Colon and Rectal Cancers
Colorectal Cancer Screening Guidelines
A number of major organizations, including The American Cancer Society, The US Preventive Services Task Force, and The American College of Gastroenterology, have created guidelines related to screening for colorectal cancer.
Overall, their recommendations are fairly similar. However, some differences exist between the different guidelines. These differences are related to:
- Which tests are recommended or preferred
- The recommended frequency at which the screening tests should be performed
- When screening should begin or end
Screening tests that are addressed in the guidelines include:
- Flexible sigmoidoscopy
- Double-contrast barium enema (DCBE)
- CT colonography (CTC), also called virtual colonoscopy
Stool test options for screening include:
- Guiaiac-based fecal occult blood test (FOBT) with high test sensitivity
- Fecal immunochemical test (FIT) with high sensitivity
- Stool DNA test (sDNA)
In 2011, levoleucovorin (Fusilev) was approved for use in combination with 5-fluorouracil (5-FU) for palliative treatment of advanced metastatic colorectal cancer. Levoleucovorin is chemically related to leucovorin (Wellcovorin), a cancer drug that has been in short supply in the United States in recent years.
Cancers of the colon and rectum, often collectively referred to as colorectal cancer, are life-threatening tumors that develop in the large intestine.
More than 80% of colorectal tumors develop from adenomatous polyps. These gland-like growths develop on the mucous membrane that lines the large intestine. They are usually either:
- Tubular polyps, which protrude mushroom-like
- Villous adenomas, which are flat and spreading and are more apt to become malignant (cancerous)
Polyps are very common, and most of them are benign. Their numbers increase with age. Polyps are found in about 25% of people by age 50, and 50% of people by age 75. Fewer than 1% of polyps under 1 centimeter (slightly less than half an inch) become cancerous. About 10% of larger polyps become cancerous within 10 years, and about 25% of these larger polyps become cancerous after 20 years.
Digestion takes place in the gastrointestinal (GI) tract, basically a long tube that extends from the mouth to the anus. It is a complex organ system that first carries food from the mouth down the esophagus to the stomach. Food then travels through the small and large intestines before being excreted through the rectum and out the anus.
The esophagus, stomach, and large and small intestine -- aided by the liver, gallbladder, and pancreas -- convert the nutritive components of food into energy and break down the non-nutritive components into waste to be excreted.
The esophagus is a narrow muscular tube, about 9 1/2 inches long, that begins below the tongue and ends at the stomach.
In the stomach, acids and stomach motion break food down into particles small enough so that the small intestine can absorb nutrients.
The small intestine, despite its name, is the longest part of the gastrointestinal tract, extending for about 20 feet. Food passes from the stomach through its three parts: first the duodenum, then the jejunum, and finally the ileum. Most of the digestive process occurs in the small intestine.
Undigested material, such as plant fiber, is passed next to the large intestine, or colon, mostly in liquid form. The colon is wider than the small intestine but only about 6 feet long. The colon absorbs excess water and salts into the blood. The remaining waste matter is converted to feces through bacterial action. The colon is a continuous structure but it is characterized as having several components.
Cecum and Appendix. The cecum is the first part of the colon and it gives rise to the appendix. These structures are located in the lower-right quadrant of the abdomen. The colon continues onward in several sections:
- The first section, the ascending colon, extends upward from the cecum on the right side of the abdomen.
- The second section, the transverse colon, crosses the upper abdomen to the left side.
- The third section extends downward on the left side of the abdomen toward the pelvis and is called the descending colon.
- The final section is the sigmoid colon.
Rectum and Anus. Feces are stored in the descending and sigmoid colon until they are passed through the rectum and anus. The rectum extends through the pelvis from the end of the sigmoid colon to the anus.
In most cases of colon or rectal cancers, the cause or causes are unknown. Defects in genes that normally protect against cancer play the major role in causing polyp cells to change and become cancerous. Some of these cases are caused by inherited genetic defects, and such patients usually have family histories of colorectal cancer. Most of the genetic mutations involved in colon cancers, however, appear to arise spontaneously (no strong family history) rather than being inherited. In such cases, environmental or other factors may trigger genetic changes in the intestine that lead to cancer.