What is colorectal cancer?

Cancer that begins in the colon is called a colon cancer, while cancer in the rectum is known as a rectal cancer. Cancers that affect either of these organs may be called colorectal cancer. Though not true in all cases, the majority of colorectal cancers generally develop over time from adenomatous (precancerous) polyps. Polyps (growths) can change after a series of mutations (abnormalities) arise in their cellular DNA. Some of the risk factors for colorectal cancer involve a family history of colon or rectal cancer, diet, alcohol intake, smoking and inflammatory bowel disease.

What parts of the body are affected by colorectal cancer?

To understand colorectal cancer, it is helpful to understand what parts of the body are affected and how they work.

The colon

The colon is an approximately 5- to 6-foot long tube that connects the small intestine to the rectum. The colon—which along with the rectum is called the large intestine—moves and processes digesting food across your body and down towards the rectum, where it exits the body as stool. There are several parts of the colon, including:

  • Ascending colon: This section is where undigested food begins its journey through the colon. Undigested food moves upwards through this section, where fluid is reabsorbed more efficiently.
  • Transverse colon: Moving across the body, the transverse colon takes the food from one side of the body to the other (right to left).
  • Descending colon: Once the food has travelled across the top through the transverse colon, it makes its way downward through the descending colon—typically on the left side.
  • Sigmoid colon: The final section of the colon, this portion is shaped like an “S” and it is the last stop before the rectum.

Illustration of the colon | Cleveland Clinic

The rectum

The rectum is a 5- to 6-inch chamber that connects the colon to the anus. It is the job of the rectum to act as a storage unit and hold the stool until defecation (evacuation) occurs.

How does colorectal cancer develop?

All of the body's cells normally grow, divide, and then die in order to keep the body healthy and functioning properly. Sometimes this process gets out of control. Cells keep growing and dividing even when they are supposed to die. When the cells lining the colon and rectum multiply uncontrolled, colorectal cancer may ultimately develop.

Fortunately, most colorectal cancers begin as small precancerous (adenomatous or serrated) polyps. These polyps usually grow slowly and do not cause symptoms until they become large or cancerous. This allows the opportunity for detection and removal at this pre-cancerous polyp stage before the development of cancer.

What if I have polyps?

There are a variety of colorectal polyps, but cancer is thought to arise mainly from adenomas and sessile serrated lesions, which are precancerous polyps. If a polyp is found during a colonoscopy it is usually removed, if possible. Polyps removed during colonoscopies are then examined by a pathologist and evaluated to determine if they contain cancerous or precancerous cells. Based on the number, size, and type of precancerous polyps found during colonoscopy, your healthcare provider will recommend a future colonoscopy for monitoring (surveillance).

What are the signs and symptoms of colorectal cancer?

Unfortunately, some colorectal cancers might be present without any signs or symptoms. For this reason, it is very important to have regular colorectal screenings (examinations) to detect problems early. The best screening evaluation is a colonoscopy. Other screening options include fecal occult blood tests, fecal DNA tests, flexible sigmoidoscopy, barium enema, and CT colonography (virtual colonoscopy). The age at which such screening tests begin depends upon your risk factors, especially a family history of colon and rectal cancers.

Even if you do not have a family history of colorectal cancer or polyps, tell your doctor if you have any of the signs that could indicate a colorectal cancer, no matter what your age. Common signs of colorectal cancer include the following:

  • Change in bowel habits: Constipation, diarrhea, narrowing of stools, incomplete evacuation, and bowel incontinence — although usually symptoms of other, less serious problems — can also be symptoms of colorectal cancer.
  • Blood on or in the stool: By far the most noticeable of all the signs, blood on or in the stool can be associated with colorectal cancer. However, it does not necessarily indicate cancer, since numerous other problems can cause bleeding in the digestive tract, including hemorrhoids, anal tears (fissures), ulcerative colitis, and Crohn's disease, to name only a few. In addition, iron and some foods, such as beets, can give the stool a black or red appearance, falsely indicating blood in the stool. However, if you notice blood in or on your stool, see your doctor to rule out a serious condition and to ensure that proper treatment is received.
  • Unexplained anemia: Anemia is a shortage of red blood cells— the cells that carry oxygen throughout the body. If you are anemic, you may experience shortness of breath. You may also feel tired and sluggish, so much so that rest does not make you feel better.
  • Abdominal or pelvic pain or bloating
  • Unexplained weight loss
  • Vomiting

If you experience any of these signs or symptoms, it is important to see your doctor for evaluation. For a patient with colorectal cancer, early diagnosis and treatment can be life-saving.

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