What Is Colon Cancer?

Colon cancer begins when abnormal cells within the colon start growing uncontrollably. Symptoms may not occur initially, but as the disease progresses, a person may notice a change in their bowel habits or dark or bright red blood in their stools. There are several factors that increase a person's chance of developing colon cancer, some within a person's control (for example, being overweight and leading a sedentary lifestyle) and some not (for example, having an inflammatory bowel disease).

Anatomy

The colon is located around the perimeter of the abdomen and is about five feet in length. The colon is divided into the following sections:

• Ascending colon
• Transverse colon
• Descending colon
• Sigmoid colon

The term colon cancer is sometimes used interchangeably with colorectal cancer because colon and rectal cancer share common features. The rectum is a part of the colon, and both organs together are called the large intestine. The rectum measures six to 12 inches long and is located between the sigmoid colon and the anus. The stool is stored in the rectum until it is ready to be evacuated from the body via a bowel movement.

Types of Colon Cancer

Based on the tissues involved, colon cancers be further broken down into various types.

Adenocarcinoma is the most common type of colon cancer, accounting for approximately 96 percent of diagnoses. This type of cancer originates in the mucus-secreting glands of the colon.

In addition to this, there are multiple subtypes or variants, meaning the tissue or cell pattern of the cancer differs slightly from a "conventional" adenocarcinoma. For example, mucinous adenocarcinoma is one subtype that produces excessive mucous and tends to be found on the right side of the colon.

Other adenocarcinoma variants include:

• Signet-ring cell carcinoma
• Serrated adenocarcinoma
• Cribriform comedo-type adenocarcinoma 
• Micropapillary carcinoma
• Medullary carcinoma

Besides adenocarcinoma, much rarer types of cancer found in the colon include:

• Soft tissue sarcomas
• Colorectal lymphomas 
• Neuroendocrine tumors 
• Gastrointestinal stromal tumors (GISTs) 

Colon Cancer Symptoms

There are many potential symptoms of colon cancer, and what symptoms a person experiences depends on factors like the location and size of the cancer.

Keep in mind that some people experience no symptoms in the early stages of colon cancer. In addition, the signs and symptoms mentioned below may be (and usually are) due to a non-cancerous medical condition, like an infection or hemorrhoids. Seeing your doctor, though, is the only way to know for sure.

Colon cancer symptoms include:

• A change in bowel habits, like experiencing diarrhea, constipation, or stool thinning ("pencil stools") that lasts for more than a few days
• Alternating constipation and diarrhea, potentially due to a partial obstruction of a tumor in the colon
• Bright red or dark red blood in your stools
• Feeling like you cannot completely empty your bowels
• Abdominal discomfort and cramping
• Whole-body symptoms like weakness, unusual tiredness, unintended weight loss, and a loss of appetite
• Nausea or vomiting
• Yellowing of the skin or whites of the eyes (called jaundice)

Causes

Most colon cancers are caused by growths within the inner lining of the colon, called polyps, that change into cancer over many years. That said, the majority of polyps do not develop into cancer. The chance of a polyp developing into cancer depends on factors, like the type of polyp (adenomatous polyps are considered pre-cancerous) or whether the polyp contains abnormal cells (called dysplasia).

There are several risk factors associated with the disease. Some can't be changed, while others may be addressed with lifestyle changes. Common risk factors include:

• Being over the age of 50
• Having colon polyps
• Drinking alcohol to excess
• Having family members with colon cancer
• Eating a high-fat, low-fiber diet
• Being overweight or obese
• Physical inactivity
• Having a personal history of other cancers
• Smoking or using tobacco 

Another risk factor for colon cancer is inflammatory bowel disease (Crohn's disease and ulcerative colitis). More specifically, having active inflammatory bowel disease in the colon for a long time can increase this risk. The majority of people with IBD, however, will never develop colon cancer.

Diagnosis

Screening tests are important for the diagnosis of colon cancer, since most people do not have symptoms in the earlier stages of colon cancer.

According to the American Cancer Society, the tests used to screen for colon cancer include:

• Colonoscopy
• Virtual colonoscopy
• Flexible sigmoidoscopy
• Guaiac-based fecal occult blood test
• Fecal immunochemical test
• Stool DNA test

If colon cancer is suspected with a screening test, a person will then undergo a diagnostic colonoscopy and biopsy. With a biopsy, a gastroenterologist removes a piece of tissue from the suspicious mass. Then, a pathologist examines the tissue sample under a microscope to look for cancer cells.

If cancer is present, other tests may be performed on the sample to better classify cancer, like those that look for mutations in certain genes. This way, treatment can be best targeted towards that specific cancer.

Imaging tests, like a CT scan, MRI, chest X-ray, or PET scan, are also used to access the stage of cancer, meaning whether (and how far) it has spread to lymph nodes or organs. Colon cancer stages range from 0 to 4.

Treatment

Based on the type, stage, and location of cancer, doctors can determine the best course of treatment to either cure the disease or better manage the symptoms if the cancer is not treatable.

Options that may be considered include:

• Surgery: For colon cancers that have not metastasized or spread to distant sites in the body (for example, the liver), surgery is the primary treatment. Surgery often entails removing the cancerous polyp during a colonoscopy (polypectomy). Sometimes part of the colon needs to be removed to treat colon cancer (partial colectomy). Rarely, the whole colon is removed (total colectomy). 
• Local therapy: Radiation therapy is a local therapy because it targets one area of the body. It may be used to treat certain people with colon cancer, often in conjunction with surgery and/or chemotherapy.
• Systemic therapies: Chemotherapy drugs target rapidly duplicating cells in the body and may be administered before or after colon cancer surgery and/or in conjunction with radiation. Sometimes other systemic therapies are used, either alone or in addition to chemotherapy. Targeted therapies bind to and block the action of specific proteins located inside or outside the cancer cells that normally help them grow. For people with advanced colon cancer or cancer that is resistant to chemotherapy, immunotherapy may be utilized. 
• Palliative care: Palliative care is an important component of colon cancer treatment, especially in the advanced stages. This type of therapy focus on optimizing comfort and managing symptoms like pain, anxiety, and bowel issues. 

Prevention

The most important step you can take to prevent colon cancer is to talk to your doctor about getting screened. The good news is that there are several screening tests available including visual tests (for example, a colonoscopy) and at-home stool tests (for example, the fecal immunochemical test).

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According to the American Cancer Society, screening for colon cancer should begin at age 45 for people of average risk. For people at an increased or high risk for developing colon cancer (for example, those with inflammatory bowel disease or with a family history of colon cancer or polyps), screening begins at an earlier age and at more frequent intervals. 

Other strategies you (and your loved one) can undertake to prevent colon cancer include:

• Losing weight, if you are overweight or obese
• Moderating red meat consumption and limiting (or avoiding) processed meat consumption
• Eating more fruits, vegetables, and fiber
• Engaging in regular moderate to vigorous exercise
• Avoiding smoking and excess alcohol consumption

A Word From Verywell

Colon cancer can be a difficult disease to wrap one's head around. It's diagnosis, staging, and treatment is often complex and intensive. The good news, though, is that the rates of colon cancer are declining as more people who are at risk for it are getting screened. Moreover, with the improvements in screening and available treatments, the survival rates of colon cancer are increasing.

Lastly, experts are discovering more and more ways for people to prevent colon cancer in the first place. Some of these simple strategies include eating more fiber and less red meat, exercising, losing weight if overweight or obese, and avoiding excess alcohol consumption.