Screening: It Works!
Colorectal cancer doesn't develop overnight. It takes many years, as long as two decades, for normal cells that line the lower intestine or rectum to transform into cancer. Before the emergence of full-blown cancer, these cells must first evolve into growths called polyps. Screening for colon cancer works by detecting these polyps before they become deadly.
It Begins In The DNA
Cells lining the intestine constantly change. Older cells die and new ones are created. During this process, changes occur in the cells' DNA. Either by chance or because of inherited tendencies, changes in DNA can lead to abnormal cell growth. Usually, the cell's defense systems keep abnormal growth in check. However, if abnormal cell growth can't be checked, a polyp begins to form.
A polyp begins as a small nubbin (lump) invisible to the naked eye. Over many years, it grows into a bump. As the bump gets bigger, it may stop growing. Polyps that remain small, about 0.5 cm (¼ inch) or less in diameter, are usually benign and remain so. However, the polyp may continue to enlarge.
From Polyp To Cancer
Most polyps that continue to enlarge are of a type called adenomas. Ninety-nine percent of adenomas remain benign. But if not removed, they may become malignant. Once cells become malignant, they increase more rapidly. During this stage, the body produces no warning signs.
Screening for colon cancer works by detecting the presence of polyps before cancer develops and before signs and symptoms appear. For example, sometimes polyps bleed. A fecal occult blood test is designed to discover the "occult" or hidden blood in your bowel movement that may indicate polyps. With a more advanced test called colonoscopy, the doctor looks directly for polyps.
Once A Polyp Is Found
During a colonoscopy, if the doctor finds one polyp, he or she will look through the entire colon and rectum for more. The doctor removes the polyps and sends them to the pathology lab for evaluation. A lab report gives details about the specimen, providing information about the degree of cancer risk. If an adenoma is removed completely, it will not become cancerous, even if there are a few cancer cells at the tip.
If a person has an adenoma, even a benign one, his or her risk of developing a new polyp increases substantially. About 35 percent of people with one polyp will develop a second polyp within five years. Therefore, a repeat colonoscopy is recommended in one to five years, depending on the size and appearance of the polyp.
The Bottom Line About Screening
Screening doesn’t really prevent most types of cancer. Instead, screening is intended to find cancer when it is at an early, highly curable stage. But screening for colon cancer is different: Finding and removing precancerous polyps can actually prevent cancer from developing. Screening for colorectal cancer will also help to find cancerous tumors when they are small and can be removed completely.