administered after surgery in case the cancer has spread to the lymph nodes; this may help reduce the risk of recurrence of this type of cancer.
• Radiation therapy: Radiation therapy is performed by using powerful energy sources, such as X-rays, to kill cancer cells that might have remained after surgery, to shrink large tumors before an operation to facilitate removing them, or to relieve symptoms of colon cancer and rectal cancer.
• Targeted drug therapy: This includes drugs that target specific cell changes associated with cancer. Examples include bevacizumab and ramucirumab which prevent tumors from forming new blood vessels that supply them with nutrients.
• Supportive (palliative) care:  Generally, the palliative care approach is directed toward improving the quality of life for people with cancer and their families.
• Alternative medicine: Art therapy, dance or movement therapy, and music therapy are all possible options that help the patient cope with the psychological effects of having cancer.

Complications of colorectal cancer may include:

• Bowel obstruction resulting from a blockage of the colon.
• Metastasis, which is when cancer spreads to other organs or tissues.
• Development of a second primary colorectal cancer (if already cured).

There is no certain method to prevent colorectal cancer, but there are plenty of measures that may help lower the risk of having it. The following are some of these measures:

• Early screening for colorectal cancer: The majority of polyps can be detected and removed before they have a chance to turn into cancer if regular screening is conducted appropriately. Screening is recommended to start at age 50.
• Lifestyle changes: This includes body weight monitoring, increasing physical activity, and proper diet. In addition, restricting the intake of red and processed meats and consuming more vegetables and fruits may help lower the risk of having this type of cancer.
• Stopping smoking: Smoking increases the risk of developing colorectal cancer, so stopping smoking may help reduce the risk.
• Vitamins and minerals: The daily consumption of a multi-vitamin containing folic acid, or folate, can contribute to lowering colorectal cancer risk.
• Non-steroidal anti-inflammatory drugs (NSAIDs): Taking aspirin or other non-steroidal anti-inflammatory drugs (NSAIDs) can contribute to lowering risk of colorectal cancer and polyps, but they be should be used under the supervision of a medical professional.
• Hormone replacement therapy for women: Prescribing estrogen and progesterone after menopause may lower the risk of developing colorectal cancer in woman.

Recovery from colorectal cancer is highly dependent upon the stage of the disease at the time of diagnosis. Generally, the chance of survival increases the earlier the disease is detected. Figures published by the American Cancer Society show the five-year survival rate is typically 90% for cancers discovered at the localized stage, 71% for cancers that have spread to regional lymph nodes or surrounding tissue,  and 14% for cancers that have spread to distant parts of the body.

An article published in the American journal Clinics in Colon and Rectal Surgery reports that colorectal cancer represents 9.4% of all cancers in men and 10.1% in women around the globe.  Colorectal cancer is the third most common cancer worldwide and the fourth most common cause of death.

Countries with the highest incidence rates include Australia, New Zealand, Canada, the United States, and parts of Europe. Countries with the lowest risk include China, India, and parts of Africa and South America. It is estimated that 394,000 deaths from colorectal cancer still occur worldwide every year.